Differnece between an RH4 and a RH10 head on an 850

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May 4, 2005
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Can anyone explain the difference between the RH4 and RH10 head. From what I have read on this forum from time to time was that the RH10 head is sought after.

I currently have an RH10 head, which has been rebuilt twice in the last 3 years, and is still causing me to have oil issues mainly leaking into the cylinder. The previous owner had ground down the valves so much that they were an 1/8 of an inch below the head surface, the fix was to install the largest oversize racing valves.

I am now considering purchasing a used head and rebuilding it. More than likely I will probably end up with an RH4. Therefore my question on what is the difference between the two.
The RH10 head has 30 mm intake ports (the carb manifolds are 32 mm at the carb and 30 at the head), while the RH4 head has full 32 mm ports. The theory on the RH10 head was the narrower ports increased fuel velocity and improved throttle response.

If you change to an RH4 head, make sure you use the 32mm to 32mm intake manifolds.

The RH4 was found on '73 850's, this was changed to the RH10 on the '74's, then back to the RH4 in '75. I have run both on my Interstate "mule", but see little "seat of the pants" difference.
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