head clearance effect?

[moontz]

Checked my head clearance the other day on a mac 67 and was .020 (used plastigage and micrometer) CMDI recommends .010-.012

Right now my plugs look good (slightly pulled out and good color) and running fine. What would I potentially gain by moving to recommended head clearance? I dont have time to run this weekend or would test but curious if there are any proven answers?

thanks,

Brian

 

[Terry Keeley]

Checked my head clearance the other day on a mac 67 and was .020 (used plastigage and micrometer) CMDI recommends .010-.012
Right now my plugs look good (slightly pulled out and good color) and running fine. What would I potentially gain by moving to recommended head clearance? I dont have time to run this weekend or would test but curious if there are any proven answers?

thanks,

Brian

More bottom and mid fur shur!

 

[Steve Wilson]

To much gap=less compression and less displacement..take it to the recomended specs and I bet you see a good diferance in it

 

[moontz]

lowest I can get with no shims is .014 (according to plastigage-yellow)

bought it used so possibly the guy i bought from could have sanded the button down from detonation or something. i dont know but doubt a mac 67 from factory would come with such high gap. or gap naturally increases through wear?

also, does the shim act as a gasket as well? can I run no shims without issues?

 

[Terry Keeley]

lowest I can get with no shims is .014 (according to plastigage-yellow)
bought it used so possibly the guy i bought from could have sanded the button down from detonation or something. i dont know but doubt a mac 67 from factory would come with such high gap. or gap naturally increases through wear?

also, does the shim act as a gasket as well? can I run no shims without issues?

Would be best to trim the head in a lathe...

 

[Steve Wilson]

lowest I can get with no shims is .014 (according to plastigage-yellow)
bought it used so possibly the guy i bought from could have sanded the button down from detonation or something. i dont know but doubt a mac 67 from factory would come with such high gap. or gap naturally increases through wear?

also, does the shim act as a gasket as well? can I run no shims without issues?

not sure about the shim acting as a gasket I have seen engines with them and without. perhaps it depends on the engine manufacturer...but i do agree that the engine you have may have been modified and you may need to modify somemore to get it back...