cilinder heads compression ratio

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Feb 4, 2008
Has anybody informations about the 1974 RH9 heads with 10.5 compression for the 850 modells ? Is it a milled standard head or has norton used domed pistons ? I want to rise my 8.5:1 up to 10+, i use 98octane fuel, so i haven't to worry about knocking !

I have never heard of Norton using anything other than a flat top piston in 850's. It was Norton's practice to mill the head to achieve a compression increase.

Although I have never seen an RH9 head, it purportedly had 32mm intake ports so was probably a milled version of the RH4.
Information regarding the RH9 and other head compression ratios can be found in Roy Bacon's "Norton Twin Restoration" book.
The heads are listed along with their compression ratios, so the ratio increase as Ron has said, will be the result of the head being machined, and not due to domed pistons.
The Norton factory stage 1 tuning notes, gives information on how to modify a standard *063830 compression head by machining 1.91mm from the gasket face in order to increase the ratio to 10:1 , along with the optimum port shaping dimensions, as well as information on shortening the push rods and cutting the necessary extra valve clearance in the pistons.

*That 063830 number doesn't appear to correspond to any standard (8.5:1 RH4/RH10) 850 head part number?
P/N 063830 is listed in my '75 Mk3 parts book as "cylinder head with guides", which would make it an RH4 part number.

Interestingly, my 750/850 parts book (1973 era?) list the 850 head as P/N 064038 which DynoDave's list does not show, but I would think should also be an RH4 head.
Mill the head, that sounds good ! If the pistons have enough thickness to make some valve pockets...or i do the job with custom pistons and internal balanced crank. I have to disassemble my engine in any case ( camshaft washer is attached the wrong way ).
Thanks everybody for your help.

If the pistons are stock Hepolites there is plenty of room to make sufficient valve pockets. My 850 cafe racer is milled flush with the first fin and has oversize valves with a 2S cam grind and there was enough meat to provide the necessary clearance.
When my '74 850 engine was rebuilt the head was milled 0.060" and the cam was replaced with a Norris SS cam with .358" lift for intake and exhaust. Valve reliefs were cut into the new Hepolites. I don't remember the size of the reliefs, but the pistons looked much like the pistons from a high compression 750.

The Hepalites have plently of material to allow for the valve reliefs.

A healthy bump in compression and a bigger cam really bring the low compression 850 to life. The bike snaps up to the redline quickly and the power continues to build all the way to the redline.
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