750 heads

Snakepit · Apr 5, 2012

What are the differences between the standard head and the combat head? Does the combat head flow better? Are the valve train parts the same?

htown16 · Apr 5, 2012

Combat head was milled to raise the compression ratio. Otherwise they are the same.

comnoz · Apr 5, 2012

Snakepit said:
What are the differences between the standard head and the combat head? Does the combat head flow better? Are the valve train parts the same?

The combat head has the intake ports opened up to 32mm from the standard 29-30. They don't flow any better however. The valve train is the same. The surface has been milled to raise the compression ratio. Jim

swooshdave · Apr 5, 2012

comnoz said:
Snakepit said:

What are the differences between the standard head and the combat head? Does the combat head flow better? Are the valve train parts the same?

Click to expand...

The combat head has the intake ports opened up to 32mm from the standard 29-30. They don't flow any better however. The valve train is the same. The surface has been milled to raise the compression ratio. Jim

Despite Jim's testing I can verify that they make 100 more horsepower, breath fire and spit out brimstone and send women and children running for cover. Other than that they are just like a stock head. :mrgreen:

bill · Apr 5, 2012

some will say that some of the combat heads were ruined from the factory with a poor port job.

comnoz said:
The combat head has the intake ports opened up to 32mm from the standard 29-30. They don't flow any better however. The valve train is the same. The surface has been milled to raise the compression ratio. Jim

conkers · Apr 5, 2012

bill said:
some will say that some of the combat heads were ruined from the factory with a poor port job.

comnoz said:

The combat head has the intake ports opened up to 32mm from the standard 29-30. They don't flow any better however. The valve train is the same. The surface has been milled to raise the compression ratio. Jim

Click to expand...

It was 40 years ago.....!
They thought they were doing the right thing back then.....

;-)

Mike

comnoz · Apr 5, 2012

conkers said:
bill said:

some will say that some of the combat heads were ruined from the factory with a poor port job.

comnoz said:

The combat head has the intake ports opened up to 32mm from the standard 29-30. They don't flow any better however. The valve train is the same. The surface has been milled to raise the compression ratio. Jim

Click to expand...

Click to expand...

It was 40 years ago.....!
They thought they were doing the right thing back then.....

;-)

Mike

I talked to an old factory guy one time who said they only did it because the customers demanded it. Everybody else was using big ports so Norton had to keep up with the Jones. Jim

Hobot · Apr 6, 2012

Yes the early Combat heads were hand hogged out, later refined narrower by casting I think. I got distinctly more pleasure with a standard small port head than the spiffed up 6 mm valve CHO head. Hope its ok for a 920 cc though. Here's more detail by DynoDave.
http://atlanticgreen.com/nhth.htm

marinatlas · Apr 6, 2012

Hi there, to keep the infos growing , what are the difference between the combat , the RH5 and the RH6 , apart the compression ratio which differs, are they better flowing ?? cause they all are 32MM.....?

bill · Apr 6, 2012

what was the saying, when in doubt hog it out :lol:

comnoz said:
I talked to an old factory guy one time who said they only did it because the customers demanded it. Everybody else was using big ports so Norton had to keep up with the Jones. Jim

swooshdave · Apr 6, 2012

marinatlas said:
Hi there, to keep the infos growing , what are the difference between the combat , the RH5 and the RH6 , apart the compression ratio which differs, are they better flowing ?? cause they all are 32MM.....?

Jim has a nice thread on this forum about head flow testing. You might want to check it out.

comnoz · Apr 6, 2012

marinatlas said:
Hi there, to keep the infos growing , what are the difference between the combat , the RH5 and the RH6 , apart the compression ratio which differs, are they better flowing ?? cause they all are 32MM.....?

On a flowbench the 32mm heads will flow a little more than the 30 mm heads. However any gains due to the higher flow are lost because of the lower velocity. On a dyno they come out about the same with the 30 mm port showing a little wider powerband.

What they should have done was leave the port at 30 mm and installed a larger intake valve. The valve size is the limiting factor. Jim

rvich · Apr 6, 2012

Jim,
Someday I will decide to rebuild my Combat motor. To improve flow is it realistic to take a standard 750 head with the 30mm ports and install larger intake valves and then mill it? Or should I just save up for a Fullauto head if I am going to worry about this stuff?

Russ

comnoz · Apr 6, 2012

rvich said:
Jim,
Someday I will decide to rebuild my Combat motor. To improve flow is it realistic to take a standard 750 head with the 30mm ports and install larger intake valves and then mill it? Or should I just save up for a Fullauto head if I am going to worry about this stuff?

Russ

To install larger valves you have to re-angle the guides. It is a lot of work and not cheap to pay someone to do it right. I would vote for the fullauto head which will run with most big valve heads right out of the box. Then you can always send it to me for a big valve conversion.

Jim

rvich · Apr 6, 2012

It is sort of a twisted world we live in where you can seldom have your cake and eat it too. If I want to improve the flow of my Combat motor, all I need to do is get rid of the Combat head. But then, I really don't have a Combat motor anymore! Sheesh.

Some days I wonder why I use stock parts at all. Except of course I already have them!

Russ

dynodave · Apr 6, 2012

I doubt any one has measured their head....I have never seen a stock 30mm 750 head. They all seem to measure 28-28.5mm ports or combat @32. I'm not 100% positive about the quite rare special 73 750 heads and it has been quite a while since I've had my hands on them. I own about 25-30 heads but only one 73.
The 74 850 head is 30mm designated as RH10. As far as I know it is the only 30mm.

worntorn · Apr 6, 2012

Is it the same Combat like crude porting job on the 32 mm 75 mk111 850 heads(marked RH4 as i recall) or did the porting get better with practice?

Glen

ILLF8ED · Apr 6, 2012

If not the head that makes a combat engine faster than previous Commandos, was it the camshaft? Not much else is that different....don't say 19T sprocket. I've owned two '72 combats and one '73 750 with RH6 head. Both combats are stronger than the '73 high compression 750. The '73 has the "standard" camshaft.

comnoz · Apr 6, 2012

illf8ed said:
If not the head that makes a combat engine faster than previous Commandos, was it the camshaft? Not much else is that different....don't say 19T sprocket. I've owned two '72 combats and one '73 750 with RH6 head. Both combats are stronger than the '73 high compression 750. The '73 has the "standard" camshaft.

Compression and cam. The 73 high compression is not combat spec compression. Jim

rvich · Apr 6, 2012

If you were going to put Amal carbs on a 750 Fullauto head, what do you need for manifolds? For that matter what do you use on the 850 heads? Are they 30mm?

750 heads

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