I have a 1974 GMC, front wheel drive, motorhome with a broken 500ci Cad motor.
What GM diesel would be the best for this conversion?
I am new to this forum, perhaps this has been discussed before.
Many thanks for any insight.

Since the transaxle has a BOP bell housing, I think your only option is a 5.7 Oldsmobile diesel.

What is a BOP bell housing. But my only thought is maybe it needs to run backwards.

Brian

I think I would stay away from the 5.7 gm diesel. I would think you should be able to find a better engine that will adapt. I think the big problem will be the tranny & holding the low end torque. But cash can make a lot of things work.

You can get a Buick-Olds-Pontiac adapter to a small block chevy and runn a duramax, isuzu 4 cylinder, cummins 4bt, etc.

You can get a Buick-Olds-Pontiac adapter to a small block chevy and runn a duramax, isuzu 4 cylinder, cummins 4bt, etc.

Now there ya go, but remember the tranny.

The 5.7 diesels were hard on that transaxle, and they were a gutless animal. Keep in mind that is a snug fit in an OE application.

The 5.7 diesels were hard on that transaxle, and they were a gutless animal. Keep in mind that is a snug fit in an OE application.

The 455 Olds and 500 Cad were torque animals although at hightr RPM. The transaxel is basicly a turbo 400. They can be built up to accept additional touque.
I have a fresh 6BTCummins (same bell housing as the 4BT) from a Dodge. (I didn't even consider using it.) BUT.. The MH does has lots of room in front of the rad and coolers. Does this sound plausable??

The 455 Olds and 500 Cad were torque animals although at hightr RPM. The transaxel is basicly a turbo 400. They can be built up to accept additional touque.
I have a fresh 6BTCummins (same bell housing as the 4BT) from a Dodge. (I didn't even consider using it.) BUT.. The MH does has lots of room in front of the rad and coolers. Does this sound plausable??

Still thinking out loud.. I know from personal experience that a Chevy to BOP adaptor is nothing more than a flat plate with the necessary holes. Assuming I use one of those, what GM engine would be worth considering?
Again thanks

any pics of it never seen this before?

1974? What does the interior smell like?

I also have a classic GMC motorhome (check out the 1977 GMC other in my garage). I considered putting a diesel in place of the olds gasser but couldn't justify it without first getting more gears out of the tranny. The original 3.07 final drive spins at 3000 plus at 70 MPH. A diesel would be best served if that were more in the 2300 RPM range. I ended up buying an 8100 out of an 03 pickup and am getting ready to put that in instead.

There is a guy who swapped a 6.5 in. He has pictures of the installation on the gmc motorhome picture archive.

I also have a classic GMC motorhome (check out the 1977 GMC other in my garage). I considered putting a diesel in place of the olds gasser but couldn't justify it without first getting more gears out of the tranny. The original 3.07 final drive spins at 3000 plus at 70 MPH. A diesel would be best served if that were more in the 2300 RPM range. I ended up buying an 8100 out of an 03 pickup and am getting ready to put that in instead.

There is a guy who swapped a 6.5 in. He has pictures of the installation on the gmc motorhome picture archive.


I answered an ad for this GMC listed for sale at $7,500. The owner had pased away since placing it..property owner wanted it moved..they GAVE it to me with all of the paper work, FREE. All I had to do was have it towed.. $150.

I rebuilt a 6BT for another project that I'm not going through with. It has zero miles and has been in my shop for over a year with no place to go.

NOW.. given the circumstances, it seems reasonable $$ wise to consider the swap. I realize the final drive ratio needs to be taller and the trans will need some upgrades. The question is..is it practicle?
What do you think?

Nice find on the coach. What kind of condition is it in?

I think the swap is possible but would need some details covered. I would find a th425 out of an old cady or toronado. Those can be found pretty easy and cheap. The trick will be getting the engine mouned up so that the axle shafts fit underneath the oil pan. That is a tight fit on a small block and I bet its a lot tighter on an I6.

Another hurdle is the weight. The GMC front wheel bearings were on the edge of carrying that much load. The added weight of the cummins may require that they be improved.

