72 LT-1 engine advice needed

[Tony 38901]

I currently have my engine out of the car. My original intent was to freshen it up w/ some new gaskets, paint & also do some underhood detailing. I pulled the heads and found some strange "scratches" on the #3 cylinder (they feel rough to the touch). I've never seen the inside of an engine before, so I'm not sure if this is normal. My compression checked out OK back when I tested it in Dec 2003 (less tha 1k miles since then). I checked it when the engine was cold and got readings of 142, 120, 143, 150. I have a thread at the CF with pics.
http://forums.corvetteforum.com/show...post1554579519
Can anybody explain those scratches?

I also found a part # on one of the pistons, 3959456. From the quick research I've done the look like GM service replacements with 11:1 compression.

I'm also looking for some tips as to how I should go about painting once I get it all back together.

Thanks,
Tony

[Mark #28455]

please post pics of scratches *NM*

[Tony 38901]

pics here

Here is a close up on one of the "scratches". There are a total of 3 like it on the cylinder walls.

[Joe L.]

Re: pics here

Tony------

As best as I can tell from the pictures, I would say thet these scratches are the result of some sort of ring problem. You will need to remove the pistons to more fully inspect for this. Most likley, if the scratches will not clean up with a light honing (a good machinest will know), you'll need to overbore.

GM #3959456 is a forging number for the 69-70 LT-1 pistons. Although no LT-1 engines were produced for the 1969 model year, the pistons were. For the 1969 model year, the pistons were set-up for floating pins. For 1970, the same forgings were machined for use with pressed pins, which all PRODUCTION LT-1 engines used. The FINISHED PART NUMBER for the pistons obviously changed with these pistons. The FINISHED PART NUMBER for the 1969 LT-1 piston (standard) was GM #3959448. The 1970 LT-1 piston was GM #3989048. These numbers usually appear nowhere on the piston other than, possibly, an ink stamp on the crown.

There's no reason to believe that your pistons are replacements. If they were GM SERVICE parts, they would be identified by the same forging number as original (assuming they were parts purchased MANY years ago). So, while they may be SERVICE pistons, everything considered, it's more likely that they're original pistons.

[Joe L.]

Correction

I just noticed that you're talking about a 1972 LT-1 here. Somehow, I thought you were talking about a 1970. I don't know what gave me that idea since you state the year clearly in the title of your post.

In this case, the pistons were definitely replaced with the 69 or 70 pistons. Most likely, someone did this to get the higher compression used in those years. The 72 pistons were a flat-top with one elongated notch for valve clearance. Obviously, these pistons are the 69-70 domed style LT-1 pistons.

It could also be that the "someone" that did this somehow screwed up the piston ring installation in a few cylinders.

[Duke W.]

Re: 72 LT-1 engine advice needed

Minor scoring, which should not be a problem for a "driver" or even a restored car if you are not after internal engine perfection.

A few years ago when I borescope inspected my Cosworth Vega cylinders I found some minor scoring in a couple of cylinders and an unidentified anomaly. A couple of years later I pulled the head to replace the valve guides and the anomaly turned out to be casting porosity hole that measured about 1/16" by 1/32". The block probably should have been rejected. Since the engine showed both good compression and leakdown numbers and good power on a chassis dyno I decided to leave the block alone. New valve guides and seals reduced oil consumption to near nill, and in a subsequent dyno pull the engine made 122 SAE corrected RWHP @ 7000. One RWHP per cubic inch on a low compression engine on regular unleaded is okay in my book, so my decision to leave the block alone despite the discrepencies was okay.

You should always run compression or leakdown tests with the engine hot. Your numbers have more that the factory specified 20 psi variation, but a hot test might prove otherwise. The bore with the minor scoring reads the lowest at 120, but it might read better on a hot test.

This is a situation that could go either way. If engine performance is okay, and you don't drive the car a lot, the engine could be okay for many years to come, but if you are a perfectionist, now may be the time to rebuild the engine.

