The Stovebolt Site Forums The Shop Area Engine Shop pitted piston

Working on my '50 GMC 150 with a 228, replacing all gaskets. I have the head off getting cleaned and magna fluxed, which has revealed a repairable crack. In the process, some pretty significant pitting has revealed itself on one piston--with less on one other. How big of a problem is this?

That dome section is pretty thick- - - -I doubt if it will cause any noticeable problems. It might allow some carbon deposits to build up, but with the fairly low compression ratio the GMC engine has, that shouldn't cause any detonation or other difficulties.
Jerry

I would be more concerned if there was any pitting on the cylinder wall. As Jerry said, those domed pistons are relatively beefy.

That could have been caused by water in the jug, so, make sure and check the cylinder walls.

Ed

James,

The exposed cylinder wall next to that pitted piston appears to be in rough shape. Is that an optical illusion or is the bore surface damaged?

One of the things that first attracted me to this forum were the questions and answers about "freeing" "stuck" engines. Is this engine one that required some unsticking due to internal rust?

Damage like that in a flathead engine is frequently due to knocking (spark or mixture), and harmless to the piston.

Ray,

The engine never had to be unstuck. It was only owned by my grandfather, who was a mechanic and machinist. Last thing he did before he died in 85 was rebuild the whole engine. After that, it has mostly sat indoors without running. It ran okay if a bit rough before breaking down the engine, but with a misfire. So I decided to break it down as much as possible without pulling the engine--that is a bit beyond my capabilities and facilities. When I pulled the valve cover the rocker arm was pretty much inundated with gunk. So I kept going and now I have the head, manifolds, radiator connections, and oil pan off and completely cleaned. I have a whole gasket set because they all look pretty rough to me and I think that forty years is long enough for them to last. I hit the top of the block and pistons with a grinder with wire brush, because they were seriously fouled. I will carefully look at the cylinder walls as Ed suggests and hope they are okay. One push rod is bent, but I think I see the way clear to remedying that.

James,

That is very cool that you have your grandfather's GMC. I hope my grandson adopts my '36 Chevy pickup someday. It's unlikely that I'll be around to see that '36 turn 100 years old but my sons and grandkids should be able to.

Since you're so far into the engine disassembly it might be worthwhile to you to inspect the condition of rod and main bearings and crankshaft. If there are any problems those components are accessible now.

Is there any rust damage in the cylinder walls?

Have fun!

Ray,

Thanks! No damage to cylinder walls, just some grime that I can get out. There is minor pitting on the block right around the edge of a couple cylinders. I've read that a good gasket sealant can compensate for that--what do you think?

Ray,

Stupid question here, but how do I check the bearings, and what am I looking for in them and the crankshaft?


James

"There is minor pitting on the block right around the edge of a couple cylinders. I've read that a good gasket sealant can compensate for that--what do you think?"

James, I'm no professional mechanic, just a very interested and motivated amateur. But from what I've seen, combustion pressure and coolant will find any flaws in casting surfaces and blow through them, making matters worse by eroding the casting surfaces and the gasket too. Failure of that type is particularly eminent when the flaw is between two closely spaced cylinder bores. One person's "minor" is another person's "oh expletive deleted". I would suggest checking the questionable block deck with a precision straightedge to verify flatness and having a really experienced mechanic look at the block surface. If there is any question about a marginal condition I personally would correct it now that you have already done so much disassembly.

"Stupid question here, but how do I check the bearings, and what am I looking for in them and the crankshaft?"

I don't think there is such a thing as a "stupid" question. How are any of us to learn if we don't ask? You check the bearings by removing the connecting rod and main bearing "caps". That will expose the crankshaft and bearings themselves. In a previous post you said "When I pulled the valve cover the rocker arm was pretty much inundated with gunk.". That same "gunk" could be in the oil galleries, on the crankshaft bearing surfaces, on the bearings and it could also be clogging the oil pump screen. A clogged oil pump screen will starve the engine for oil with near instantaneous disastrous results. The crankshaft and bearings should be smooth and free of abrasion damage. With everything exposed you can check the bearing clearances with "plastigage", a plastic product that looks like mono-filament fishing line and is crushed between the bearing cap and the crankshaft. The smaller the clearance, the flatter it is crushed. The paper container has a scale printed on it that you hold against the flattened plastic to see what the clearance is in thousandths of an inch indicated by the flattened plastic. It's a very simple product to use and really accurate if manufacturer's directions are followed. It works!

I know from restoring my '36 Chevy pickup in the early 1970s, including assembling the engine, that a properly assembled engine in a weekend-driven toy will outlast the guy who assembled it. So if you get it right now you won't have to mess with it ever again. But you probably already heard that from your granddad plenty of times. I think he would really like what you are doing now with his GMC.

Have fun!

Machining the head to remove those rust pits is commonly done, doing the same thing to the block is more involved(engine must be removed and stripped), but is well worth it in removing future problems.

Ed