Head Clearance: Finding the sweet spot

[Jeff Torgalski]

Hey everyone,

I have 3 .21 K&B engines, and on two of them I have changed the head clearance. In the oldest one, which was manufactured around 1980, I removed the shims and ran it a few times (in very cold weather on 15% nitro). It was pretty weak, to say the least. It sputtered pretty bad on the low end.

I suspect that I set the head clearance too low.

I have two questions:

What happens when the head clearance is set too high?

How do you know that you set your head clearance too low?

Any takers?

Thanks!

JT

P.S. I know my Nitro was way too low, but I'm just trying to understand the steps in troubleshooting the problem so I can better perfect a remedy.

 

[TimD]

Jeff,

If your head clearance is too high the motor won't make much power. Particularly with 21's. Compression = power.

If the clearance is too tight the piston hits the head

When I build 21's I aim for 6 thou absolute min. and generally don't go any higher than 12 thou max. 8 - 10 thou is a good figure for k&B's I'm told.

Raising head clearance to reduce detonation is common practice but the better method is to fit a head with a larger volume bowl and maintain the same clearance. A couple of different head buttons can be handy for this. Alternatively you can buy software to calculate a trapped Compression ratio and it will give you the dimensions needed for the head button.

Hope this helps.

Tim.