bbc clearances

Ron Gusack · 11-18-2015, 08:43 PM

My buddy's engine is coming up a little tighter than I've been.
Drag only 427+.030 bbc with GM 1077 aluminum heads, steel rods, older Manley 12.5 pistons turning 7300 tops.

Piston is out .003 and he wants to use a .039 compressed gasket. Piston to wall clearance is .005 -.007, is this quench too tight?

He has loads of piston to valve clearance except radially. He used clay and has .030-.040 at the center of the piston, where the little triangle is. Too tight?

Thanks for any help

Alan Roehrich · 11-18-2015, 09:59 PM

The quench is too tight. I can't get away with closer than 0.042" on a 4.165" bore with 0.0055" clearance. That's with a light piston, a light Crower rod, and a really good wrist pin. I'm near touching at that.

For a bracket engine, the valve clearance is too tight for comfort, too.

Tom Goldman · 11-19-2015, 12:19 AM

Like Alan said ,Piston is gonna kiss the head a little . I'd use a.050 or .055 " Cometic gasket .
I'm assuming it's the exhaust thats tight on the radial clearance , you can cheat a little and shift the head with offset dowels and avoid having to cut the pocket.
I did that more than a few times back in the old TRW piston days.
I've run valves that tight myself,but i wouldn't recommend it . they will kiss the piston .

11-18-2015, 08:43 PM	#1
Ron Gusack Member Join Date: Feb 2009 Posts: 244 Likes: 1 Liked 7 Times in 6 Posts	bbc clearances My buddy's engine is coming up a little tighter than I've been. Drag only 427+.030 bbc with GM 1077 aluminum heads, steel rods, older Manley 12.5 pistons turning 7300 tops. Piston is out .003 and he wants to use a .039 compressed gasket. Piston to wall clearance is .005 -.007, is this quench too tight? He has loads of piston to valve clearance except radially. He used clay and has .030-.040 at the center of the piston, where the little triangle is. Too tight? Thanks for any help

11-18-2015, 09:59 PM	#2
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,066 Likes: 1,487 Liked 1,630 Times in 378 Posts	Re: bbc clearances The quench is too tight. I can't get away with closer than 0.042" on a 4.165" bore with 0.0055" clearance. That's with a light piston, a light Crower rod, and a really good wrist pin. I'm near touching at that. For a bracket engine, the valve clearance is too tight for comfort, too. __________________ Alan Roehrich 212A G/S

11-19-2015, 12:19 AM	#3
Tom Goldman VIP Member Join Date: Jan 2003 Location: Allentown,PA Posts: 2,468 Likes: 874 Liked 838 Times in 232 Posts	Re: bbc clearances Like Alan said ,Piston is gonna kiss the head a little . I'd use a.050 or .055 " Cometic gasket . I'm assuming it's the exhaust thats tight on the radial clearance , you can cheat a little and shift the head with offset dowels and avoid having to cut the pocket. I did that more than a few times back in the old TRW piston days. I've run valves that tight myself,but i wouldn't recommend it . they will kiss the piston . __________________ Tom Goldman 1500 SG , 1506 STK