View Full Version : 8:1 or 9.5:1 Compression,compression...

11-05-2004, 06:55 PM
Hey guys,

I am having to have new jugs and pistons installed on my 50 FL. I kept loosing compression after installing new heads. Come to find out the guy that had it before me never rebuilt the top end when it needed it. He just install .20 over rings in it, and sold to some poor sap. That sap was me. Anyway.. the upper half of my motor is 74 ci, 77 Shovel, rightside case is Pan, left side case is Shovel. I'm going to with new 80 ci jugs and pistons. Any opinions on going with 8:1 or 9.5:1 compression pistons? I'm thinking 9.5:1. More horsepower for the same price as the 8:1.


11-05-2004, 07:11 PM
Howdy man

True 9.5 will give you more power but remember that it's harder to kick over & will run hotter.

The choice is really yours, I have around 9.0 on my panhead without any problems.

Hope this helps 1beer

11-05-2004, 07:25 PM
Wides got y'a covered. I vote for 9:5

drunks bottomsup


11-05-2004, 07:50 PM
one thought...

as you already know, the hemi head/domed piston design of the shovels and pans is dated by today's "squish" technology. putting pop-ups in there to raise the compression will further block the flame path of a single plug head design.

you might want to consider either having the stock heads drilled for an extra plug, or what would be better (at least imho) get ahold of a set of std dual plug heads. the std's come with hardened seats and they have a 3-bolt ehaust flane that keeps the exhaust quiet.

just a thought....


11-06-2004, 12:48 PM
Thanks for the reply guys.

Junior, I'm with you on the dual plug heads. I just can't float the bills for it yet.

I think I am going to go with the 9.5:1. Something about more horsepower... it just makes me all warm inside. :huh:

11-07-2004, 03:43 AM
The 80 inch jugs will bolt up to your 74 inch heads, however, the pistons for the 80 inch will not work. Your 74 stroke is 3 31/32". The 80 inch stroke is 4 1/4". Your compression will be on the order of 5 to 1 or so, because the compression height on the pistons are different. If you are using NEW 80 inch jugs the pistons you need are 3 7/16'' + .060" 74 pistons....like a 74 engine bored 60 thousands. Pistons for the 74 up to 100 thousands over used to be available but are hard to come by now. A lot of the distributors don't handle these big overbores anymore.
If you eventually decide to dual plug your shovel heads, drop me a message. I do that fairly regularly and with my dual plug setup you don't have to have a special spark plug socket or drop the pushrod tubes to remove the right side plugs. I also install a steel insert in the head to avoid stripping out the short threads on the right side.

11-07-2004, 05:45 PM
Hey Soretailcat,

Thanks for the advice! That is definatley something to check on.

I am having the work done by a very reputable shop. The guy doing it has been wrenching on Harleys for 30 years (I hope he knows what he's doing) :( . I would imagine he has thought of that but, I will ask him anyway.

I am getting a warranty with the work and, they have never screwed me in the past. I hope there isn't a first time <_< . At least with the warranty, if something is not right, it will be up to them to make it right.

I'm keeping my fingers crossed!

11-08-2004, 01:04 AM
Glad that you have a reputable shop in your area that you have confidence in. By all means use them if they can perform the work you require. If I can be of service, let me know.