With a few exceptions, most 1982-'85 Eldorado (and Seville) models running around are equipped with HT4100 engines. Even with the many small changes and improvements made each year to correct the myriad material and design weaknesses (the '83-'85 TSB book is entertaining reading on this), there are plenty of reasons you might want to swap in something a little more reliable or more powerful.

The cars that have survived tend to either have been well-maintained, or had corrective repairs/replacements performed years ago, so with a little TLC, they should continue to perform better than their reputation. I'll give them this, they're the smoothest V8 going. That said, even on the best day, the 135HP (83-85), 12.6 sec 0-60, is barely adequate motivation in one of these cars. Unlike their RWD counterparts, you can't just drop in a big block and THM400. But a satisfactory swap is still within reach if you're determined....
 

There's a lot to take in, so the swap guide is divided into the sections listed here.

1. Start Here 2. Parts 3. Donor Cars 4. Awareness 5. HT4100 Engine Removal Procedure 6. Engine Installation Pointers 7. Olds Rebuilding Tips Electrical Swap Section (warning lights, dash wiring mods, etc) Cruise Control Torque Converter Lockup

1. Start Here:

The biggest drawback to engine swapping a 4100 car is how to handle the integrated electronics. You'll lose Fuel Data either way, but you can still make Climate Control work, and there are workarounds for cruise control and torque converter lockup. You will find detailed info in the Electrical Swap Section. Also, See Here for a custom Fuel Data solution for '82-85 Eldos and Sevilles.

There are several good engine options when doing a swap, with the Olds 307/350 being the most popular due to cost and availability. Most everything you need has already been made by the General in one form or another and is available if you look hard enough (more on that below).

If you're power-hungry, the stock trans will handle a 403 (w/shift kit), but anything larger is asking for trouble. Fuel Injection can be added, either with something aftermarket, or a DIY approach using a first-gen Seville intake and your electronics of choice, but for many a carb is just fine. If you really want to run a big block Olds or Cad, a trans upgrade to a THM-425 is required, and with that, custom mounting, firewall modification, custom half-shafts, cooling upgrades, and deep pockets. It can be done, but it's overkill. A 403 already has more power than can be reliably put to the ground.

2. Parts.....What Fits?

You're best bet is an Olds small block. The 307/350/403 are virtually identical externally, and will bolt right up to the stock trans.

A Cad 368 will also work if you find an 80/81 FWD parts car, though if you're pulling an engine from an '81, be advised those have the V864 valvetrain/computer setup which adds complexity (you'll need both engine, ECM, cylinder controller, dash harnesses, etc.) A pretty involved job. An '80 isn't much easier. Because of that, This swap guide will focus on the Olds transplant. Remember, an Olds 350 has nothing in common with a Chevy 350, other than displacement.

Whichever Olds engine you chose, make sure it's from a FWD car, or you'll need to track down the additional FWD parts listed below to swap over.
I highly recommend buying a parts car, it'll end up cheaper in the long run, provide an ample supply of other body/interior parts you might want, and should still hold its value in scrap when you're done. You might even find it has superior suspension parts to swap over.

Bare Necessities from an Olds FWD (307/350/403)

