cylinderhead compatibility

Joined
May 6, 2006
Messages
18
I have a 74 MKII, it needs a new head, can I use any other heads besides the RH10 that's on it now??
 

Ron L

VIP MEMBER
Joined
Feb 27, 2004
Messages
3,107
Country flag
dmellish,
You can use an RH4 (1973 or 1975 850) head but you need the 32mm to 32mm intake manifolds (P/N 062711) due to the larger intake ports. The 750 heads will not fit.
 
Joined
May 19, 2006
Messages
1,612
Country flag
You can fit any 850 head. I have never seen an RH9 which is listed as "High Performance" for 1974 It had allegedly 10:1 compression so would have been skimmed.

With any replacement head, you should check rocker angle / pushrod length, valve to piston clearance and that the springs are not becoming coil-bound.

My 850Mk 111 has a 32mm 8.5:1 RH4 and these seem to be the most common as are the straight 32mm inlet manifolds to go with them.

The most important thing when buying is to make sure you get one that hasn't been "got at" by an amateur tuner. They are almost invariably worse than standard and you can't put the metal back.
 

Ron L

VIP MEMBER
Joined
Feb 27, 2004
Messages
3,107
Country flag
If it is more compression you want, mill .075 inch from your RH10 and voila! you have an RH9. In 30 years I have never seen a "factory" RH9.

My experience is the RH10 gives a slightly better mid-range than the RH4. This was testing on my 850 Interstate with 2S (combat) grind cam and a 22-tooth countershaft sprocket. I attribute that to the smaller intake tract which probably fills the cylinder a little better.
 
Joined
May 19, 2006
Messages
1,612
Country flag
Ron L said:
My experience is the RH10 gives a slightly better mid-range than the RH4. This was testing on my 850 Interstate with 2S (combat) grind cam and a 22-tooth countershaft sprocket. I attribute that to the smaller intake tract which probably fills the cylinder a little better.

I believe that you're probably correct here although I have never had the opportunity to perform a back-to-back test. I know some tuners are not keen on big valves on the basis that the smaller inlet gives higher gas velocities and better cylinder filling.

I'm stuck with the 32mm ports of course which reinforces my suggestion that you're best finding a head with as much metal as possible and certainly not one that has been "ported".

RH4 heads are much more common in Europe. Did they always go with the "black box" models ?

Norton had a bit of a habit of making changes which conflicted with their previous practice.
 
Top