Combat or RH6S Head

S

Stephen Hill

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I have the head off my 1972 Combat. The head is an RH6S in pretty good shape. Might need a couple of guides, not sure yet.
I also have a Combat head that has been totally done: new guides, valves, etc.
Which head should I put back on the bike: the RH6S head or the Combat head?

Somewhat related, a friend also has a 1972 Combat with an RH6S head and he thinks his bike came from the factory that way. Thoughts?
 
RH6 is the high (but not as high as the Combat) compression post-Combat (32mm intake port) head.

Combat heads generally have a large 'C' stamped in the area under the head steady.

If your friend's Commando had an RH6 head from the factory then it isn't Combat specification and what camshaft does it have?
The Combat series ended from engine number 211110 so any "1972" Commando with a later serial number is unlikely to be an original Combat and also unlikely to have the Combat camshaft.
 
So what is the difference between a combat "C" stamped head and the RH6?
Compression height I am guessing?? Did these heads have 0.040" thou removed ?
 
CanukNortonNut said:
So what is the difference between a combat "C" stamped head and the RH6?
Compression height I am guessing?? Did these heads have 0.040" thou removed ?
Click to expand...

Approximately half that amount.
Combat or RH6S Head

"RH6...as RH5 less .020" off face..."
 
Cast ieon guides are not good when a snall piece of iron comes off and changes the shapr of the valve seat.
 
SH quote: "I have the head off my 1972 Combat. The head is an RH6S in pretty good shape. Might need a couple of guides, not sure yet.
I also have a Combat head that has been totally done: new guides, valves, etc.
Which head should I put back on the bike: the RH6S head or the Combat head?"

Stephan,
what cam do you have? I am assuming a 2S???
 
L.A.B. said:
Approximately half that amount.
Combat or RH6S Head

"RH6...as RH5 less .020" off face..."
Click to expand...
L.A.B.,

Looks like I was writing as you posted. I see some part number differences. I'm happy to update mind and send it to you so you can post it. Or, I'm happy to send you the Excel file and you can update it.

Also, I've been wondering about Combat heads. They have the "C" but no RH number. We always say they have been skimmed, but there were no street 750 heads with 32mm intakes that I know of to skim? So, is the "C" just another way of saying RH3. In other words, were there RH3 heads that were not skimmed? The same question comes up about RH6.
 
You have a Combat. I’d use the Combat head.

If the fuel in your area can handle it, I’d leave it stock. If it can’t, I’d play with head / base gasket options to lower the CR a bit.

A .020” copper base gasket from JS would put the Combat head back to the same CR as the RH6 according to the above info.

Seems we’re luckier than some here in the U.K., I run my 11:1cr 920 on pump fuel without issue.
 
Frame number 203629. Has a disc brake. From measuring the cam lift I am pretty confident it has the 2S cam. I think we can say it left the factory as a Combat.

I guess the question is which head should I use: Combat or RH6S?

I have been led to believe that the Combat heads have been ported out too much and produce less torque in the lower rpms?

Other than the .020 vs. .040n skim, what differences can we point to between the combat head and the RH6S head?
 
Stephen Hill said:
Frame number 203629. Has a disc brake. From measuring the cam lift I am pretty confident it has the 2S cam. I think we can say it left the factory as a Combat.

I guess the question is which head should I use: Combat or RH6S?

I have been led to believe that the Combat heads have been ported out too much and produce less torque in the lower rpms?

Other than the .020 vs. .040n skim, what differences can we point to between the combat head and the RH6S head?
Click to expand...
AFAIK that ‘over porting’ claim is related to the fact the ports are 32mm vs 30mm. So in that regard, your Combat head and RH6 are the same.

Dyno Dave and Jim Comstock have both cited how they agree that 30mm is better for the street, even with an 850. But you’ve got what you’ve got, and the bigger ports do suit the bigger cam for higher rpm ‘fun’.

I guess you could always try JS Motorsports intake sleeves if you wanted to see what 30mm ports do.
 
Last edited:
marshg246 said:
They have the "C" but no RH number. We always say they have been skimmed, but there were no street 750 heads with 32mm intakes that I know of to skim?
Click to expand...
The Combat head was the standard head skimmed and ported to 32mm as far as I'm aware, so there was no 32mm intake port "street" head before the Combat.

marshg246 said:
So, is the "C" just another way of saying RH3. In other words, were there RH3 heads that were not skimmed?
Click to expand...

As far as I know, the Combat head was (eventually?) designated 'RH3' when things started to get complicated and the factory began stamping the RH number on heads at the end of '72 after the introduction of the 750 RH5 and RH6 heads and then 850 RH4 etc., but not marked as such beforehand as far as I'm aware, only 'C' on the Combat head and the standard comp/30mm head became the RH1.

No unskimmed RH3 heads as far as I'm aware as RH3 is the Combat specification head, RH6 is the -0.020", 32mm intake port head and RH5 low compression 32mm intake head, according to the available information.
The compression ratios could vary according to the thickness of the head gasket and whether a base gasket was used, for instance, according to the factory service release information, the Combat compression ratio was only the full 10:1 when no base gasket was used with a .030" head gasket.
 

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