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Message started by hotrod on 02/25/17 at 12:41:18

Title: Compression
Post by hotrod on 02/25/17 at 12:41:18

No money but big plans. One day I would like to install the higher compression piston, but wondering if anyone ever had the head shaved to get more compression. How much to take off ? Danger of contact with the valves ? Where to have it done ? Thanks for any information.

Title: Re: Compression
Post by LANCER on 02/25/17 at 15:53:03

537A76767E692A1B0 wrote:

Hotrod, I had some work done on my engine some years back. The head was shaved .015" and the cylinder .025" as I remember, for a total of .040". I have had no issues with piston/valve contact, and that is with the High Compression Piston and a Performance Camshaft. There is plenty of room for the procedure.
That said, by removing .040" of material from the head and cylinder only raised the compression ratio about .3-.5, from 8.5:1 to maybe 9:1 at most, and it may be less than that. I do not consider "shaving" to be all that effective. You would likely not notice the difference when riding.
There are better things to spend you money on.
Focus on getting the jetting and tuning the carb done really well, and finding a good flowing muffler with moderate baffling. Those two things will have a noticeable effect on the engine. It will run better, have more power and sound better.

Where are you located Hotrod ?

Title: Re: Compression
Post by batman on 02/25/17 at 16:32:43

You might research intake tuning as a cheap way to better hp/torque in certain RPM ranges.

Title: Re: Compression
Post by hotrod on 02/25/17 at 16:36:07

Thanks for the info Lancer. I live near the east coast, Belhaven NC.

Title: Re: Compression
Post by Ruttly on 02/25/17 at 16:37:35

Milling head & cylinder ? Won't that throw your cam timing for a loop ? I would rather do a H/C piston. Did on my BSA but it's a push tube engine and made up the difference with valve adjusters .

Title: Re: Compression
Post by Armen on 02/25/17 at 17:14:36

We had a bit of a chat about this a while ago.
Judging by the work Lancer did, I'd be more inclined to just cut the barrel, and not the head. Merely shaving the head only raises compression. Cutting the barrel would raise compression, and tighten the squish band.
Just guessing, .035" might be a good number to shoot for.
Considering that Lance cut as much as he did off a barrel with a high compression piston, and the motor works, that tells me the squish band must be huge.
You might start out by dismembering the top end and measuring the actual compression ratio as well as the squish band.
As was noted by Ruttly, your cam timing will be retarded a bit when you cut the barrel. You can make some adjustment there by juggling the crank sprocket.
Anyone with a decent lathe and a bit of skill should be able to help you.

Title: Re: Compression
Post by Ruttly on 02/25/17 at 19:04:50

Armen I was hoping you would jump in. I always listen to your opinions. Knowledge is key and I gotta learn from those who have more of it than I do.

Title: Re: Compression
Post by LANCER on 02/26/17 at 04:45:54

0E29282830255C0 wrote:

Milling head & cylinder ? Won't that throw your cam timing for a loop ? I would rather do a H/C piston. Did on my BSA but it's a push tube engine and made up the difference with valve adjusters .

I agree, a High Compression Piston is the way to go. 10.5:1 adds a real kick. You can feel it.

Title: Re: Compression
Post by hotrod on 02/26/17 at 05:29:34

Thank you everyone. I guess I'll wait until the HP piston is available , and hope I have the money at that time. As for cam timing , in most overhead cam bikes that I have had, usually the cam sprocket mounting holes could be enlarged to rotate and get to the cam lobe center specs.

Title: Re: Compression
Post by Armen on 02/26/17 at 08:13:35

Hotrod,
Find the conversation about playing with the cam timing via the crank sprocket. The cam sprocket doesn't want to be twiddled.
I strongly suggest measuring the actual compression and the squish band before machining.
Looking at Lancer's numbers, I'd guess there is about 2mm (.080" squish AT LEAST). Way too much for a performance motor. Which of course, this isn't ;)