Combat or 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?

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Is the engine number the same?

I recall that Dyno Dave thought the port shape on the later 32 mm heads such as the 850 rh4 were much better than the shape of the Combat head 32mm ports, which by modern standards were " ruined".
Glen

worntorn said:

Is the engine number the same?

I recall that Dyno Dave thought the port shape on the later 32 mm heads such as the 850 rh4 were much better than the shape of the Combat head ports, which by modern standards were " ruined".
Glen

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That’s interesting Glen, can you share that ?

Here is Dave's info

The 71/72 head. This would be the 2nd series of commando head,. as a standard 71and 72 small port they have original port size of 28.5mm. In 72 the head would be marked with a "C" in the center top of the head to indicate the combat conversion was done. The combat RH3 was cut .042" to raise the compression. It had a 32 mm porting job. It appears the factory learned more about the porting business as time went on. First hand inspection of several "combat" porting jobs show how crudely (bad) they were done. Big.... yes. By today's standards these heads have been ruined. The "BIRCO" on the right and the casting number faintly visible on the left side are negatives (depressions in the surface).

And this

A 73&75 RH4 850 32 mm port head. The porting job out to 32 mm seems to have improved immensely over the combat. The 850 head types are marked above the right hand exhaust rocker cover. They show a much bigger head gasket pattern compared to all the earlier head styles.

Thanks Glen, that’s very interesting and I didn’t know that, kinda makes sense that they should learn and develop over time though.

worntorn said:

Here is Dave's info

The 71/72 head. This would be the 2nd series of commando head,. as a standard 71and 72 small port they have original port size of 28.5mm. In 72 the head would be marked with a "C" in the center top of the head to indicate the combat conversion was done. The combat RH3 was cut .042" to raise the compression. It had a 32 mm porting job. It appears the factory learned more about the porting business as time went on. First hand inspection of several "combat" porting jobs show how crudely (bad) they were done. Big.... yes. By today's standards these heads have been ruined. The "BIRCO" on the right and the casting number faintly visible on the left side are negatives (depressions in the surface).

Click to expand...

Interesting. Sounds like an original head with the compression raised by other means might make a Combat better. I wonder if the RH6 porting was the same as the RH3?

Maybe the rh 4 isn't so bad afterall.

My 850 with rh4 seemed to do pretty well against a nicely hotrodded 750, and that was a top end test, not really the intended forte of a MK3 850. That 750 has a Comstock ported head with the full auto style d exhaust ports added , hot cam and high compression.

I am going to try the JS sleeves though, got them here now.

I am looking for my Commando Road test Gold portfolio. It contains tests of several Combats. 1/4 mile et results were about the same as the earlier bikes, iirc. One would expect the Combat to be a lot quicker in this type of test as it is really a test of top end power. Maybe that crude port job Dave described is a bit of a hold back.

Glen

worntorn said:

Maybe the rh 4 isn't so bad afterall.

My 850 with rh4 seemed to do pretty well against a nicely hotrodded 750, and that was a top end test, not really the intended forte of a MK3 850. That 750 has a Comstock ported head with the full auto style d exhaust ports added , hot cam and high compression.

I am going to try the JS sleeves though, got them here now.

I am looking for my Commando Road test Gold portfolio. It contains tests of several Combats. 1/4 mile et results were about the same as the earlier bikes, iirc. One would expect the Combat to be a lot quicker in this type of test as it is really a test of top end power. Maybe that crude port job Dave described is a bit of a hold back.

Glen

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Glen,

Looking forward to that test, really has my interest.

Worntorn, the engine number is also 203629.

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...

Hi Stephen,
I have been playing with my 201881 combat roadster for 26 years. It has the “C” combat head as original, but some previous owner put a standard Commando cam in it. As the fuel here got worse I started having engine ping issues. Seems the combat head needs the combat cam profile as that profile has more valve open overlap bleeding off some of the cranking compression that caused the ping. Use the combat head.

ILLF8ED said:

Hi Stephen,
I have been playing with my 201881 combat roadster for 26 years. It has the “C” combat head as original, but some previous owner put a standard Commando cam in it. As the fuel here got worse I started having engine ping issues. Seems the combat head needs the combat cam profile as that profile has more valve open overlap bleeding off some of the cranking compression that caused the ping. Use the combat head.

