The Stovebolt Site Forums The Shop Area Engine Shop main bearing caps - busted

I am working on 54 chevy 3/4 ton. 235 engine. allegedly running when parked. I am trying to improve my chances of getting it running when I reassemble everything.

I pulled the engine. took head to machine shop for valve job. I was inspecting the block and noticed the front and rear main bearing caps are damaged/cracked/busted. I believe these are referred to as lower; it is piece where you find front and rear oil pan seals.

the truck had some front end damage which likely explains it. can I buy main bearing caps at FLAPS? any thoughts on what else might be damaged inside the block by a force that would crack the main caps?
thanks

no you cant go and buy a main bearing cap at FLAPS....if its just the lip for the cork seal then just glue the seal and forget about it, they get broke from time to time from mishandling the block with the caps still on the block. shouldn't be a big deal, I have seen many blocks like that and its not a big deal. hope this helps you.

How exactly are they damaged?

thanks for the replies.

I removed the front cap and it is broken in two pieces - break is horizontal right above the two smaller screws that presumably were holding my oil pan at one time (oil pan came off months ago). the grove for the seal is busted up; I suspect it is useless.

the rear cap appears to be more consistent with the wear that baldybenny described. I will pull it out and see if it is worse that first impressions.

any thoughts on where I can source the front cap? can I order them new or do I need to find used parts? thanks

It may be more cost effective to find another block. If you need to replace main caps you are going to need the block line bored.

Beings that a 1954 uses shims to adjust the bearing clearance it may not be necessary to align hone the cap if replaced. I would test a replacement cap first.

PM me if you can't find a cap locally.

Thank you all. Great stuff and happy to hear I have some options.

Pre 68 Dave - Will do. Thank you

I didn't think a 54 235 had shims, but I've been wrong before.
Jay D.

1954 Main Bearings - Adjust (shims)

1954 Main Bearings - rebuild - shims

I have never dealt with broken main caps. GMC is my thing, and they had precision bearings as far back as 1939. I have helped guys shim Chevs with Plastigauge to get correct clearance. But I wonder if a cap that came off of another engine would match side to side line bore. I wouldn't sink money into an engine that had replacement main caps and no line bore done. And what caused them to break in the first place.

[quote=crenwelge]I have never dealt with broken main caps. GMC is my thing, and they had precision bearings as far back as 1939. I have helped guys shim Chevs with Plastigauge to get correct clearance...[/quote]

Years later the other shoe finally drops... I always wondered why my engine machinist just scoffed when I mentioned checking things with plastigauge as I picked up the block and head of my 270. :blush:

I can tell you on a bearing insert engine, you can not just change a cap. When I was younger, I broke a Pontiac 455 rear main cap by installing the wrong seal. I pulled a cap from a second block and plastic gauged the bearing and did everything I though was right to finish the assembly. The motor ran about 1 1/2 minutes be locking up. Turned out the rear cap was smaller at the sides then at the top even though it measured correctly. If I would have measured at the deck I would have seen it wasn't going to work. A simple line bore cured it. Point of my story, measure all the way around ! Second point, find out why it broke.

Pontiac 455 is a different beast, no shims for final adjustment. I believe that these older engines required shims due to less than precise machining capabilities of the time. This comes in handy for making a cap off of another engine work.

I wonder how many of the people commenting on this thread have actually made a living as an engine machinist? Being successful swapping main caps without line boring afterwards is about as likely as finding a chicken with lips. The only thing shims do on stovebolt main bearings is make slight changes of the vertical position of the bearing shells to adjust oil clearance. They do not alter the roundness of the block/cap relationship.

Bottom line- - - -the life of a set of main bearings after a main cap swap is done without line boring the engine can be measured in minutes!
Jerry

I am wondering how they got broken. If the PO mishandled the block, maybe dropped it or something, there could be more damage.

Thanks again to all. Good stuff. I continue to learn something everyday which is the benefit of buying a lemon. The truck had front end damage - apparently the brakes failed and it rolled into something. I suspect the force was greater than described but my rookie mistake.

More good news on the learning front - it does have shims which explained another mystery when I removed it.