1. FWD Oil Pan, Pickup & Dipstick       Any 79-85 FWD Olds block.
2. FWD Olds Exhaust Manifolds / Downpipe       Any gas 79-85 FWD Olds block. See Exhaust Section below.
3. Shift Bracket and rod that mounts on driver-side rear of engine.  Found on '79-only Riv, or 1979-85 E / 80+ K body with an Olds block, including diesels. 1980-85 Riv & 79-85 Toro do not work.
4. Olds Final Drive Support Bracket.       Any 79-85 FWD Olds block including diesel. You may need to elongate the bolt slots depending on vintage of bracket. Pre-'82 mount flush to the block while the later brackets are pushed an 1/8" out to account for the new motor mount bracket.
5. RH Output Shaft & Bracket Assembly       Any 79-85 FWD Olds block including diesel.         (4100, 368 and V6 will not work)
6. FWD Flexplate GM p/n: 562949.       Any 79-85 FWD Olds block; diesels may be different.       These were available new from internet sources some years back.          (slightly smaller diameter than RWD)
7. Carb Bracket for Throttle & Trans T.V. Cable    1980-85 Olds Shown at Right. See TV Bracket Section #11 below.
8. Front "Engine Mount" Bracket '82-85.      (Ideally: '82-85 Toro, Riv or Diesel Eldo/Seville) 1979-81 for 350 is a 2-piece setup like the HT4100 and can be made to work by drilling matching hole(s) in your 82-85 front crossmember. 1982-85 rubber mounts are the same L&R, but not compatible with the 4100 or earlier Olds brackets.
9. FWD Oil Filter Mount Make sure you use the matching gasket as it's different from the RWD version.    Any 79-85 FWD Olds block.
10. Power Steering	1980+ GM cars use a steering gear that takes metric o-ring -AN6 fittings. If your PS pump is '79 or older, use adapter kit Borgeson 925122 which allows use of your flare-fitted pressure hose. Since the return hose is not under pressure, use whichever matches your steering gear and attach to your PS pump with clamp per manufacturer. You can also move the metric 4100 pump over to the Olds bracket, but the 4100 pump pulley requires a special tool for removal. The Olds PS pump brackets mount to 2 studs on the front cover, and the front- most engine mount bracket hole on the driver side of the block. I am convinced this PS 'gusset bracket' changed in '82 to coincide with the new horseshoe engine mount bracket. The older style gusset bracket will work, but you'll need to get out the angle grinder to 'round' the squared-off tab as well as remove interfering material from the backside so it will sit flat against the engine mount bracket. Check pulley alignment and add washers as needed.
11. Brackets & Cables	The 4100 transmission TV cable is too short b/w eyelet and mounting flange. Try to use a 1980-85 Riv/Toro TV/Accel Cable bracket (#7 above) and matching '80-85 TV cable (such as CA-1943). A '79 Riv bracket will work with the TV cable, but uses a unique accelerator cable which is no longer available. These new cables seem especially susceptible to heat so keep it routed from the exhaust. I even added heat wrap after the first failure. The 4100 accelerator cable is too short to reach the Olds carb/TBI; as mentioned above, avoid the '79 bracket unless you have a good cable to use. Match an '80-85 cable with an 80-85 bracket. For some reason, Toro & Riv differ by 1.5" in some listings. The '79 Eldo EFI accel cable also works with the '80-85 bracket & carb.
Optional: Fan shroud if needed, Y-Pipe/Down Pipes to save time/money on exhaust, engine harness from donor, radiator/mounting parts if larger	-A '79 Eldo fan shroud and cap will bolt right up to the '85 header. -Replacement exhaust flange bolts/springs are available as Dorman 03137. -Exhaust manifold flange are Walker/Fel-Pro 60360. -The bed-style cats have an offset inlet as compared to the later style, so some  exhaust work may be required depending on the year.

3. Donor Cars:

No single donor will have everything you need, but assuming you're swapping out an HT4100 car (1982-85), an '82-85 Riv or Toro parts car will provide most of what you need. Of course there are plenty of 'gotchas', and you might already be sitting on a box of Olds parts, so here are some things to consider....

• INTAKE MANIFOLD: Any FWD/RWD Olds 307/350/403 stock intake should work. Out of the box, an Edelbrock Performer will sit too high once a carb and air cleaner are mounted. First gen Seville & 79/80CA Eldos have multiport intakes that will also work if going with EFI; be sure to grab the matching air cleaner. The early version of this intake is cast iron but has larger bores. The later is aluminum, but corrosion can be a problem. For air cleaners, the EFI cars do NOT have thermac valves but will fit a carb.
   
