I'm building a 383 LT1 right now and picked up my block from the machine shop on monday. After file fitting the rings I went to check for the .0015 clearance the KB hyper pistons call for and its not there. I measured the pistons bores with a digital caliper and found them to be from 4.024-4.026 each. The KB pistons on the bottom of the skirt measure 4.028. The pistons fit in the bore but they are very snug. I have a 'bottle brush' honing tool and drill should I try to open the clearance with that or bring it back to the machine shop? I'm just worried the pistons with expand and the skirt will wipe out the finish on the cylinder walls.

Jason

 

Take the block in and have them mic the bore and piston in front of you with a dial bore guage and a .0001 mic. A veniers caliper is not accurate enough to make those measurements + you are using two different blades on the calipers to make your measurements.

 

I know the clearance isnt large enuff as I cannot get a .0015 feeler gauge in between the piston and block. Since I paid to get it bore and honed I'll bring it back in.

Jason

 

Is the bottom of the skirt where KB recommends you measure their pistons? Make sure you are measuring where they say to.

Yes use a mic, the calipers will not give a good enough reading.

 

Is 1 and 1/2 thou all it gets?
Ya won't be able to get a feeler guage in it.
Take it back.

 

I know everyone is telling you to take it back..... but how you do that will help you in the long run.

If you didn't give the shop the pistons to hone the block to in the first place then this is not their fault... (well other than they should have asked for them to do the job in the first place) If you did give them the pistons then have them show you how they measure clearance.

The way you measure the clearance is with a 4" micrometer and a bore guage. To get the clearance you need to either set the bore guage with the mics used to measure the pistons and do the math subtracting the piston measurement from the bore measurement OR what I prefer is to mic a piston and then set the bore guage to that measurement and then measure the bore. This gives you the clearance.

You very well could be within spec, and just running in there and telling them that all the guys on the net told you your block was machined wrong is NOT going to make you look very smart, especially if the shop did it the proper way.

Figure out how they did it and go from there.... if they did it i'm sure they will show you on your parts in front of you so you know the clearance.

Bret

 

It's odd that a shop would only take the bores out .024-.026 over stock. Even without the pistons in hand. You bore & hone it out to a true 30-over (4.030") and, if the pistons are manufactured correctly, the clearance should be very close to correct.

Agreed, having the pistons in hand for an exact match is the best way, but I've done it the "wrong" way enough times it's kind of embarassing, not a problem yet.

You'll notice your KBs are spec'ing out .002" shy of a perfect 30-over (I've used KB hypers myslef many times and found they nail that same spec almost always). That's where the clearance comes from. A piston designed for a 30-over bore SHOULD, in theory, drop into a bore that's exactly a true 30-over (4.030") with the recommended piston-to-wall clearance perfectly.

4.030" bore - 4.028" piston = .002" piston to wall clearance.

Last comment.... if your bore was actually .002-.004" SMALLER than your pistons like you claim you couldn't shove it in the bore. You measured your bore wrong.

BTW, here's where you measure your KB pistons with you mic for a true measurement (I think you probably already got close enough for a correct reading):

 

.0015" is pretty tight, it's not like .004 on a forged piston where the piston will rattle right through the bore. A feeler gauge is not a very accurate way to get an idea on your clearance but your pistons should slide in without any real resistance. Might see a little drag because of the piston profile getting a good bit of bore contact once it's in the bore. Definitely don't use a hone to open up the clearance just yet. Find out for sure what it is, if it's within a couple tenths you might use a brush type hone but if closer to a half thousandth I'd definitely opt to have them rehoned with the right equipment so everything stays straight and parallel. Who knows, they might be right on the money, get them measured properly and then you'll know

Good luck, Michael

 

I think the problem is in his measuring tool, by using the inside and then the outside part of the verniers caliper, you are not using the same tool to measure both parts like Bret pointed out in the proper way to measure clearance, I do it the same way - mic the piston and set the dial bore gauge to that number with that mic, then what ever clearance number the dial bore gauge shows is your number.

 

I know realize that I messed up when not giving he pistons to the machine shop! The measurement on the pistons is taken at the balance pad and the 4.028 was dead on with each piston. The pistons are a little tight at the bottom of the skirt going into the bore but once in slide up and down freely. Like stated I think its b/c I'm using two different sets of blades on the caliper. I dont own a bore dial gauge so I'm SOL there, so its either run it the way it is or bring it back. Judging by the comment made about not getting the piston in the bore I'm leaning to thinking I have good clearance. Is it possible that since I can only measure about 1/2 down the bore with the caliper that the bore opens up more after that? Like I said the piston only feels tight going into the bore once its in it'll move up and down very freely.

Jason

 

Jason,

Problem here is that you need to know the exact measurement to the .000X" of a inch, not .00X"

Bret