deck clearance question

[Donmcgowen]

So I am doing a mock up and clearance check of my 377 small block Chevy. I'm checking deck clearance to determine head gasket thickness. Anyway, I am finding the clearances in the center of the piston is different from outside of the piston. I'm getting 0.10 under zero deck at the center of the piston but more 0.014 at the outside of the pistons. Is this normal. The block has not been decked and if clearances vary too much I may have the block decked. I'm just checking if its normal for this variation between the center of the piston and the outside of the piston.

[jed]

If I understand your post you are going from a -.010 to a +.014. That is a total of .024.
That is out side of the normal range. Is the +.014 consistent at 12 o'clock 3, 6 and 9 o'clock position?
Is it the same on all Pistons?

[Donmcgowen]

I can deal with 0.010 clearance and use a 0.025 gasket to get 0.035 quench distance. Just not sure if I measure the largest gap or just from center.

[jake197000]

isn't that due to piston rock?

[jake197000]

sorry.average it?

[jake197000]

I think its normal.depending on piston clearance.id ck the deck height from crank centerline.u could have variance in rods or pistons.

[user-612937456]

Look at your combustion chamber and measure in the areas of the piston where they could potentially contact the flats of the chamber. When you measure the outsides of the piston combustion surface is it possible that it is pivoting on the wrist pin causing a skewed reading? Or are you measuring in a lateral line of the cylinder bank? which would be more accurate

[Steve Haaf]

That would be piston rock from the clearance between piston and the cylinder wall. Your most likely are using a forged piston, which means that there has to be at least .004 clearance. This allows the piston to rock in the bore on it`s pin, especially if you put the piston in dry and with no rings. Most people measure from the center, where there is less rock. This is how you get that .004 difference between center and the outside. Deck the block .010 to get your 0 deck and use a Fel-Pro .039 thick head gasket and you`ve got your piston to head clearance. Question; how did you open up those bores without decking the block? Just interested.

[MadBill]

If the piston is dished/domed, measure at a flat portion at the front or rear. (Better yet, front and rear. I was checking out a BBC many years ago and found a 0.010" difference front to rear in one cylinder due to a bent rod.)

[Donmcgowen]

Some answers to questions.
1. Pistons in with no rings
2. Measuring with feeler Guage at center of piston .010 and outer piston I get .014"
3. I wanted to be clear on quench distance. If I'm .010 under zero and use a .025" gasket that would give me .035" quench correct? Am I wrong with this math? I thought .035" was ideal.
4. I don't know 100% the block wasn't decked when bored. Somebody here told me to check if the desk had the factory swirl marks on the deck and I beleven the deck looks factory. This is why I'm checking deck clearance and variations to determine if the deck is too far off.

This is my first real engine build so I'm not always clear. That's why I'm here asking questions because I always get great answers. Even if everyone has their own way of doing things.

[cgarb]

How you checked the deck with a straight edge? When I check piston to deck I put at least one ring on there to help keep the piston somewhat in the center. I find TDC and level the piston in the bore by hand, feel for the piston to be even in the bore as far as piston rock and check my clearance as close to the center as I can. With the factory deck my 377 was off .010 on one side but it was front to back cylinders. Now that its been decked its the same. If the block has the factory deck there will be some factory stampings on the ends of the block on the deck surface. If those numbers are present good chance its never been cut.

[Wolfplace]

Forget the 14thou number unless you are dealing with a dome & averaging hi to low (difference in the same piston not difference cylinder to cylinder) which in your case should come out to the .010 you see in the middle
Theoretically at the edge it would be your .014 below rocked one way added to a .006 above rocked the other equaling .020/2 making it .010 down average
To me you are correct, piston height is done with the piston square in the bore which in your case would be your .035 quench number

[Donmcgowen]

So just an update, I installed a ring on each pistons an reinstalled them to measure deck clearance. With the ring, my clearane went down to .004" or .003" giving me close to zero deck height. I guess having the rings in changes the geometry of the head of the pistons. Anyway, I think I'm going with a .035" head gasket plus the .004" in the hole should give me .039" quench which is just about ideal.

All of my rod bearing clearances are at .003"-.00275" so I think those are good.

The main bearings come in at .003-.0035" so I also think I'm good. This motor is going together much easier then expected.
Is there anything else I need to look out for?

I'm starting to fee pretty confident about this 500+hp SBC build.

Also, the Sprortsman II heads have been ported very nicely. Cleaned up all casting marks and opened up the runners a bit. Did some work on short side and bowl and polished the CC and exhaust runners. Should be good for an extra 25hp hopefully.

[wyrmrider]

Don
I have not rearead your post but did you check deck at all 4 corners or was the block squared up already?

looks good

[Steve Haaf]

Don, if you look at the front left side of the block`s deck, there is a tab. If you can see numbers stamped on the tab, then the block still has the factory deck. You redeck that block and those numbers will be gone. The deck maybe kinda flat but in mass production, the decks could have a pitch or roll to them. You just don`t know where you stand until they are decked, and that`s why I asked how were those bores punched out. It`s important to know.