• HEATER NIPPLE: Located at the rear passenger corner of the intake, Cadillac used a passive threaded nipple and their vac-operated hot water valve was hose-mounted inline. The diesel cars, and Olds version typically used a combination valve that was part of the nipple, often with a provision for a blower motor turn-on switch. Either will work, but you may want to simply reuse the 4100 inline valve and grab a nipple at the parts store/home center. The blower turn-on switch is incompatible with the 4100's climate control, so no advantage there.
   
• EXHAUST: The HT4100 Y-pipe diameter & arrangement is very restrictive; you'll want to use the donor car setup. Somewhere along the line ('84?) the old bed-style cat converters were changed to the smaller modern type. The bed-type uses an angled inlet, so some exhaust work may be necessary if you're using the earlier Y-pipe arrangement with the later version cat, or vice-versa as alignment is slightly off. The down pipes / Y-pipe assembly is no longer available new, so try to find a good example, or fab will be necessary.
  
  The Diesel exhaust will bolt-up and is a larger diameter which also eliminates the cat. I recently pulled one from an '83 Eldo and it appeared to take into consideration the offset inlet. Diesel exhaust manifolds are completely different and best avoided. Diesels and the EFI cars do not use a butterfly valve in the exhaust for warmup.
   
• ALTERNATORS: Diesels & the EFI Cadillacs use an 80A alternator, a nice upgrade. To make this work, you need the matching brackets. And because the alt body sits higher, you also need the matching air cleaner or the snorkel will interfere. Just an FYI, the EFI car snorkels do not have thermac valves, and the diesels are a different incompatible style, altogether. I suspect a RWD Olds with heavy duty charging would have a thermac-equipped compatible air cleaner. The thermac is probably not needed unless you do a lot of winter driving.
   
• TRANSMISSIONS: Since you're converting an '82-85 HT4100, these all have 4-speed transmissions. If you really want to put the THM-325 3-speed in, I suspect you could swap the rear trans brackets to match the 82-85 frame which uses donut mounts. The 4-speed will need an electrical connection for torque converter engagement. See the Electrical Swap section for details. Furthermore...The 1981 trans has a "3^rd Gear Output" switch for the V864 MD system. (*1981 V6 cars supposedly have a 325-4L)
  
  Some will argue with the 325 is stronger than the 325-4L (200-4R); between the chest-beaters and the tear-down experts, the bottom line is...it's a wash.
  
  The '82-'85 4-speed is slightly different between diesel and DEFI. Besides different torque converters, the DEFI trans has a switch that opens when in 4^th gear to let the ECM know when it's ok to engage the TCC. The Diesel cars have a governor output that closes above ~40MPH.
   
• BRACKETS: Final drive support bracket and the RH output shaft/bracket were covered in the intro section; if it has an Olds block and it's FWD, they're basically all the same. A TV (throttle valve) cable is required for the trans to operate correctly, and should be matched to whichever bracket you're using. If running the EFI intake, use the EFI bracket arrangement. If running a carb, an 80-85 Toro/Riv accel/TV bracket is your best bet.
  
  The shifter bracket is another matter. '79-85 Toros use a cable shift, as do the '80-85 Riv's. It's a superior system but requires a steering column swap to take advantage of it. Otherwise, you need a bracket from a '79 Riv, a 79/80 CA Eldo, or a diesel FWD Eldo/Seville. Here's the Cable-Shift setup.

1979-81 Trans Mount Brackets Welded to Frame 1982-85 "Donut" Mounts at Rear Example of Toronado Shift Cable Arrangement - Circled in Red

