Race Prep Miata race-only chat.
Sponsored by:
Sponsored by:

Piston to Head Gap

Old 12-21-2014, 04:02 PM
Junior Member
Thread Starter
iTrader: (4)
Join Date: Sep 2010
Location: Australia
Posts: 167
Total Cats: 12
Default Piston to Head Gap

Hi guys,

Rebuilding a race motor (high compression Nat. Aspirated) that I picked up as a spare. The engine was built by a guy here in Australia with a good reputation, and was a quick motor on the track.

It has some interesting stuff in it, but the most interesting thing we found was how little quench the car was set up with. Measured at the pin, it had less than .020" clearance to the head (counting the .040" head gasket - so the Pistons were sitting around .020" above the deck). With everything at workshop temp (72 degrees F), there was around .010" of piston rock as well, meaning that under load the quench gap might have even been less.

There is evidence of the Pistons hitting the head, but not hard.

Everything I have read is that running a piston to head of less than around .036" with steel rods is asking for trouble... Is this also the case with BP engines? This engine had 30 hard racing hours on it and did not grenade... And frankly, I am surprised.

Engine has a stock crank in a nb8b bottom end bored to 85.5mm with Supertech 11:1 pistons.
zossy1 is offline  
Old 12-21-2014, 05:07 PM
Senior Member
iTrader: (1)
Join Date: Jun 2006
Location: Sweden
Posts: 1,378
Total Cats: 60

The "safe" guideline I've heard is 0.1mm per 1000rpm. Run a little closer if you once you have successfull test data.

The rods on BPs is no different than other engines.
NiklasFalk is offline  
Old 12-21-2014, 06:31 PM
Junior Member
Thread Starter
iTrader: (4)
Join Date: Sep 2010
Location: Australia
Posts: 167
Total Cats: 12

So my math makes that around .035" at 9,000rpm (for safety).

Pretty much in my ballpark. The figures I was working on were general figures often used on engines with less main bearing support of the crank than a BP... Thought that may have made a difference.
zossy1 is offline  
Related Topics
Thread Starter
Last Post
Build Threads
03-16-2019 08:07 PM
Supercharger Discussion
09-12-2016 08:17 PM
Suspension, Brakes, Drivetrain
04-21-2016 03:00 PM
09-28-2015 11:18 AM

Thread Tools
Search this Thread
Quick Reply: Piston to Head Gap

Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.