Author Topic: Small valves, high comp  (Read 1009 times)

czakky82

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Small valves, high comp
« on: December 04, 2022, 03:38:40 PM »
I’ve got this bitsa motor that I’m going to build. As far as I can tell it has Lemans 1 barrels and pistons with small valve (round) heads.
It’s also got a different cam.

Any reason why the pistons won’t work with the heads?

I’ll obviously check piston to valve clearance.

Thanks
Bill

Offline Frenchfrog

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Re: Small valves, high comp
« Reply #1 on: December 04, 2022, 05:23:42 PM »
The small valve head has a smaller and shallower combustion chamber too....chances are that with those higher compression pistons you will not have enough clearance.Some folk did the opposite ( LM heads on low dome standard pistons) and ended up with much lower compression.If the set up you are suggesting does cleat the compression may well be astronomic!
I'd be very interested what you find !

Offline John Croucher

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Re: Small valves, high comp
« Reply #2 on: December 04, 2022, 07:41:49 PM »
I put v11 sport pistons in a California engine.  The squish band is/was to narrow on the Sport pistons due to the smaller combustion chamber on the Stock head. I machined the piston dome diameter smaller.

I got over 200 psi compression.  Lower duration cam and smaller valves are stock.  2003 hydro engine. 

czakky82

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Re: Small valves, high comp
« Reply #3 on: December 05, 2022, 06:36:42 AM »
That’s good info John. I might have to do a bit better f machining off these pistons.

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Re: Small valves, high comp
« Reply #3 on: December 05, 2022, 06:36:42 AM »

Offline guzzista

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Re: Small valves, high comp
« Reply #4 on: December 05, 2022, 06:51:35 PM »
I put v11 sport pistons in a California engine.  The squish band is/was too narrow on the Sport pistons due to the smaller combustion chamber on the Stock head. I machined the piston dome diameter smaller.

I got over 200 psi compression.  Lower duration cam and smaller valves are stock.  2003 hydro engine. 
How many thousandths did you cut off the dome to fit ?
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Offline John Croucher

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Re: Small valves, high comp
« Reply #5 on: December 06, 2022, 08:11:02 AM »
How many thousandths did you cut off the dome to fit ?

The specifications for the squish band very.  There is a lot of information on the www regarding how t calculate and adjust. 

You will need to run high octane fuel and may need to retard the ignition to avoid spark knock. 

Offline Turin

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Re: Small valves, high comp
« Reply #6 on: December 06, 2022, 10:46:45 PM »
I looked into raising the domes on a set of small valve heads. It ended up cheaper tracking down a set of real 850 lemans heads. ( I have a 77-78 850 LeMans Engine that I plan on building. it only had one remaining small valve head on it and the other piston shows signs of eating a valve. )

The difference between the small and medium valve heads are pretty minor and are not immediately apparent to the naked eye. If the heads are off, I highly recommend some port work. I have an 850T that is mildly hopped up. The work on my T heads was minimal and made the bike pretty happy.

Maybe you could get away with port work, and thicker head and base gaskets. We used thin copper head and base gaskets on my 850 T when we set the squish to .040




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« Last Edit: December 06, 2022, 10:51:05 PM by Turin »
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Offline Frenchfrog

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Re: Small valves, high comp
« Reply #7 on: December 07, 2022, 03:53:40 AM »
If you need to give a bit more clearance ,HMB in Germany make and sell these spacers that between the barrel and crankcase.https://hmb-guzzi.de/Cylinder-Spacer-for-round-engines

Offline John Croucher

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Re: Small valves, high comp
« Reply #8 on: December 07, 2022, 08:56:42 AM »
That’s good info John. I might have to do a bit better f machining off these pistons.

The 1100 sport piston is much taller than the one you pictured.  Here is a picture of the piston I installed.  Since it has more material/heavier, I removed some weight from the under side skirt and machining the top squish band.  I am within a couple of grams of the oem piston. 





This is the compression reading at first assembly and no start up.  I have not checked it since riding several miles.    Very high.  I need to have the engine dyno-tuned mapped.  It is a f.i. engine.  It runs significantly better. 




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