Now, about those Donor Cars

• 79 Eldo has an Olds 350 and is pretty close to a direct swap, except it originally had EFI, the same system as used on the 1^st gen Sevilles. You can use a compatible small black Olds carb/intake and call it a day (and a lot of these have been converted). Or, if you want to keep the EFI, you have a couple options. The harness unplugs at the firewall, and you could get out the soldering iron and extend it to use with an aftermarket system like MegaSquirt (see EFI section). You could also retain the original analog EFI computer, but because the harness is integrated into the dashboard wiring, grabbing a standalone harness from a 75-79 Seville would make life easier. The Seville harness connects directly from the engine to the ECU through a dedicated firewall grommet and is virtually plug-and-play. CA-Spec 1980 Sevilles and Eldos also have this drivetrain, but use a different ECU and an O2 sensor. See Tech Info Page for ECU P/N's. You will need to drill your front crossmember for the front motor mount spacing on the '79, and you'll want to use your existing trans. If you use the old 3-spd, then swap over the rear mount brackets.
   
• 79 Toro/Riv: Will have a carb'd Olds 350 if not diesel and not turbo. Everything should swap over like the Eldo. The Riv will have the correct shift bracket. Both have the old style motor mount brackets.
   
• 80 Toro/Riv could have either a 350 or 307. Check the emissions label or better yet, the displacement will be cast in large numbers on the side of the block near the freeze plugs.

• 80 Eldo: Cad DEFI 368 unless ordered for CA emissions, in which it's an Olds 350 w/EFI (see '79 caveat above). If you really want a 368 without the EFI headache, you could use the carb/intake from an '80 RWD Cad. However, this guide does not cover that swap.

• 79-85 Eldo/Riv/Toro/(80+)Seville with a Diesel. Has many good external mounting parts, especially if 1982+. This includes the cruise module if it's a Cadillac (even RWD). But the diesel T.V. Cable bracket is different compared to the carb version, and the shifter caveat above applies to the Toro/Riv. I'm told the diesel flexplate is different, but haven't confirmed. The Diff bracket, RH output shaft and oil pans should swap.

• 77/78 Toro: Good source for an Olds 403, but the FWD parts are different than 1979-85. Note: these engines have a computer for spark control and a crank trigger. You can drop in a standard Olds HEI and omit the rest of that system if you desire. The spark curve of a 350 distributor will greatly improve the 403's running.

1979 Toronado 1982-85 4100 Eldorado

Note: HT4100 has smaller exhaust pipes; both feed offset cat inlet until debut of smaller inline cat.

4. Other Considerations...

• Now is a good time to figure out your fuel line strategy. The trans donut mounts make it difficult to route fuel lines to the engine while avoiding exhaust heat, and the passenger side is especially close to the exhaust manifold with the offset engine. The 4100 cars run supply along the driver's side, and return & evap on the passenger side. Evap then crosses under the engine and pops up adjacent to the charcoal canister.
  
  For a carb conversion, I retained the 4100 flex line, adapted to couple with new hard lines up over the intake to the mechanical pump. For a 40 PSI EFI conversion some years ago, I want to say I installed new 3/8 hard line from the tank (and new external pump) along the passenger side. I also noted that diesel cars use a different line arrangement along the passenger side, doing their best to skirt the hot exhaust with some extreme bends.
   
• Final drive ratios on these cars are all over the place depending on the original engine, trans, year and optionality. You can mix and match for your preferred package. '79 is as numerically low as 2.19:1, while an '85 Eldo can be 3.15:1 (3.36 in a T-Type Riv!) to take advantage of the OD trans. When in doubt, look for the buildsheet, or the stamped code on the final drive 'tab'. Speedometer error can be addressed by a combination speedometer-driven-gear swap and an inline speedo cable gearbox. See the Tech Info Page for details on codes and gear colors.
   
• The rear trans donut mounts are actually bolted to the frame from the outside, with square nuts welded to the flange itstelf.
   
• Riv T-Type: I excluded these from above as they're a special breed you're not likely to find as a cheap parts car. These have turbo 3.8L V6's. If a guy wanted to swap everything over to an Eldo, I don't see why it wouldn't work, but have no useful info to add.
   
• The Buick 4.1L V6 was offered on all 3 E-bodies (and RWD Cads) from mid-1980 through '82, and other than the cruise module (Cadillac only), would be quite worthless as a parts car. Word is that the V6 cars got the 4-speed trans prior to 1982. Would be interesting to prove/disprove that.
   
