Need advice for swapping 429 for 500 in '66 DeVille
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500, 472, 425, 368 Discussion, Need advice for swapping 429 for 500 in '66 DeVille in Cadillac Engine Technical Discussion; I have a 66 Deville convertible and I've reached the limits of my patience with the engine. I bought the ...
  1. #1
    Unky is offline Cadillac Owners Member
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    Need advice for swapping 429 for 500 in '66 DeVille

    I have a 66 Deville convertible and I've reached the limits of my patience with the engine. I bought the car back in '96 and had the stock 429 rebuilt two years later by a substandard engine contractor for Kragen. Later I found out this was exactly the wrong thing to do. I've been able to survive since then with a moderate but consistent level of blue smoke from burning oil plus I'm on my third intake manifold and the junkyards are running out. I had a mechanic replace the valve stem seals in spring of last year and the smoking seemed to stop for about 6 months before returning to it's previous state. I've had many mechanics look at the engine over the years always coming back to the same discussion of whether the problem is associated with the valve guides, piston rings or both.

    So I just got the results back from the leak-down test and it is indeed indicating a ring problem no pressure loss at the top of the engine at all. The interesting thing is that the engine seems to run fine and has for years... if not for the problems with smoking and exhaust. The car consumes about twice as much oil as under normal conditions. I know that indications are never completely accurate until the engine is opened up to observe the problems directly; however as I said, my patience is running thin with this engine. The prospect of rebuilding it a second time from the ground-up doesn't seem worth it. The long efforts to keep the car all-original have just become too much trouble for the lack of a reliable car.

    I belong to the Caddy club in Nevada and some of the members recommend buying a 500 8.2L V8 engine from a '74-76 El Dorado. The good folks at cad500parts recommended getting an engine that lacks a smog pump, includes an HEI distributor and a rear sump oil pan. If I can find an engine that looks good I plan to do a gasket reseal, replace the timing chain, and valve springs. I'm certain I'll need new motor mounts. There's a company that makes an adapter plate that will convert the bolt pattern on my '66 TH-400 to a 70's 500 engine or I can try to buy a compatible transmission and other powertrain parts from whatever junk yard has the engine I'm looking for. At the very least this would leave me with an engine that has certain level of 3rd party "support".

    To those folks out there that have done this type of thing before, ...is this a good solution? Are there other things to consider? What are the proper techniques for assessing the viability of an junkyard engine still installed or removed? I was hoping to get some advice from forum members about what approach to take here. I'm currently between jobs so I am making financial considerations as well.

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  3. #2
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    I have not done that swap, but it should work well. I expect your biggest problems will be the accessories & exhaust system. I'd recommend an HEI and internally regulated alternator anytime over the 66. Possibly you could redrill 2 holes in the trans to fit a 500 BOP pattern. You have a switch pitch trans, good to keep. There are several pans, find the one that works. Bruce Roe CLC # 14630

  4. #3
    Unky is offline Cadillac Owners Member
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    Does anybody else have anything to add?

  5. #4
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    MTS used to supply all sorts of things for various Cad engine swaps. I haven't talked to them though since the change of ownership. Certainly there is always a lot of detail stuff to deal with (hoses, wiring, fuel lines, air cleaner, etc) once the new engine is bolted into place. Bruce Roe CLC # 14630

  6. #5
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    I used to have a '66 Olds Ninety Eight LS with a 425 and that thing would really fly. It was a Camaro killer in the early 80's. You couldn't kill that engine. It outlived the car. I would think the Cadillac engineers were just as capable of making a 429 engine that would have lots of reliable horsepower and torque and could take a lot of abuse. Was your engine re-bored by this substandard contractor? If not, then there's your problem. If they did bore it they must have screwed something up with the rings. If it was re-bored then it would be worth it to take it apart and inspect the cylinders and rings to see what is wrong. It may just need a new set of rings due to improper clearances.

  7. #6
    steelybill is offline Cadillac Owners Enthusiast
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    I had a '66 Coupe DeVille with that engine. I rebuilt it myself. Sounds to me like a decent overhaul would be a lot less hassle than a complete engine change.
    The valve stem seals were not very good at ll, but that can be remedied if an automotive machine shop replaces the valve guides, and installs the teflon seals in place of the stock parts.
    Maybe a decent hone job with new rings could clear up the bottom end problems.

  8. #7
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    The 429 has a different aluminum front cover for each year, replacements not available. They contain an oil pump which wears out and cannot be repaired. I used to drive some exciting 60s engines, but I got fed up with their problems. Now I drive a very unexciting late 70s engine with electronic alternator, HEI ignition, valve rotators, and ability to perform on todays' crap gas. It just runs, and runs, and runs. I am totally sympathetic to anyone with similar issues. Bruce Roe CLC # 14630

  9. #8
    Unky is offline Cadillac Owners Member
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    Some interesting suggestions. I have identified a working engine in a '76 El Dorado. The engine has a rear sump oil pan because the car was front wheel drive. It also lacks a smog pump which is good. I'm trying to determine whether it would be better to purchase an adapter plate to convert the stock TH-400 trans in my '66 Deville to a BOP pattern to fit the new engine, or try to purchase the trans from the '76 Eldo along with the drive shaft and whatever other parts I might need. I'm unsure whether these extra parts will fit or are fully compatible with my '66. I'm also unsure as to whether I can use my air conditioner after the 500 is installed.

    Further I had the '66 trans completely rebuilt in fall of 2007, cost me around $1500, I've heard from many people that the TH-400's from 64-67 were widely appreciated because of the variable pitch design. 'would be a shame abandon it. Any opinions about what the best approach is here?
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  10. #9
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: Need advice for swapping 429 for 500 in '66 DeVille

    The 76 Eldo front driver is not compatible with a rear wheel drive car. I would certainly keep the rebuilt THM400 switch pitch trans that you had rebuilt (if working well). I think the trans case could have 2 mounting holes redrilled to fit a BOP engine, use an adapter plate, or find a suitable case and transfer your present internals into it. You can see some more info on this subject at

    http://s93.photobucket.com/albums/l71/bcroe/

    (that is a lower case "L"71, not an upper case "i"71)

    click on an Album at left
    click on a picture to enlarge

    I believe someone said on a 429 to 500 conversion, the engine mounts were a little off. So they split them (drilled rivits) slid them a bit, and welded back together.

    If you wanted to add to the switch pitch performance & smoothness, you might want to add an electronic switch controller. Bruce Roe CLC # 14630

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