The Stovebolt Site Forums The Shop Area Engine Shop recurring problem on third rebuild

about a year ago i deciced to rebuild my chevy strait six 250. we went 20 over and shortly after start up piston slap developed in number one. No bearing damage , crank or rod. No damage anywhere in the engine except for the passenger side of number one. There the piston had eat enuf out that apon rebuild number one was sleeved cause it wouldnt clean up. The first time we had the machining jobbed out. the second time we did the machining cause we felt maybe faulty machining was the culprit. This time the engine was run longer , harder , and further. Piston slap and damage occurred in the exact same spot only not as badly. Well everything else was fine as far as we could tell again so we were guessing that maybe the intake had a leak due to warping and had caused the damage by running the number one lean. both the intake , exhaust and head were milled to correct what we thought was the problem. a new water pump and radiator were also installed and the operating tempature had dropped abbout 15 to 20 degrees running now from 190 to 210 all looked good for about 3500 miles. I've got piston slap on number one again. oh and this is the kicker. number 3 was a new block we machined ourselves again. the number one on that engine was significantly more scored than the other 5 we discovered after teardown. The only thing i can think to check is the head which was purchased rebuilt from a good place. Is there anything peculiar to these sixes that might explain this? Does anybody know a good shrink?

Unless if you are a Machine shop expert, I would let a pro completly go though your engine and rebuild it back from top to bottom. Have all items inspected and make sure all tolerances are followed as per Shop Manual.

But if you were a machine shop pro, you wouldn't be here making this post.

The only engine repairs I do are regular maintence and tuning. As far as machining blocks and heads, better leave it to the experts...........

Whats your skirt-to-wall clearance after machining?

What was your ring end-gap?

Cast and forged pistons require different skirt-to-wall clearances. What kind of pistons, rods, rings, and crank were you using?

What was your rod brng clearance and rod big-end (side) clearance? What was crank end-play?

All these things affect cylinder lubrication and piston fitment.

chip

#1 always takes a beatting it is closer to the thermostat and take more heat the tapper on #1 is always greater

I assume you are using the same distributor and wires on all three engines? Could it be from preignition? Check the valve seat area and combustion chamber of the head for any sharp edges that may be heating up. If you go for a forth engine, try a cooler sparkplug and a 180 or 160 thermostat. See if it will live a bit longer. I find it hard to believe the piston right in front of the water pump is the hottest one, should be the farthest from it you would think! What about a slightly bent connecting rod. Just where on the piston is the scoring or rubbing? With 12 O-clock towards the fan, 1, 2, 3, etc...
If the rod is common on all three engines, I would really check it over or replace it.

These are just some ideas I had and most of them should of been eliminated by changing blocks and internals.

The 250 and 230 have a weak spot on the number one cylinder where the drivers side front headbolt is. They are prone to crack from stress.

Good luck and keep us posted, Joe.

If you have used the same crank and/or rods I would have them checked and rechecked. Unless the crank was reground each time, I would consider it might be miscut or bent. Take it to a different shop and have them check both the crank and rods.

Actually, on a six cylinder #1 is closest to the water pump and it runs too cold (and too lean) if the water flow is too fast or there is no shield on the pump impeller.

Are you using a thermostat, strayfarer? If you aren't then you are getting too much water flowing through the pump and overcooling the #1 cylinder. This affects the clearances a lot. If you are running a thermostat (180 or 195 degree is best) and your engine is running that hot then you need a new and/ or larger radiator. You might also use a 292 waterpump, it has a shield behind the impeller that directs water out away from the first cylinder.

Think about it #1 cyl is the coolest at the bottom hottest at the top

You did not say what the Piston and rings looked like when taken out. Sounds like CHIPHEAD was on the right track.

Hi all!
I'm the machinist on this job, and it's getting a little frustrating! I've got 45 years in the trade, ASE master rated in every category they offer, 22 certifications in all, including Master Machinist! The original block was bored .060" by a local shop, and was not, in my opinion, done very well. Among other things, the main bearings had not been align bored, and the bearing clearances were incorrect. We repaired that before the first assembly job. The piston was badly scored and the wrist pin was tight on the first teardown. I sleeved the #1 cylinder, fitted a new piston with a little extra clearance, and got about the same mileage as before when it tightened up and scored #1 again.

Got another block, sleeved #1 due to excessive wear, and bored all the holes .030" Speed-Pro Hypereutectic pistons this time, for a 307 V-8. (Same dimensions as a 250, but flat top, not dish top pistons.) I'ver done this before on several engines with good results. Milled the intake manifold for a leak-free fit to the head. Milled the head .050" also. Performance increased dramatically, but it's knocking again, haven't done another teardown yet. I build roundtrack race engines, 600 HP or more, and they stay together and win races! Am I missing something here?
Jerry

Do you have pictures? Where exactly is the scoring.? How much is "a little extra clearance" on the one rebuild? How much clearance did you give the Hypereutectic pistons?
.050" off the head and flat top pistons? How far past 10:1 are you? Joe H mentioned a week spot under front driver side head bolt. You can see it from the water pump cavity.
Someone -strike that. Everyone wants to know if a thermistat is being used. 3500 miles before the problem shows up. Ya, well, maybe, maybe not, maybe so.

A blue oval guy in bow tie country :p
Found another great bunch of guys the other day at lincolnsonline

You have to have pretty high compression with the .050" and flat top pistons. That cylinder could just be running lean. Its unlikely due to the two valves in one port head configuration. 1 and 2 would both be lean if that was the case. The water could be doing it, its been the constant thru this whole thing. All though, won't a cold cylinder run rich?? Is it the same camshaft? could it be ground wrong and be doing something funny to that cylinder?
That would be my next check, degree it and the one next to it.

Keep us posted, Joe H.

OOPS!
I started a new thread by mistake! "More info on bad rebuild" Sorry bout that!
Jerry

Twisted rod

what Rusty46 said

Sorry, guys! I checked the rods on a Sunnen rod aligner the first time it gave problems, and again when we switched blocks. No problems. I'm leaning toward something that's causing #1 cylinder to overheat and sieze the wrist pin in the piston, just can't figure what's doing it. The head's coming off for a pressure check next, plus an inspection of the cylinder & piston, again! I'm pretty sure I'm going to find another locked-up pin/scored piston, but won't know until we tear it down again. It's got the classic piston knock that lessens when the plug is shorted out to that hole. Piston clearance was set up at .0025", lots more than minimum for a hypereutectic piston. The head is the only common factor to all the buildups, so it's my prime suspect.
Jerry

i have seen a 377 (400 w/ 350 crankshaft) score the sides of the cylinder wall before, that was caused by the wrist pin not having enough clearance in the piston. the piston should pivot freely on the wrist pin.