I'm hauling to a machine shop next week. More later.

235's are not like most newer engines in that only the rear main cap is held centered by dowels.
The other three caps are free floating, only centered by the crank.
If the clearance is checked with Plastigauge it is almost certain to be fine.
One could doubble check by putting Plastigauge at the center and near both sides to confirm the clearance.

If you suspect the caps might have been damaged from the front end damage, then maybe the crank is also suspect? Hope that is not the case.

Dennis

I worked on a Chrysler 426 hemi once that would lock up the crankshaft every time the main bearing cap with the thrust flanges was torqued. If we removed the thrust-flanged bearing shell from the cap and replaced it with a non-flanged bearing, no problem. We found that the cap was bent, and tightening it eliminated all the end play on the crankshaft. The car the engine came from had been involved in a serious front end crash and the crankshaft had taken a hard lick on the vibration damper. The center of the cap would move back .015" as it was torqued down. A different thrust bearing cap and a line bore solved the problem.
Jerry

I am afraid I am going to be faced with that same situation. I bought a 302 GMC engine from a guy in Austin that turned 360° when it was sitting in his garage. I was going to back up his drive way, but his wife insisted he roll the engine to the end of their drive way which was cement and slightly down hill, and then I pull my trailer along side of the drive. She was afraid I would back into her flower bed. About 6 feet from the end of the drive, one of the castors on his Harbor Freight engine hoist broke off like glass and the whole engine hoist turned over and the front of the engine hit the concrete. The engine is now locked up and I just haven't felt like dropping the pan and seeing what happened. There is enough other stuff on the engine that is worth the purchase price like a 6 bolt 6 volt flywheel and small clutch housing, but the whole thing is still sickening. My wife was along and she had no doubts that I could back up the drive way, but she keeps reminding me to be glad that neither the seller or I had our feet in the wrong place. And it gives me just one more reason to not even consider Harbor Freight stuff.

I hope he gave you the engine after that! I think I would have left it sitting there just for the satisfaction of making him clean up his own mess!
Jerry

I actually bought it on eBay and had already paid for it. I got it for cheap because he didn't want to ship and that kills bigger items on eBay. There is enough other stuff on it worth what I paid. And it turned out he only lived about a mile from where my daughter lives. I didn't try to turn it until I got to my shop with it. If I hadn't already paid for it, I would have just got in my pickup and left. I think that I was just astounded to see a swivel castor break off like glass and then the leg on the hoist fold under. The hoist was rated at 2,000 and I doubt that a 302 weighs over 600 lbs.

Hey Dave,
Hotrod's on us pretty hard today (maybe rightly so, as he knows his stuff and has handed me my a$$ in he past ). Do you know if the shimmed engines were align bored when built at the factory? I thought they had different diameter main bearings making it difficult if not impossible to align bore, thus requiring shims for final fit.
Carl

actually it's not far from 800lbs

Starting in 1948 Chevrolet started precision aline boring the blocks and using precision inserts. Before 1948 the aline boring was done to the bearings after pouring.
It is true that the main bearing sizes are stepped but that only means a special set-up is used for this job.

I have line bored several engines with different-size main bearing bores. It's just necessary to set up a cutter for each bore size, and either make multiple passes through the block, or have a boring bar with positions for several cutters and do it all in one pass. The trick to any line bore is to take a whisker-thin cut out of the block to minimize the change in timing gear or chain fit, and take most of the material out of the cap. I mill .010" off the cap mating surfaces first to shrink the hole, then cut it back round and the proper size. .008"-.009" is cut out of the cap and only .001"-.002" is taken out of the block.
Jerry

I think your confusing or combined two similar machining terms that each mean something completely different. Line boring is the process in which the main saddles are machined by a boring bar to bore the housing bore/bores to a predetermined size. All engine blocks are line bored as either a primary or secondary operation when manufactured. Align honing is the process in which the housing bores are honed to a predetermined size when a smoother finish or plateau surface finish is required. All blocks can be line bored to correct or qualify their housing bores, while not all blocks can be align honed, as this requires all the housing bores to be the same size to do it correctly.