• 81-up Toro/Riv use a computer carb. Not a bad system once understood, but you'll need the wiring/ECU if you want to retain it.
   
• The front engine mount bracket changed in '82. The 4100 uses a pair of L-brackets that attach to the front cross member. Ditch those in favor of the Olds U-shape bracket from 82'-up which'll fit the 307/350/403 and the studs will drop into the existing holes. If you find yourself with a 79-81 bracket, it will fit the block, but you'll need to drill new holes in the cross member as the stud spacing is different. I like the U-bracket as it mounts to the sides of the engine rather than blocking the front cover. The matching motor mounts use single bolts and are the same p/n for L or R.
   

  1979 Eldo Olds 350 Bracket Arrangement - Cad Parts Book

  Olds 350 Bracket Arrangement - 1984 Buick Shop Manual

5. HT4100 Engine Removal:

Alright, here we go! This isn't too bad a job, just take your time and plan ahead and everything should go smoothly. If the car's outside, I leave the hood on and disconnect everything I can until it's time to actually pull the engine, and I wait to pull the heater/radiator hoses until AFTER I'm done crawling around underneath unbolting things so I'm not laying in antifreeze. Also... DO NOT CUT ANY ENGINE WIRING.

*If you need to swap out/repair your power antenna, now is the time to do it as the passenger side hood hinge has to be removed to get the antenna out. See the Electrical Swap Section for details.

Front & Rear brackets securing engine & trans- See Buick Dwg Above	A high angle of attack is a must for removal/install.	Rear brackets are visible here; life is easier if removed before installation

6. Engine Installation Pointers:

• With the HT4100 out of the car, you'll want to carefully remove the two engine harnesses to reuse with the new engine. Quite a lot obviously won't get connected, and wires can be stagger-cut, taped/shrink tubed to prevent shorting. But don't do anything just yet, you might want to re-use some of that wiring for gauges/lights. Please see the Electrical Swap Section section for full details.
   
• Ground Strap: If a strap was present, be sure to re-install it between engine and body/frame. If not, someone removed it; add one. Otherwise, currents will find their own path back to the frame, either through sensitive electronics, or even engine bearings, and resistance can affect aftermarket gauge readings.
   
• The HT4100 cars use metric steering gears; see info above in Section 2.
   
• If your trans cooler lines are shot, new ones can be made from 60" pre-made stock. One is exactly 60", the other a couple inches shorter, but there's enough 'play' that 60" can work, avoiding any cutting or flaring.
   
• Before you even think about putting the THM325-4L back in the car, there are a few things you should do. First, order a Transgo SK-325-4L kit and follow the instructions to-the-letter. This will improve all of the factory deficiencies, improve shift quality, and give your trans more life, especially with a more powerful engine. Even if you decide to do a basic rebuild, include this kit in your plans.
   
• You might want to replace the RH output shaft bracket bearing. Timken 6206-2RS is the official sub. OD is perfect, but I have yet to get the ID to fit the original shaft. Dimensions are simply too tight, and exceed allowable press-fit design clearance. A head-scratcher as the bearing ID numbers are a match.
   
• When you go to install the new engine/trans package, remove the rear transmission mount brackets from the trans. Set your leveler so that the chain cover tucks in under the firewall, then support the rear of the trans with a jack and wood block. Lower the front down so the studs drop into the cross-member, then jack the rear of the trans up to allow reinstallation of the rear u-brackets. Doing it this way will save hours of frustration. The driver side bracket doesn't have enough clearance for a ratchet, but a Gearwrench fits, and you can jack that side up as needed to get the bolts in and started.