I'm not sure if the torsion bars can be twisted up to compinsate for the added weight. It may also be possible to swap in some out of a newer 1 ton.

You will also have to either add a secondary gas tank or update the generator to diesel as well.

It would be fun to hear a diesel churning away under the hood while running down the interastate. The added power wouldn't be bad either.

On the previous post I was assuming that the existing engine and tranny were in running condition. The spare tranny I suggested was to be used for fit up and experimentation. You could see if it mates up without going through the work of pulling out the original unit.

On the previous post I was assuming that the existing engine and tranny were in running condition. The spare tranny I suggested was to be used for fit up and experimentation. You could see if it mates up without going through the work of pulling out the original unit.

The coach has 61K miles, doesn't leak and has a brand new refrig. It has been sitting for a number of years, needs brakes and air bags at the very least. The ad stated the engine (a 500 Cadillac) needed some work. I won't know the condition until I get in to it next week. I hadn't even thought about the oil pan clearance, thanks.
Where can I find the adaptor?

1974? What does the interior smell like?

ORANGE BLOSSEMS!!
Actually the interior is clean as a pin, no mildew.

Keep us updated.

I think I would be very tempted to just rebuild the caddy motor. You can even build them up now. Aftermarket parts are available to pump up that caddy.

I think I would be very tempted to just rebuild the caddy motor. You can even build them up now. Aftermarket parts are available to pump up that caddy.

With only 61K on the clock, even pulling a motorhome around, it probably doesn't need a rebuild; I'd just diagnose the problem, fix it and drive it.

With only 61K on the clock, even pulling a motorhome around, it probably doesn't need a rebuild; I'd just diagnose the problem, fix it and drive it.


True, I was going off of "broken". I know when I say broken, it means broken and not "sick" LOL! Problem with it being in a motorhome is that the caddies do not like Hi RPM's. 65 K on an older 500 caddy in a motorhome is not anywhere near low mileage. But that doesn't condemn it either. Worth diagnosing for sure.

THE REST OF THE STORY...
I was going by what I was told by relitaves of the PO. Now that I have it home, it turns out the engine is the origional 455 Olds and the trans is from a Caddy. So far the engine seems to be in reasonably good condition.
Preliminary measurements indicate the front floor would require serious modification in order to install the 6BT. That and other "problem areas" have me reconsidering the diesel conversion. Besides, I'm getting a little old for these kinds of projects.

THE REST OF THE STORY...
I was going by what I was told by relitaves of the PO. Now that I have it home, it turns out the engine is the origional 455 Olds and the trans is from a Caddy. So far the engine seems to be in reasonably good condition.
Preliminary measurements indicate the front floor would require serious modification in order to install the 6BT. That and other "problem areas" have me reconsidering the diesel conversion. Besides, I'm getting a little old for these kinds of projects.

OH Yeah.. The method of engine removal from one of these things involves erecting two "A" frames, inside the coach, supporting an "I" beam running from the windshield down to the door. You can imagine the rest of the procedure.

THE REST OF THE STORY...
I was going by what I was told by relitaves of the PO. Now that I have it home, it turns out the engine is the origional 455 Olds and the trans is from a Caddy. So far the engine seems to be in reasonably good condition.
Preliminary measurements indicate the front floor would require serious modification in order to install the 6BT. That and other "problem areas" have me reconsidering the diesel conversion. Besides, I'm getting a little old for these kinds of projects.

I believe the Olds and Cad transmissions are identical. I also think you are better off with the Olds engine, much stronger bottom end, better heads, more after market parts available, etc. The engine, trans, differential package is like nothing else. The gear ratios are matched to the torque of the engine (somewhere north of 400 # ft at a relative high rpm). A small diesel will not produce the same amount of torque, and what torque it does provide will be at a lower rpm, so transmission ratios will be wrong. As far as bolting in, a 6.2 / 6.5 would probably fit pretty easily, but these weren't noted for their high torque output. The biggest drawback to the 455 is fuel consumption, but you can buy a lot of regular unleaded for the cost of a conversion. Just my opinion.

