TRW piston clearance?

rod · 06-15-2012, 06:33 PM

will be putting together a new short block for a bracket racer. he has a set of older TRW forged SBC pistons [12 to 1] the previous block had .007" clearance. i believe that is too much for a drag engine. however with the AZ heat, i also believe .0055" is too tight.
you're experience and or your thoughts please.
thanks rod in AZ

Billy Nees · 06-15-2012, 06:40 PM

I've run the old TRWs in smallblocks with up to .010 wall clearence. It'll go faster than stink but will wear the rings in a hurry and be noisy. Also, don't run your piston to head clearence too tight.

Mike Taylor 3601 · 06-15-2012, 07:11 PM

What Billy said is right, I have ran them at .003. Usaully as example say it was a 350+ .030 skirt measured 4.026 then it was made to be @.004 min.
If pistons have been ran be sure to check skirts were bad to be collasped on those pistons
Mike Taylor 3601

CFMCNC · 06-16-2012, 12:57 PM

Exactly what Billy and Mike said as tight as .003 and as loose as .010 and even looser at .012-.013 when customer had no money for block or pistons.The barometer for piston condition is skirt collapse and Rockwell B scale test,Current 2618 material starts out at 75-83 Rockwell B scale ,We test all pistons at rebuild ,When they go below Rockwell 50 they are junk.Bill C.

Alan Roehrich · 06-16-2012, 02:17 PM

They seem to "like" about 0.004" to 0.006" on a 4.0" to 4.125" bore size.

The most important thing to know about those is the pin is not great, and the pin fit is too tight. Replace the pin if possible, and open the pin clearance up by about 0.0002 to 0.0005" from where it starts. Way too many of those things have seized a pin, with ugly results.

GTX JOHN · 06-16-2012, 04:05 PM

I mike piston middle or a little lower on skirt and 90 deg from pin....
is that how you guys do it?

Thanks

06-15-2012, 06:33 PM	#1
rod Member Join Date: Dec 2009 Location: Kingman, NW AZ. in the middle of the longest stretch of Route 66 Posts: 314 Likes: 0 Liked 20 Times in 15 Posts	TRW piston clearance? will be putting together a new short block for a bracket racer. he has a set of older TRW forged SBC pistons [12 to 1] the previous block had .007" clearance. i believe that is too much for a drag engine. however with the AZ heat, i also believe .0055" is too tight. you're experience and or your thoughts please. thanks rod in AZ

06-15-2012, 06:40 PM	#2
Billy Nees VIP Member Join Date: Oct 2002 Location: On a hilltop in Pa. Posts: 4,485 Likes: 3,587 Liked 7,682 Times in 1,730 Posts	Re: TRW piston clearance? I've run the old TRWs in smallblocks with up to .010 wall clearence. It'll go faster than stink but will wear the rings in a hurry and be noisy. Also, don't run your piston to head clearence too tight. __________________ Billy Nees 1188 STK, SS I'm not spending 100K to win 2K

06-15-2012, 07:11 PM	#3
Mike Taylor 3601 VIP Member Join Date: Oct 2008 Location: Somerset,Ky Posts: 1,369 Likes: 348 Liked 293 Times in 101 Posts	Re: TRW piston clearance? What Billy said is right, I have ran them at .003. Usaully as example say it was a 350+ .030 skirt measured 4.026 then it was made to be @.004 min. If pistons have been ran be sure to check skirts were bad to be collasped on those pistons Mike Taylor 3601

06-16-2012, 12:57 PM	#4
CFMCNC Junior Member Join Date: Apr 2012 Location: Somwhere in NJ Posts: 72 Likes: 4 Liked 3 Times in 3 Posts	Re: TRW piston clearance? Exactly what Billy and Mike said as tight as .003 and as loose as .010 and even looser at .012-.013 when customer had no money for block or pistons.The barometer for piston condition is skirt collapse and Rockwell B scale test,Current 2618 material starts out at 75-83 Rockwell B scale ,We test all pistons at rebuild ,When they go below Rockwell 50 they are junk.Bill C.

06-16-2012, 02:17 PM	#5
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,105 Likes: 1,564 Liked 1,789 Times in 408 Posts	Re: TRW piston clearance? They seem to "like" about 0.004" to 0.006" on a 4.0" to 4.125" bore size. The most important thing to know about those is the pin is not great, and the pin fit is too tight. Replace the pin if possible, and open the pin clearance up by about 0.0002 to 0.0005" from where it starts. Way too many of those things have seized a pin, with ugly results. __________________ Alan Roehrich 212A G/S

06-16-2012, 04:05 PM	#6
GTX JOHN VIP Member Join Date: Jun 2009 Location: Boulder City, Nevada 89005 Posts: 2,731 Likes: 2,794 Liked 2,471 Times in 696 Posts	Re: TRW piston clearance? I mike piston middle or a little lower on skirt and 90 deg from pin.... is that how you guys do it? Thanks