    This is also a good time to replace any seals, such as:

• Front Diff Output Seals – Timken TM3907 is a good sub for these.
• Torque Converter Seal, Dipstick Tube seal (ATP brand SO-35), Shift Shaft o-ring, TV cable.
• New trans filter with o-ring, and add a small magnet square to the pan to catch clutch material.
   -> One expert claims torque converter drainback was an early issue on these; the official fix was a filter with a check valve in it, but I guess not all replacements take
        this into consideration.
• "Do as I say, and not as I do" and add a drain plug to make life easier down the road.

If you changed transmissions or final drives, this would be a good time to sit down and recalculate your speedometer error. Different rims/tires will also affect this. Depending how far off you are, a road test with a GPS speed app on your phone can help dial in error. Remember, GM made quite a few different speedo output gear ratio options that will drop into your trans; if you can't find exactly what you need, you should be able to get pretty darn close with a speedometer cable ratio change gearbox and a matching trans gear. Lots of flexibility there.

Depending how handy you are, you could also expand the speedo face to something beyond 85MPH, printed on good quality vinyl or UV-stable backer. See the Speedometer Swap Page for more.

7. Olds Rebuilding Tips:

When it comes to Olds engines, I'm far from an expert, but having just gone through the rebuild process, perhaps the following will be helpful. Especially since most machine shops seem to only be familiar with Chevy and Ford mills.

• The cup plugs in the heads (2 per) are 1-1/4 and should be the 'deep' type. Melling MPC-18B is a perfect fit.
• At the rear of the block is a 29/32" cup plug. This allows access to an internal pipe plug at the end of the driver-side oil gallery. Both should be removed before hot-tanking. The pipe plug (if original) takes a 5/16" square head drive. You can grind down a 3/8" extension, or pick up an 8mm "Oil Drain Plug" socket which I was able to find locally a lot easier than a 5/16" one. The pipe plug is special in that it has a 0.040" hole in the center for oiling the distributor drive. You'll want to make sure you've got daylight there and re-use it.
• At the front of the block, behind the cam gear, are two threaded plugs; they look the same but are different sizes. They are not NPT. Clean and re-use them. One also has a 0.040" hole for oiling the timing chain. When you first pull the front cover, you should see an oil slinger on the crank. Sometimes these go missing.
• My Fel-Pro gasket set came with a rope seal for the rear main; apparently most sets do. A Ford 292 Y-block neoprene seal is a perfect fit for the SBO. Some Olds sellers offer a neoprene/rubber seal as well, but these are just the Ford seal with a healthy markup.
• Push Rods..... when you walk into a parts store, or pull up Rock Auto, the stock length is listed as 8.234". My '77 & '79 shop manuals list stock length at 8.265". To complicate matters, Olds replacement camshafts (even if stock spec), are usually ground on reduced-base circle blanks. Edelbrock suggests using big block Chevy intake pushrods which are ~0.030" longer. The safest thing to do is check lifter pre-load. I used Sealed Power RP-3182 which are indeed BBC intake PR's, and had pre-load in the 0.030" to 0.040" range. The originals were closer to 0.010" with the new cam.
• I have been told the Edelbrock Olds cams are actually made by Melling and re-boxed with a markup. Unfortunately, it was all I could find available at the time as many shops & online sources no longer stock hard inventory and are simply middle-men .
• My intake manifold only had one threaded port for a temp gauge. I'm now kicking myself for not drilling and tapping at least one of the other two adjacent flats. While the rear heater hose port looks like a decent spot, it's a 'dead end' for coolant flow and may not be entirely accurate.
• Olds factory head gaskets are in the neighborhood of 0.017" thick, though some claim they changed in the late '70s. If yours is this thickness, be aware that the aftermarket replacements are 2x as thick and will drop your compression. There are a couple companies (like SCE) making shim stock Olds gaskets that are close to the original spec when compressed, but they're a little spendy.
• If you can get your hands on an NOS dipstick tube, do it. The only other alternative are the aftermarket chrome jobs...I'm told these will require filing of the two crush ribs to fit the block properly, and at least one brand is too loose. I don't believe the stock dipstick will work with these, either.

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