Can I run .020 over pistons in a .021 over bore?

[2slow4utsi]

My block is .021 over and I want to keep my pistons at .020 over. The machine shop says it should be fine but wouldn't there be excess play of the pistons with the added thousandth? I don't want to have them do anything else until I know for sure and I have yet to buy my pistons cause I was unsure of the size. The shop suggested .030 over but I really don't want to go that route.

I am having this block machined for my HX40 build and plan on running at least 30psi if not more. If you guys have any REAL insight not just assumptions and guesses please let me know. I really don't want to go any more than .020 over.

Thanks,

Danny

 

[delta448]

Your first mistake was not having the block bored to match the individual pistons you'll install. When you're at about 0.001" of ptw clearance, you're waaaay too tight for forged pistons.

You need to have the pistons measured by the shop doing the block and let them bore to the correct PTW clearance that the manufacturer of your pistons mandates.

 

[2slow4utsi]

The block has already been bored...it's already .020 over...

I told the shop I wanted to be able to run .020 pistons. I didn't buy the pistons before I knew what the block would clean up to. They already measured and said it has been punched over and is actually a little over .020 over. Does that clear things up? Why would I purchase a set of pistons before knowing what the block would bore to?

 

[delta448]

I told the shop I wanted to be able to run .020 pistons.

Did you tell them what alloy the pistons would be or that the engine would be turbocharged?

I didn't buy the pistons before I knew what the block would clean up to.

That's fine.

They already measured and said it has been punched over and is actually a little over .020 over. Does that clear things up?

Yes. But you should be more exact.

Why would I purchase a set of pistons before knowing what the block would bore to?

Not a problem. All you need to do is have the block precisely HONED now to the exact desired PTW, assuming that the block hasn't already been bored out too far to get the PTW you want with the 0.5mm oversized pistons you're installing.

 

[95blackGsTurbo]

The block has already been bored...it's already .020 over...

You can bore any block .020" over, but if the pistons aren't used during the process to check piston to wall clearance, you need to have it redone. I would go .030" over and make sure it's all done right.

 

[JRSTANGE]

I'd say depends on the piston bore and recommened ring gap for those pistons to give you the right clearances you need. Find that stuff out and do the math and go that route.

 

[2qik4u]

i agree with figuring out what pistons AND ring gap you are going to run before boring. I bought a set of wiseco pistons that were .020 over stock and had the shop bore to exactly what wiseco recommended for the piston and ring package with the ring gap they reccomend to run. go on wisecos website at wiseco.com and look at their FAQ's section. they have some very useful info on piston to wall clearance among other things. hope this helps!

 

[JRSTANGE]

What i stated and 2qik4u is the reason to have the pistons or parts first before having the machine work done on the block. As other have noted there is different recommened dations for each brand and manufacturer. I'm honestly suprised the shop did the work with out a piston first or at least ask you about the pistons. But, then again i've never tried dropping the block off with out the rotating assembly or at least the pistons. best of luck to you.

 

[BogusSVO]

ok... .020 over pistons will be fine, a STD bore 4g63 is 3.346 + .020 = 3.366
if using a forged piston, your Piston to wall clearance should be .0035,

Now when you get your pistons they will NOT mic out at 3.366 but more like 3.3625-3.363
Pisto to wall clearance is "built into the piston

also stock clearances are for stock engines, or slightly mod ones, the more you build the engine, the more clearance you will need
an extra .001-.002 piston to wall clearance will not hert, but in some cases keep you from frying an engine.
if you run lean, you make more heat, more heat means more thermal expantion, more expantion means a need for more clearance.
this will apply to piston to wall, ring gap, and rod side clearance.
but there is a fine line between just enough clearance and too much.
sine I would bet your car/engine will see more street miles than race miles a slight increase in some of the primary clearance will be ok

if your piston to wall clearance is supposed to be .0035, you will be fine as far up .005, more than that and you may get piston slap on clod starts till the engine is up to op temp

so what I am saying is order your .020 over pistons, and measure them 90* to the center line of the wrist pin, then use a bore gauge and measure your bores at 6 points, top, middle, bottom inline with the block and then 90* across the block.
then do the math and make sure you will have the min clearance for your pistons at all 6 points in all 4 bores

also if you plan on running your engine hard for more than a mile at a time, I would add about an extra .002-.006 to your ring gap on #1 and #2 ring to help insure that the ring ends to not "butt up" and break ring lands, and live with a slight puff of smoke every now and then.

 

[hubz91talontsi]

This is all blah blah blah. Buy a catch can put them in. Turn up the boost =P

 

[2slow4utsi]

Lmfaoooo buy a catch can. I figured out what it had to be. I talked to Ross then I called the shop. Thanks guys for all the information.