gtpvette

iTrader: (2)

I just got my short block back from my builder and was measuring the piston/deck numbers. In doing so I started rocking the pistons top to bottom and I知 seeing huge swings in the numbers. I知 running JE pistons that call for .004 clearance which is quite a bit more that the GM specs.

Anyone have an idea of what the swing in the numbers should be when I rock the pistons? Something tells me I値l need to have my bores checked


Thanks!

ss1

iTrader: (32)

.004" is fine with a forged piston. They require more piston to wall clearance than a cast or hypereutectic piston. I know it will seem like the piston is rocking too much, but it's probably fine.

Have you measured the bore yet? You can probably spec out the JE piston and find the exact size, then compare the two.

gtpvette

iTrader: (2)

Thanks,,,, I'm feeling better but still really concered. This block was just bored/honed, decked etc so my assumption is the machinist did it to spec.

The JE Specs call for .004 clearance,,, with a 3.905 bore. I checked my spec sheet and also just called JE tech to verify. He also said anything over .006 may cause problems.

I have a dial indicator close to the bottom (outside of block) of the piston while it's at TDC. In rocking the piston top to bottom I'm seeing changes between .015 - .018 which seems like WAY to much,,,,, but I'm really not sure.

Thanks!

ss1

iTrader: (32)

I was messing around with my racecar engine (sbc) on sunday and was rocking the piston in the bore. It has Wiseco forged pistons in it. I didn't measure it, but my guess it was rocking about .020".

I'm waiting for someone else to chime in on this one.

machinistone

That is normal - You are measuring at the top of the piston...piston to wall clearances are measured at about 1/2" above the bottom of the skirt, pistons wider at the bottom than the top to allow thermal expansion from the heat of combustion at the top of the piston.

MrDude_1

iTrader: (4)

im confused on how you guys are measuring this??

Adrenaline_Z

It is normal for all pistons to rock as they are oval in shape until they expand.

If you are measuring the clearance, it is done at the skirt, not the crown as
Machinistone has stated.

There have been a few threads discussing this. Do a search for more info.

briannutter

there are two primary things that determine piston rock. taper and overall length. The overall length is somewhat a function of compression height (center of pin centerline to deck of piston), so that leaves us with skirt taper to keep rock to a minimum. An average piston may be .004 smaller than the bore at the bottom, but averages about .050 smaller than the bore at the top of the top land. There are a couple other things that play into it like material and pin offset, but not as much. The top 3 lands see a lot more heat than the area below the oil ring groove, so they'll be quite a bit smaller than the measurement directly below the oil ring groove.

gtpvette

iTrader: (2)

Thanks for all of your thoughts guys. I did load up the short block yesterday and took it to another machine shop to have the measure the cylinder bores. Looks like they are OK,,,, maybe just a tad bigger than they should be .0005. I also had him spec the deck height on one cylinder. This brings up another question.

When checking deck/piston numbers,,, where is the correct location to take the highest reading from? He said that the center of the piston would be fine but I also had him take a reading at the bottom (outside of the block) and I rocked the piston and it was .007 higher there. I壇 like to run a fairly tight quench but I also want the motor to live but I知 not sure what numbers to use. Do I average these,,, use the smaller or higher number???

Thanks!



Update - I did a search and found this:

https://ls1tech.com/forums/showthrea...measuring+deck


So it looks like an average is the way to go. That said,, do you need to maybe run the quench a little higher to account for the rock? I'm shooting for somewhere between .035-.040.


Thanks!!

CNC BLOCKS N/E

If your block was prepared properly and it was and was bored of the main line and not off the UNSQARED DECK as from what I have seen with the decks on GM blocks they should be squared and decked first if your setting a boring bar on the decks. At our shop we are set up to bore of the main line as some customers want to save the vin numbers on the deck.

If this block was plate hone and your checking it with out a torque plate bolted to the deck you being misled buy how much the piston is moving in the cylinder.

JE pistons are a top of the line piston and should be put in a cylinder that has been plate hone to ther spec and there should not be any issues with ring seal and and scuffing of the skirts due to non torque plate honed cylinders.

redtail2426

iTrader: (10)

i already know the rock on a forged ls1 would scare me,because the rock on my stock shortblock with 15k miles scared the crap out of me,they rock a lot.

briannutter

Pistons may or may not have offset pins like (Wiseco's or O.E.'s are offset) and rotate around a "slightly" different axis than the center, so you may get two different readings on the major and minor thrust. The best way to measure deck is to put a "bridge style " dial indicator on the topmost portion of the piston and rock it for and aft. Say your measurement here is .005 negative and .003 positive....That gives you an average of -.001 "in the hole". Then you go to the extreme opposite side of the piston with the indicator and you might be the measurement -.004 negative and .006 postive (that gives you .001 standing proud). These two 'averages' would give you zero deck.

In reality, the pressure you apply to the piston (usually with rings on them for support) is a bit subjective and it's tough to get a repeatable measurement down to the 1/2 thousandth.... .002 is more like it. Also, it's very possible to have .020 "up and down" total measurement depending on the taper in the piston..just maintain your averages. The best piston companies have minimized skirt taper to decrease noise and rock-yet not so little taper that it would seize from thermal expansion.

If you don't have a bridge indicator or at least a magnetic base indicator (a pain in the butt to use on aluminum blocks), the needle of a dial indicator in the center of the piston would at least give you some idea..but it's not nearly as accurate.