What head is this?

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pdl999

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I have a 750 Commando built by Norvil in UK in 2002. The head is stamped RH4S. Now all the lists I have seen say that RH4 is an 850 head but the chambers on mine are a perfect fit to a 750 head gasket.
Perhaps this question is relevant to the recent long conversations about where did Norvil get all their heads from to build their "new" bikes?
 
Interesting. The S suffix is supposed to indicate a 750 head with 850 valve guides, but, as you point out, the RH4 is supposed to be a 1973 850 head to start with, so an S suffix would be meaningless. You mention that the bike is a 750. Is the bolt pattern on the head 750 or 850? I have seen (and built) 750s using cylinders and heads with 850 bolt pattern, so maybe Les was doing that in 2002? Does the cylinder have through bolts like the 850 cylinders? Someone was making iron 750 cylinders with the 850 through bolt design back then, and I think Norvil might have carried them. I always assumed they were 750 bolt pattern, but never had one in hand to check. Maybe they also did them with 750 bore and 850 bolt pattern. I've also seen 750s built for racing by sleeving an 850 cylinder to 750 bore size and using 850 heads, so that's also a possibility. Plenty of possibilities, but none of them explain why the head would be stamped RH4S.

Ken
 
pdl999 said:
Ken
This one has the 850 bolt pattern
Click to expand...

Even more interesting. Sounds more like you have an 850 engine, not a 750. On the other hand, it could still be an 850 cylinder sleeved down to 750. I guess you would have to pull the head to be sure.

Ken
 
kraakevik said:
Ken

The 750 cylinders with through-bolts from that era used 750 heads--I bought one in England fifteen years ago that's still on my Combat



Tim Kraakevik
kraakevik@voyager.net
Click to expand...

Thanks, Tim. I've wondered about that ever since I first saw them advertised.

Ken
 
Even more interesting. Sounds more like you have an 850 engine, not a 750. On the other hand, it could still be an 850 cylinder sleeved down to 750. I guess you would have to pull the head to be sure.
Click to expand...

I had the head off a few days ago and it is definitely a 750 (I measured the bore as 73mm). Shame I put it back on before starting this thread as I can't now re-inspect what I have but here is a photo of the barrell.
What head is this?

Ken, in case I am being dumb can you confirm if I am right to say this is an 850 bolt pattern?
thanks
David
 
pdl999 said:
John
I used a composite gasket (the AN product sold by RGM)
David
Click to expand...

Hi
and by the looks of the rings on the barrel it was/is a 850 gasket, as the 750 bolt spacing would not fit!
You teasing us David?
JohnT
 
pdl999 said:
Ken, in case I am being dumb can you confirm if I am right to say this is an 850 bolt pattern?
thanks
David
Click to expand...

Definitely an 850 bolt pattern. That seems to mean that the head is indeed an RH4 850 head, because a 750 head couldn't have been bolted to this cylinder block. Still don't know why the S suffix was added to the head. I can't tell for sure from your picture, but it looks like maybe the cylinders were sleeved down to 73 mm.

Ken
 
This is really helpful guys as I think we are getting to the answer!
I confirm that this is 750. Apart from measuring the bore, If you lay a 750 gasket on top it fits, but an 850 gasket is way too big. Laying the gasket over the head, showed that only the 750 gasket fits the combustion chamber diameter.

Does sound like an 850 sleeved down to 750, but as for the head markings?? Could it be an 850 head where the combustion chamber is blank and has been machined to be a 750? Are they normally cast with blank chambers and then machined?
 
The cylinder is almost certainly a 750 through-bolt version with those dimensions and the handed cutouts on the pistons--I'd cast a vote for disregarding the head markings and considering this a 750 head on a 750 barrel--like the one on my Combat


Tim Kraakevik
kraakevik@voyager.net
 
The cylinder is almost certainly a 750 through-bolt version with those dimensions and the handed cutouts on the pistons--I'd cast a vote for disregarding the head markings and considering this a 750 head on a 750 barrel--like the one on my Combat
Click to expand...
I also bought a 750 'Thru bolt' barrel from Les Emery in November 2002. So now everyone tells me I have an 850 and some will not accept the explanation! Cannot explain your '850' head no though.
 
Surely if a 750 gasket fits, it is actually a 750 bolt pattern....and 750 bore spacing and size.

But the one thing we have not seen is the head? is the head as it left the factory or could it be that an RH4 was used and the combustion chamber and bolt hole pattern has been modified to fit a 73mm 750 barrel? Seems a lot of work to do really unless there is a good reason for it.

What size are the inlet ports, 32mm?, and carbs, 32mm?

Of course it could be said....if it is working well enough does it really matter until you strip it next time?

But it seems like you want to know what you have got, right?
 
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