Engine questions - 1970 LT-1

[Ed H.]

I have the engine mostly apart (crank and cam still in). It has not run in over 30 years, but I rebuilt it a few years earlier.

Bores, checked with a dial bore gauge, vary between 4.001 and 4.003 in either direction (no individual bore varies by more than .001 with respect to taper, most by .001 with respect to out of round, none over .003). These numbers are calculated by the maximum - the minimum dimension, accounting for taper.

The pistons, Chev P/N 3959460 measure 3.998 (I confess to measuring only 2) in dia, perpendicular to wrist pin, in middle of the skirt.

The bores look remarkably good, the cross-hatch from honing still shows.

I'd rather not bore and replace the pistons. Comments?

Thank you!

[Joe L.]

Re: Engine questions - 1970 LT-1

I have the engine mostly apart (crank and cam still in). It has not run in over 30 years, but I rebuilt it a few years earlier.

Bores, checked with a dial bore gauge, vary between 4.001 and 4.003 in either direction (no individual bore varies by more than .001 with respect to taper, most by .001 with respect to out of round, none over .003). These numbers are calculated by the maximum - the minimum dimension, accounting for taper.

The pistons, Chev P/N 3959460 measure 3.998 (I confess to measuring only 2) in dia, perpendicular to wrist pin, in middle of the skirt.

The bores look remarkably good, the cross-hatch from honing still shows.

I'd rather not bore and replace the pistons. Comments?

Thank you!

Ed------

Assuming that all of your pistons measure out like the 2 you measured, you should be fine. You should be well within the acceptable piston-to-bore clearance for the LT-1 engine. I would, of course, replace the piston rings with good Federal-Mogul moly rings or Total Seal rings.

[Duke W.]

Re: Engine questions - 1970 LT-1

I concur. It's not perfect, but shouldn't create serious problems. If you have a known reputable high quality engine shop nearby, I suggest you take your data to them and get their opinion on whether honing the cylinders with the greatest taper and/or out of round will reduce those values without increasing piston clearance out of the acceptable range. Nominal clearance for the OE forged pistons is about .0035", but they will probably tolerate up to .005".

Duke

[Patrick H.]

Re: Engine questions - 1970 LT-1

I would consider checking with a torque plate installed. You might find that the numbers change enough to influence your decision.

[Joe L.]

Re: Engine questions - 1970 LT-1

I concur. It's not perfect, but shouldn't create serious problems. If you have a known reputable high quality engine shop nearby, I suggest you take your data to them and get their opinion on whether honing the cylinders with the greatest taper and/or out of round will reduce those values without increasing piston clearance out of the acceptable range. Nominal clearance for the OE forged pistons is about .0035", but they will probably tolerate up to .005".

Duke

Duke------


The acceptable range for 1970 LT-1 is 0.035"- 0.060" with 0.035"-0.040" preferred.

[Ed H.]

Re: Engine questions - 1970 LT-1

Thank you!

I know of four, maybe five good shops near by, plus a friend who only does aluminum stuff. I'm trying to drag him over here for a double check, but he's off on vacation this week.

I don't have such a plate, but doubtless the shops do.

Piston clearance .035"? Just checking.

In any case, new rings, bearings, bolts, etc. I'll be back with more questions!

[Duke W.]

Re: Engine questions - 1970 LT-1

Duke------


The acceptable range for 1970 LT-1 is 0.035"- 0.060" with 0.035"-0.040" preferred.

Joe - you misplaced the decimal point... should be .0035-.0060" and .0035-.0040" preferred.

Duke

[Ed H.]

Re: Engine questions - 1970 LT-1

What's an order of magnitude between friends.

[Joe L.]

Re: Engine questions - 1970 LT-1

Joe - you misplaced the decimal point... should be .0035-.0060" and .0035-.0040" preferred.

Duke

Duke------


Yup, I sure did.

[Duke W.]

Re: Engine questions - 1970 LT-1

One more thing, Ed. Only allow the machine shop to remove metal if absolutely necessary. This applies to grinding the Tufftrided forged steel crank and align boring the main bearings, and, of course, decking the block. If the cranks spins freely then there's no need for align boring, and if you decide not the do any block honing it doesn't need to be removed. If you decide to replace the rear main seal then I suggest new bearings on #5 since removing the cap relieves the crush.

IMO the "pink" rods are okay to use with the OE bolts. No new bolts, which require resizing the rods are necessary, but you should install new bearings for the reason stated above.

I have a technical paper on the subject, and given that you have taken precision measurements of the cylinder bores, the rest of the measurements I recommend will be fairly easy.

Send me an email through the TDB, and I will send you the paper.

Duke

[Mark E.]

Re: Engine questions - 1970 LT-1

If you decide to replace the rear main seal then I suggest new bearings on #5 since removing the cap relieves the crush.Duke

So when a RMS is replaced, the bearing should also be replaced?

[Duke W.]

Re: Engine questions - 1970 LT-1

...not absolutely necessary, but a precautionary measure for any plain bearing. As manufactured they are slightly out of round, and tightening the cap deforms them slightly into round - what's called "bearing crush". It's a one shot deal so if you remove the cap and reinstall without changing bearing halves the bearing is not held as tight, so it will there will be a greater chance that the bearing will "spin", especially if there is momentary oil starvation.

The bearings are held in place primarily with the locating tabs, but crush adds to total clamping force that keeps the bearing from spinning. If the bearing show no signs of stress and are not removed, it's probably okay to reuse, but I would not reuse a bearing after it's been removed.

Duke

[Dan D.]

Re: Engine questions - 1970 LT-1

Duke, interesting, reducing bearing crush - this is news to me - but I see what you mean. My question is - by that is it saying that you should not use Plastigauge to check bearing clearance? Is this correct?

I put my whole engine together using Plastigauge (has not been run yet).

-Dan-

[Duke W.]

Re: Engine questions - 1970 LT-1

Checking bearing clearances with Plastigage is a good idea. As long as you don't remove the bearing halves from the caps when you remove them to check the Plastigage (and there is no reason to do so) and all the bearing bores are within the proper dimensional range it's very unlikely to be a problem.

Duke

[Dan D.]

Re: Engine questions - 1970 LT-1

Okay, thanks Duke,

I should be okay.

-Dan-