Click to expand...

Mileage/ results vary. I did the opposite with my '71 750. Milled the head .040, Ron Fratrulli (sic) ported the head for 32mm Mikuni's and I left cam stock. Performed quite well. I went back to Amal 32mm's for easier starting, lost a little top end. Timing with points was 28 degrees advanced per Dunstall tuning manual.
Only noticed overall performance suffered once "modern" fuel was only thing available. Ah, the good old days prior to gas with ethanol.
It may be anecdotal, but allegedly John Gregory of Sunset Motors Hogslayer fame said he was involved with assisting the Norton factory with porting in the Combat era. May be why it was a top end motor along with the 2S cam. Dragsters aren't really designed to perform well unless WOT.
I can't comment about what they decided to port the Combat head for a stock street engine.

I had a 73 750 with the RH6 head, and what the factory called the high compression version. 32mm carbs. There was also a lower compression version where the head was not milled. Mine had a std. camshaft installed from the factory. That was a nice engine. I later installed a Megacycle cam and bored the carbs to 33mm.

Got the barrel off. Cam looks fine. A couple of lifters look a bit rough, but no mirroring of damage on the lobes. I will put the lifters in the lathe and with a toolpost grinder, dress the faces. See how it goes.

Interesting observation, the original lifter rubbing faces are not surfaced perfectly square to the length of the lifter body. One of four is good. On the other three I by eyeball I can see a 2 to 6 thousands gap along the width of the lifter face. Either they were build out of square from the factory, or they were resurfaced out of square at the time of the last build.
BTW, we are not talking about wearing out of square, as the noncontact surfaces on the edges are not square. Hope that makes sense.

I also confirmed that the valve lift at TDC on both sides, and both intake and exhaust, is relatively symmetrical. I think this just confirms an earlier observation that the cam lobes are not worn unequally.
RH intake max lift at TDC .393"
LH intake max lift at TDC .388"

RH exhaust max lift at TDC .328"
LH exhaust max lift at TDC .329"

I suppose that the .005 difference on intake could be wear, but the lobe surfaces look perfect, so I doubt it.
I am inclined to attribute this to sloppy manufacturing.

your measurements are at the valve stem or off the cam lobe?
combat spec is .390” intake lift and .346” exhaust lift at the cam lobe. If you are taking measurement at the valve stem, check the exhaust again. Looks very worn. Rocker ratio is 1.13 to 1.

milfordite said:

I had a 73 750 with the RH6 head, and what the factory called the high compression version. 32mm carbs. There was also a lower compression version where the head was not milled. Mine had a std. camshaft installed from the factory. That was a nice engine. I later installed a Megacycle cam and bored the carbs to 33mm.

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The Commando before my current of the last 26 years was a ‘73 750 high compression version (RH6). As you said it had a standard camshaft with higher compression, but not as high as the combat. I also owned it up to 1987 so issues with ethanol had not yet come around.
That’s it in my avatar.

worntorn said:

Maybe the rh 4 isn't so bad afterall.

My 850 with rh4 seemed to do pretty well against a nicely hotrodded 750, and that was a top end test, not really the intended forte of a MK3 850. That 750 has a Comstock ported head with the full auto style d exhaust ports added , hot cam and high compression.

I am going to try the JS sleeves though, got them here now.

I am looking for my Commando Road test Gold portfolio. It contains tests of several Combats. 1/4 mile et results were about the same as the earlier bikes, iirc. One would expect the Combat to be a lot quicker in this type of test as it is really a test of top end power. Maybe that crude port job Dave described is a bit of a hold back.

Glen

Click to expand...

I think the 1/4 mile prowess of the combat and some of the earlier Commandos was due to the gearing (19T counter sprocket). My first combat was at 103mph at the end of a quarter and had plenty more acceleration left.

I just spent 10 minutes with a pair of spring calipers in the inlet side of both the Combat and the RH6S heads. The first (outboard) 2 1/2 inches appear identical in shape and dimension.

When Jim Comstock works his porting magic on the RH10 heads he doesn’t change the main part of the port at all, all of his time and efforts go into changing the valve area of the port (and it makes a HUGE difference).

So I assume this is where the difference is between the early 32mm heads and the RH4 ?