Another thing you could do is measure bore wear at the top of the cylinders, If it is no more than .003" the engine still has some life left in it.

What's the total mileage?

The other thing to consider since the pistons were replaced with the early high compression design is what CR will you end up with when you put the heads back on.

What kind of gasket was installed and what is the compressed thickness? (I hope you didn't throw them away already.) Did the engine show any detonation problem?

Chances are if you install your heads with a typical .038" composition gasket the compression will be no more than 10.5:1, which should be okay for unleaded premium.

Duke

[Clem Z.]

Re: pics here

some time these scratches show up when using "file fit" rings if you do not use a fine honing stone to "break" the sharp edges on the "filed" ends of the ring after "filing" them to get the correct "end gap"

[Tony 38901]

Re: 72 LT-1 engine advice needed

The part # on the head gasket reads 7733PT-2. Its a Fel-Pro brand.
The engine has 73k miles on it.
I have a friend who is much more knowledgeable that I come over to inspect the block later. My major concern is not doing anything now only to find out later that more damage had been done.

[Duke W.]

Re: 72 LT-1 engine advice needed

If you measure the gasket thickness and measure your deck clearances, you can compute what the CR was and what it will be with any different gasket thickness. With only 73K miles I don't think that bore wear is near the .005" limit.

You might also be able to find the nominal heat gasket clearance on the Web.

You can use the online CR calculator I referenced in a thread last week. The dome volume of the 350 SHP high compression pistons is +2.4 cc.

Duke

[Tony 38901]

Re: 72 LT-1 engine advice needed

I have 993 heads, so I plugged in 75cc and the rest of the head gasket info I found on the Net. What would the piston deck clearance be?

Enter Cylinder Bore Size = 4
Enter Piston Stroke Length = 3.25
Enter Head Gasket Bore Diameter = 4.125
Enter Compressed Head Gasket Thickness = 0.039
Enter Combustion Chamber Volume In CCs = 75
Enter Piston Dome Volume In CCs Negative For Dished Pistons = 2.4
Enter Piston Deck Clearance = ???

Also, forgive my ignorance...but is it normal to have the head gasket bore diameter larger than the cylinder bore?

[Clem Z.]

Re: 72 LT-1 engine advice needed

1/2 the stroke is 1.625,rod length 5.700.piston compression distance added together = 9.000" and the std SBC deck height is 9.025 so the deck clearance should be .025 BUT that varied to as much as .040

[Tony 38901]

Re: 72 LT-1 engine advice needed

I made a mistake on the Stroke length, should be 3.48

[Duke W.]

Re: 72 LT-1 engine advice needed

You have to measure piston-deck clearance. It varies based on how high the block was machined relative to nominal.

Use a dial indicator to find TDC of each cylinder, then use a machinist bar and two feeler gages - one on each side because piston rock can cause a false measurement if you just measure one side.

Head gasket bores are wider than nominal OE bore diameter to take into account overbores. Measure the opening diameter of the installed gasket and use that.

With the large chamber heads you should not have a problem with the actual CR being too high for current pump gasolines.

Duke

[Clem Z.]

Re: 72 LT-1 engine advice needed

chevy blocks also vary from end to end. i have seen as much as .012. a lot of people say to use a thick head gasket to lower the CR BUT this can cause detonation because of the amount of fuel trapped and not burning evenly. i would never go over .045 total deck clearance including the head gasket.

[Duke W.]

Re: 72 LT-1 engine advice needed

I think you're referring to "quench clearance" - the distance between the piston crown and head, which is the sum of deck clearance and head gasket thickness.

According to Taylor, quench clearance doesn't reduce detonation tendency until it is reduced to .005 times bore or less, which would be .020" on a SB, which is below the .035-.040" minimum quench clearance recommended by Chevrolet.

As far as deck taper is concerned, most that I have measured or have good data for are reasonably parallel to the crankshaft axis, but you've probably seen a lot more than I have.

I think sixties vintage engines may be dimensionally better than later built engines.

Duke