[GMR]

Could someone explain the significance of head gasket bore size as it pertains to a street motor please (I understand it needs to be larger than the cylinder bore). For example, a particular head gasket for a sbc 265-400 with a compressed thickness of .040 can be had in bore size 4.060 or 4.165. If I have a 350 that is .030 over, which gasket should I use and why?

[mrdreex]

Originally Posted by GMR

Could someone explain the significance of head gasket bore size as it pertains to a street motor please (I understand it needs to be larger than the cylinder bore). For example, a particular head gasket for a sbc 265-400 with a compressed thickness of .040 can be had in bore size 4.060 or 4.165. If I have a 350 that is .030 over, which gasket should I use and why?

One thing that jumps right out is the fact you will have slightly higher (~.1) compression with the smaller bore size gasket.

[techinspector1]

Originally Posted by GMR

Could someone explain the significance of head gasket bore size as it pertains to a street motor please (I understand it needs to be larger than the cylinder bore). For example, a particular head gasket for a sbc 265-400 with a compressed thickness of .040 can be had in bore size 4.060 or 4.165. If I have a 350 that is .030 over, which gasket should I use and why?

What I do is measure the bore at the very top, where the machine shop has added a chamfer to facilitate starting the rings into the cylinder on assembly. I'll use a gasket with at least that diameter or larger. I don't want any gasket overhang into the bore to act as a hot spot to begin detonation. But I also don't want the gasket diameter so large that I create a "shelf" cavity between the bore and the head. Small spaces like that are a "dead zone" that will not light on ignition and will contribute to dirty emissions on a motor that has to conform to State requirements.

I might be all wet, but that's my take on it.

[GMR]

Thanks fellas.

[Double_v23]

The most important thing is to make sure the gasket bore is larger than the engine bore. A 4.060 gasket is used for anything up to a +.040 bore on a 350. The 4.165 you specified is for use on 400's with a bore of up to 4.155. (which is a .030 over 400)

If you have a gasket that is smaller than the engine bore the gasket will fail. The best idea is to find a gasket that is the right thickness and has a bore that is the closest to the actual bore of the engine without being smaller then the cylinder.

[OHD]

The 4.060 gasket

[GMR]

Thanks Double_v23 and OHD.

[engineczar]

Also measure the cross section of the combustion chamber and how it sits in relation to the bore. If it is larger than the bore of the gasket or is offset from the bore then you may need to run a larger bore head gasket. I've seen some heads that the chamber does sit outboard of the bore due to either core shift or machining tolerances. Just something more to check.

[GMR]

Good tip engineczar, didn't know that could happen, thanks.