I was thinking the caddy motor sounded a little strande. The Olds make perfect sence. I have seen those in there before. The diesel conversion would be nice, maybe a 6.5, would be easier. I still think I would stick with what it has in it. FYI that is not exclusively a caddy trans axle. As you can see it was also in the olds toronado. The 455 is a decent engine. I had one in a boat I had a while back. The valve train just sucks. It is non adjustable, but the after market can fix that.

I agree with you both.
To make the decision even easier, It RUNS!
I removed the plugs, filled the cylinders with Marvel Mistery Oil and let it sit for a few days. Bypassed the fuel tanks with a hose directly to the fuel pump from a portable tank. Cleaned all of the electrical connections, changed points, condenser, coil & plugs and it seems to be running smooth. The power steering and alternator are working and the radiator isn't leaking...so far.
I had to remove all of the brakes in order to get it to move out of the spot it where it had been setting for 10 - 15 years (according to the relatives).
It was FREE.. what did I have to lose?
Many thanks for the thoughtful responses from all of you.

Sounds you may have scored! I hope your luck holds out with it.

Sounds good. Plan on replacing the brake master and wheel cylinders and calipers; after that long in storage, enough moisture was absorbed in the brake fluid to rust the cylinders. Enjoy your find!

I am pretty sure you will have leaks pop up, once you start driving the old girl. I had a 1978 Itasca that only had 26,000 on it when I bought it. And bot did I pay for that several times over. It sat and was barely used, they hate that. It is better to use them, than to let them sit.

It took me three days to dig it out. I had to remove all six brakes just to make the wheels turn. Moisture distroyed two outer bearings, they were replaced. All six tires held air and the onboard air pump inflated the rear air bags. The frame has surface rust but it looks straight and has no rest holes.

I drove it yesterday (around a 1/4 mile circular drive). I'm impressed! It seems to be worthy of spending some time and $$. The interior and all of the appliances are in really nice condition as well as all of the cushions and upholstery. My wife said "retro is in". She doesn't care for the yellow floral design.

Now to get it up in the air.

A couple things to check when its in the air:

1. Rubber pads between the frame and body. They can compress over time leading to a rough ride and pinched fuel lines.
2. Strut rod going corner ways from the frame to the front control arm mount. They should be straight and could throw off your measurments if they aren't.
3. Cracks in the control arm welds
4. Make sure the cones on your air bags are made out of aluminum and not plastic. The plastic ones explode and you don't want to be around when they do!!

I'm sure there are quite a bit more things to look for, but thats a start.

My neighbor, has had one of those sitting, in her yard for at least twenty years. Before her husband died, he told me it had had an interior fire. I guess they were prone to that? He's been dead for over a dozen years, but if I remember the conversation correctly, it was some sort of wiring issue, up near the engine?????

A couple things to check when its in the air:

1. Rubber pads between the frame and body. They can compress over time leading to a rough ride and pinched fuel lines.
2. Strut rod going corner ways from the frame to the front control arm mount. They should be straight and could throw off your measurments if they aren't.
3. Cracks in the control arm welds
4. Make sure the cones on your air bags are made out of aluminum and not plastic. The plastic ones explode and you don't want to be around when they do!!

I'm sure there are quite a bit more things to look for, but thats a start.

I found a huge amount of useful information on line. I truely appreciate your input.
Today I'm moving it to a concrete pad and will have it up in the air this afternoon.
Then the fun begins.
Thanks again...

My neighbor, has had one of those sitting, in her yard for at least twenty years. Before her husband died, he told me it had had an interior fire. I guess they were prone to that? He's been dead for over a dozen years, but if I remember the conversation correctly, it was some sort of wiring issue, up near the engine?????

Thanks for the heads up...That's one I hadn't heard of.

The LT1- 6 speed is a great combination. I have an LT1 in my 49 Mercury, but it's an automatic.

Thanks for the heads up...That's one I hadn't heard of.

The LT1- 6 speed is a great combination. I have an LT1 in my 49 Mercury, but it's an automatic.

NEXT DAY.................
I looked into this and you are correct! Early coaches did have a problem with the alternator wiring harness. I have the FIX.
Many thanks for bringing it to my attention.