Inliners International Forums General Discussion Engines How Far Down Are Pistons in Bore on 250 at TDC?

Hi all, this is my first post. I have been reading a bunch and have learned a lot of new things. I am new to the inline world, with my '64 Chevy II wagon having a 250 six in it. It has a Clifford intake and header, Holley 390cfm carb but otherwise a fairly stock rebuild.

Recently I ran into a problem where is overheated enough to cause the #6 wrist pin to get tight (not seized) and cause the piston skirt to be gouged and slight in the bottom of the bore.



Since I have things apart, I am checking everything. One thing I noticed is how far down the pistons are in the bore when at TDC. I measured this at almost a 1/4" (.24x). I have been reading and really haven't found the answer.

On a 250, how far in the hole should the pistons be at TDC?

Thanks!!!!

There really isn't an established set deck height on any of these engines, but typically on the 250's it has been found to be as much as .050"-.060" below the deck to as little as .020" below the deck. Depending on how many times the engine has been rebuilt in the past and if the block has been resurfaced a few times before you ended up with it. Your measurement of almost a 1/4" of an inch makes me suspect you have a 230 and some one installed 250 pistons into it. That would explain the excessive deck height you have. See if you can find a part # somewhere on the piston somewhere to verify someone didn't make this mistake.

Thanks for the info. I am seriously suspecting something is wrong. I'll need to start checking numbers on thing. I wasn't able to find a part number on the piston, only 040 which I assume means .040 overbore. Number mic out that way.

This would definitely explain the detonation issue I have been trying to get rid of.

I don't have a close up of a bore but if you look at #5, you can see the deposits at the top of the bore.

Detonation? With so much piston/deck clearance one would expect the exact opposite . . . low compression ratio and limited power.

It's just the opposite

MBHD
.

Okay, I am at a loss... I checked the crank number and is shows in the book as a 250 crank (460407). Rod measured 5.7" Pistons are marked 040 and the bore shows it to be .040 over stock. Top of piston to center of wrist pin is 1.64.

How can the pistons be sitting so low in the bore?

Nevermind... I'm a Dumb A$$... Somehow I didn't get the pistons to TDC (using #3 and 4 as I have 1 and 6 out). They are about .005 down, so all it good.

Thanks for coming to my aid and making me double check numbers.

LOOKS TO ME AS IF YOUR PASSING WATER, LOOKS LIKE RUST ON TOP OF PISTONS 123&5. I,D DO SOME MAGNIFLUXING BEFORE REASEMBLE IF IT WAS ME. AND THAT URNNED PISTON IS JUNK. INSTALL SOME FLAT TOP 307 SBC PISTONS. ALSO BET THAT CYLINDER WITH A BURNNED PISTON NEED BOREING/ A SLEEVE. BET YOU HAD LOW WATER BECAUSE OF A CRACK AND THIS CAUSED SIEZED PISTON. JMO
BOB

ALSO IT LOOKS LIKE A HEAD GASKET PROBLEM BETWEEN #4&5 CYLINDERS

Let me add this pic of the head.



Measured the bad piston last night and found it .012 under. The #6 bore is matching the rest a 3.915. Seems that piston got hot and shrunk when it cooled.

What are you seeing in the pics that is wrong? My machinist/auto buddy came by last night and didn't see anything wrong on the head, gasket or block that would indicate a problem.

Thanks for taking the time to reply everyone.

Also let me add, I had been driving about a 100 miles on the freeway, running between 65-75mph with a little jaunt to 100. When I got off the freeway I heard the knocking sound that I thought was the exhaust leak that had gotten worse (loose header to down pipe connector as I found in disassemble). About a mile later I got the temp light and shut it off immediately.

When I popped the hood, I found the coolant overflow was full and bubbling. With a rag, I carefully loosened the radiator cap and had steam coming out. Let it cool of and put a gallon of water in to get me the 1 mile home.

I ran compression test and all were between 145-160psi. Leakdown test showed no leaking of any cylinders. Ran a block test and no exhaust gases were found in the cooling system. I never lost power and outside of the noise, it was running well, outside of the mid range throttle ping/detonation.

The orange color(rust, ie. water) of the valves and combustion chamber definately indicate a strong presents of water. Note the total absence of any carbon traces on the chambers and valves.....
That could have also led to the piston galling by washing all the oil off of the cylinder. Magnaflux the head for sure, and make sure the deck surfaces on the block and head are resurfaced also, especially after running it hot like you said.

One thing I noticed when pulling the head bolts, other than one in the rear, none of them seemed to have any thread sealer. My understanding from what I read in "the book" is all of these bolts go into water jackets and are to have sealant on them.

Could coolant be pulled from the threads and into the cylinders?

BTW, this engine only has 5300 miles on it since being rebuilt. I put a 1000 on it (have receipts from PO).

with that low of miles Their shouldn't be any carbon build up to start with. Esp. with todays fuel. And if you had No smoke (white)coming out the tail pipe then I don't think you had water
in the chambers.
But you should still flux everything. Esp.the head, An if you had seats installed in the head They Could?? have been cut to deep and maybe??? hit the water jacket?? To me from the photos the gasket looks to have had a good seal.

Never had any smoke of any kind out the pipes. I know California gas is crap compared to other areas. O2 enriched crap.

The thing I think that caused the coolant leaving the radiator is it was converted to a recovery tank setup by the PO. I was recently reading that there are two types of radiator caps and the original style is not the correct one for a recovery system. I don't think the PO ever changed the cap and basically it was acting as a one way valve. It pushed out coolant and was not pulling it back in. In turn, this caused the overheating and hence damage.

At least that is a theory. Open to any and all ideas to why this occurred on such a low mileage engine.

Mike - if you need a GOOD shop to mag and pressure test the head, I highly recommend Gary Camp. He took over Ed Hale's shop after working for Ed and Pat for the past 25-odd years. The sign over the quansit reads GMC AUTO MACHINE I think, but still looks the same inside the shop, same location on Wintergardens in Lakeside.

I can certainly feel your pain, having been snakebit on a 3/4 race 235 with low miles. PO's shop left out the baffle in the crankcase for the road draft tube for starters. Then a Crower valve spring broke, dropping a intake valve. Mag reveled a crack above the water jacket on the FULLY PORTED HEAD.:( #6 was also 20-lbs. down on compression, so tore it down for the baffle, rings and rod bearings. NEVER had great oil pressure, but it got sick 1-1/2 years later on I-5 in San Clemente. It had spun #5 rod bearing, but never knocked - just ran hotter, oil pressure dropped to around 12 lbs. I call this mill THE JINX !!! I have a couple freebie motors, so I just might build it again with a used crank and bearings!:)

Tear the 250 down and RECHECK EVERYTHING!!! Gary can help with your build.

Good Luck, Tim

Thanks Tim for the recommendation. Glad to hear Ed Hale's shop is still going strong after his death. Ed sure knew engines!!!

Yeah, I am pulling everything apart tomorrow. Going to double check everything. Sounds like I need to take the head out and at least get it checked, as this seems to be the general consensus here. Still learning a lot about these sixes. I'm a SBC guy.

do the block too, where you galled the piston definatly need honeing at least. jmo