.030 Head Gaskets in the HO 103's - Road Glide Forums
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post #1 of 16 (permalink) Old 03-17-2016, 12:18 PM Thread Starter
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.030 Head Gaskets in the HO 103's

How much compression gain is realized from using the .030 head gaskets in a HO 103? The stock compression is supposed to be 9.7 to 1. The old compression ratio was 9.5 to 1 and most results show a .2 to .3 compression ratio increase. I haven't found any specifics for the HO 103's. I'm in Denver so all of the compression numbers are a little skewed to start with.
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post #2 of 16 (permalink) Old 03-18-2016, 07:49 AM
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Your figures are probably pretty close, .25 is the norm. The real value of a .030" HG is the tighter squish = better burn.
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post #3 of 16 (permalink) Old 03-18-2016, 09:55 AM
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When you pull your heads, check how for down in the hole each piston is. Put piston at top dead center and take a straight edge across the top of each cylinder and take a feeler gauge and measure the distance between the piston and the straight edge. Each side could be a little different. Usually not much though.This is called measuring the deck height. The volume of how far down in the hole the pistons are and the volume of the combustion chamber of the heads will give you how many cc's your total combustion chamber is. This now can be figured into what size head gasket can be used to get the compression ratio you desire. I will look for the conversion of cc's to compression ratios and post up again. Most shoot for around 83 t0 85 cc's.


Here you go: http://www.bigboyzheadporting.com/TwinCamComp.htm

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post #4 of 16 (permalink) Old 03-18-2016, 10:03 AM
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You can see if you play with the numbers on Big Boyz site things change quickly. Such as if you heads cc at 83 instead of 85 it increases compression ratio very quickly. Also when you measure deck height and enter these numbers the effects are pretty dramatic.

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post #5 of 16 (permalink) Old 03-18-2016, 11:29 AM
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The reason the 103 HO engine has more compression than the earlier 103 engine is that HD used a different cam. The cam will change the compression ratio more than a head gasket will.
The increase of compression with a .030 head gasket isn't worth it IMO why tear down the heads and do all that work for such little gain in performance. Now if you had to tear down the top end to do a R&R or you were putting in a big bore kit then it makes sense to use the .030 head gasket. JMO


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post #6 of 16 (permalink) Old 03-18-2016, 01:32 PM Thread Starter
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Thanks for the responses. I'm not ripping into the motor. I did it last year. My buddy and I were discussing the best build for him. He'll probably do the same thing I did. Mild port/polish, valve springs, cam and pushrods. The question is really will the thinner head gasket help his performance? I say it can't hurt.

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post #7 of 16 (permalink) Old 03-18-2016, 03:46 PM
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Quote:
Originally Posted by ironmark View Post
The reason the 103 HO engine has more compression than the earlier 103 engine is that HD used a different cam. The cam will change the compression ratio more than a head gasket will.
The increase of compression with a .030 head gasket isn't worth it IMO why tear down the heads and do all that work for such little gain in performance. Now if you had to tear down the top end to do a R&R or you were putting in a big bore kit then it makes sense to use the .030 head gasket. JMO
The cam has absolutely zero effect on compression!!!
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post #8 of 16 (permalink) Old 03-18-2016, 06:35 PM
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Quote:
Originally Posted by ironmark View Post
The reason the 103 HO engine has more compression than the earlier 103 engine is that HD used a different cam. The cam will change the compression ratio more than a head gasket will.
The increase of compression with a .030 head gasket isn't worth it IMO why tear down the heads and do all that work for such little gain in performance. Now if you had to tear down the top end to do a R&R or you were putting in a big bore kit then it makes sense to use the .030 head gasket. JMO
Ironmark, I want to hear the explanation as to why a cam would change the compression of of a build. I understand the term corrected compression but as for our compression which is the determined by the cc's of the combustion chamber which is the size of bowl in the heads and the deck height.

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post #9 of 16 (permalink) Old 03-18-2016, 06:37 PM
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The cam has absolutely zero effect on compression!!!
He may have it confused with cranking compression which is determined by the cam lobe and overlap.

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post #10 of 16 (permalink) Old 03-18-2016, 06:51 PM
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Quote:
Originally Posted by Darreldanger View Post
The cam has absolutely zero effect on compression!!!
Quote:
Originally Posted by Smarty View Post
He may have it confused with cranking compression which is determined by the cam lobe and overlap.

Yes......

Camshaft does not affect static compression, but will affect dynamic(cranking)
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