The Stovebolt Site Forums The Parking Lot Big Bolts replace 270 with 250?

I am replacing a 270 in a GMC DUKW. One of the options I'm considering is a later GM 6-cyl, either a 250 or a 292. I'm hoping to find a bell-housing that will connect the 250/292 (small block Chevy bolt pattern) to the existing Clark 204 transmission.

First try: The stock cast iron bell housing for the 250: It's the same length as the 270 housing and making an adaptor ring for the centre hole is no problem, but the 250 housing doesn't extend far enough down. There's no metal where I'd have to drill for the Clark mounting bolts.

Second try: There was an old Chevy school bus behind my barn. The small-block had been pulled years ago, but the bell-housing was still attached to the transmission. It seems to have 2 sets of bolt holes on the back. Neither of them is what I need, but there's plenty of metal to drill another set. Problem is, when I examined the engine end, it's not a small-block pattern. It looks like one, but slightly bigger. Did some of the small-blocks use an adaptor plate to mount a larger bell-housing? Anyway, it won't fit.

Are there any other possibilities that I might find in my local junk yard, or is a custom bell-housing the only solution? If it is, then some of the other options start to look attractive.

Hy uglydukwling, welcome, yes there are adapters on some of the larger flywheel/clutch/transmission applications. Does the truck employ a flat mount at the rear of the bellhousing, or is it an angled mount? I don't think I've ever seen a flat mount adapter/bellhousing setup. If you were to use such an adapter/bellhousing that would mean re-locating the engine front mount forward of its present location due to the increased width of the adapter/bellhousing setup. Have you considered swapping to a different transmission, or are there p.t.o.s involved? Hope that helps.

What you have on your bus bell housing is a sae bell housing.There is a thin adapter ring that bolts to the block and the bell housing bolts to the adapter ring. I'm converting my 46 COE over to this style of bell housing with a new process 5 speed tranny. If you go this route you would also need a different flywheel.

The old style bellhousings are a pain in the neck to R&R, need to remove all the engine mounting bolts through the retainer hole, a REAL pain if the engine is stuck. There is a gentleman in WA who makes an adapter to bolt a modern GM bellhousing to a 235/270/302 etc series. Look at Inlinersinternational, forgot his name.

However, if going 292, etc., the NP-540 series 5 speed as was already mentioned, is a great choice, nice close ratios, bolts to any GM bellhousing w/ 5.125 retainer hole, get the one with the short input shaft, few new parts available. Some were available w/ OD, however, at the expense of a big gap in ratios.

If the bell-housing had been circular, I would have assumed it was an SAE, but it isn't. It's about the same shape as the small-block pattern, but bigger. In fact, I thought it was a standard small-block housing until I held it up to the engine. The adaptor ring is long gone. It went with the engine decades ago, so I don't have access to it. In any case, I probably couldn't use this bell-housing with an adaptor, because it's already about 1 1/2 inches longer than the 270 housing and a ring would make it still longer.

I'm not sure what the bus transmission is, except that it appears to be a 4-speed and it's even bigger than the Clark 204.I'm pretty well stuck with the Clark unless I can find a replacement that has a pto in the right place.

I'll try to find the gentleman in WA who makes the adaptor you mentioned, and see if has anything to go the other way (modern engine on 235/270/302 bellhousing)

Another of the several options I'm considering is a 302. It has both advantages and disadvantages compared to the 250 series, but if it really is a bolt-in as I've been told, that could be the clincher. Comments?

The original plan was (is) a Perkins 6-354, but it looks like a tight fit. The main problem is going to be getting the crankshaft centreline low enough for the transmission to line up with the transfer case. This is still the preferred option, but in case it turns out to be more trouble than it's worth, I'm exploring other possibilities.

Plan D is doing whatever work the original 270 needs and bolting it back in. At least I know it fits, but I just can't ever seem to leave well enough alone.

Given the options you mentioned, replacing the 270 with a 302 is a no-brainer- - - -it's virtually a bolt-in swap. If you happen to find a military surplus 302, you'll need an electric fuel pump or the hole for the mechanical fuel pump will need to be cut out at the proper location in the block, and there's a place on the block that needs to be ground away to clear the starter, but both of those are relatively minor modifications compared to adapting a different block bolt pattern. Good luck!
Jerry

The 302 option looks promising. I have a graveyard of M-135 series trucks and I hope there's at least one good engine among them. On the other hand, I could waste a lot of time trying engines out. I also have several 235's, mostly from '55 Chevys, if any parts off them are useful.

Since my 302's are all military, there are a few more questions that come up:

They are 24 volt. Generator is no problem, since I plan on a 12-volt alternator anyway. Starter will be a problem. Will the 12-volt starter off a 235 work, or will the 302 starter have to be rewound for 12 volts? I can use mechanical gauges, so no voltage problems there.

The military engines are all automatics. Will the 270 flywheel bolt on in place of the flex-plate, or did they do something nasty, like use a different crankshaft flange?

The military engines have sealed ignition systems. Good for an engine that has to operate in a wet environment, but a mixed blessing. For one thing, replacement parts cost several times as much as parts for a conventional system. For another, having all the connections inaccessible makes troubleshooting a lot harder. What conventional distributor will fit if I give up on the sealed system?

The DUKW has unique clearance problems caused by the front driveshaft and axle tubes. This results in an oddly shaped oil pan. Will the 270 oil pan bolt onto the 302, or will I have to cut and weld the 302 oil pan until it looks like the 270 pan? This is something I'd probably have to do the oil pans of any of the other alternatives, so it's not an argument against the 302.

270 pan will bolt to 302, just use 270 oil pickup instead of the 2 story pan and pump.

Modern Even-fire GM V6 or 230/230/292 Chevy HEI distributor will drive 302 or 270, or distr off any Chevy.

270 flywheel and clutch will work on 302.

Uncertain if 'any' GM starter will work on 302, however, 270 6 volt starter should work on 302.

Other issues mentioned above should work.

A few more things I've found out.

It's good to know the 270 pan will fit, since I pretty well have to use it. I took a look at the military 302 oil pan and it's about 13" deep. It would be even harder to fit than the Perkins.

Also good to know the later distributor will fit. I probably have a couple around, just haven't compared them with the military yet. The late 6's are so common I've actually bought complete engines just to get the HEI.

The flywheel off my 270 will not fit. It's a 4-bolt mount as opposed to the 6-bolt on the 302. I've heard a flywheel from a later 270 or 302 will fit, though. What years use the 6-bolt mounting?

Starter may not be a big problem. I was amazed how interchangeable Delco starter parts are when I needed a 24 volt starter to replace the pup engine on an old Cat D-6. I took tracings of the ring gear and mounting flange and a few other measurements to my local starter shop and they were able to pull a snout, drive, armature, field coils and body off the shelf and assemble a starter. A lot more expensive than finding one in my junk pile, but not nearly as bad as I had expected, either. So it should be no big deal to assemble a 12 volt starter for a 302 if all else fails.

On some forum, I read that the military engine uses side mounts at the front, and it will be necessary to change them. I don't know if it's true of all of them, but the first one I looked at has a front mount like the 270. I'll have to get better access to measure and see if it's identical, but at least it's the same style. The bosses on the sides near the front are used to mount the generator and air compressor.

I didn't know wartime 270's had 4 bolt flywheels, 6 bolt, 11 or 12" clutch flywheels are sort of rare, make sure you don't find one for a 14" clutch, or could drill new clutch mounting holes, if it has the correct starter ring gear. Probably all angines after the war are 6 bolt flywheels, don't know for sure, however. Later flywheels will fit. Some have redrilled V-8 flywheels.

270 and 302 pans have lumps in the sides for each rod because of the longer stroke engines, I'm sure your 270 does, might check.

Another update on the 302:

The first one I looked at was shoved in the back of a shed and hard to get a good look at. The second one is on a military storage stand and easier to see. The stand is bolted to a vertical surface on each side of the outer bell housing. The first one I looked at only had the inner part of the housing, so no place to attach rear mounts. At the front, The stand is bolted to the bottom of the brackets that mount the air compressor and generator, so it does have side mounts. The cradle at the front of the engine, that I took for a front mount, looks like the front mount on the 270, but isn't drilled for mounting bolts. It looks like it could be used as a basis for building a front mount, though.

I haven't looked at any of the engines still in trucks recently enough to remember details. We just had more snow, so it may be a month before I can get a good look at them again. For now, I'm assuming that the mounting system in the truck is the same as on the stand.

Hmmmmmmmm- - - - -just how many of those 302's are there, and are they for sale?
Jerry

There are the 2 take-out engines I described, and I believe 6 more still in trucks. A couple of the trucks are too good to scrap just for the engines. Three could be restored, but may not be worth it, and one "fell out of an airplane" (no kidding, that was the story that it came with).

They may all be for sale, if I don't end up using one myself. If I do, then I'll have to keep a couple of others for spare parts. The problem is, it may be a year before I know which, if any, I want to keep. If I end up using the Perkins or the 250/292, I won't need a 302.

I actually tried to sell them all, along with 3 other engineless trucks and a quantity of new-in-the-army-crate parts, about a year ago. Now I'm glad nobody was ambitious (crazy) enough to buy them as a lot. I may end up using an engine and possibly other M-135 parts in the DUKW project.

If somebody can just help me figure out how to make the Perkins fit, there'll be a lot of 302 stuff for sale.

By the way, if anybody is interested, I'm in south-west Ontario. In US terms, I'm about 50 miles east of Detroit.

"Fell out of an airplane"- - - -yes, I believe that! My father's former business partner had been the US Army NCO in charge of a repair depot in France that was tasked with salvaging and rebuilding battle-damaged trucks after the Normandy invasion. Some of those trucks had been dropped off ships into the ocean during cargo unloading operations. That man was the most talented frame-straightening guy I've ever met. He's been gone about 10 years or so- - - -died in his early 90's.
Jerry