1972 Combat Roadster valve clearance

I'm pretty sure my motor is a Combat, which my book says .008" intake, .010" exhaust. When I performed an adjustment today, it appeared they were set at .006" intake and .008" exhaust, apparently non Combat settings. The exhaust were spot on at .008", the intakes a little tight at about .005". Previous owner said motor was rebuilt by a Norton guru in Miami and I'm wondering if possibly the motor was "detuned" with a milder non Combat cam and that's why those settings were used. I set them at .006" intake and .008" exhaust figuring that was the safer thing to do and having some faith in the engine builder.

Do you like my assumption and should I stick with the .006 intake and .008 exhaust settings? Should I verify which cam I have in case I should use the looser Combat settings? I have a dial indicator so checking lift shouldn't be too difficult, or is there an easy way to read a cam stamping?

Check the lift with the gauge. You don’t need to be super accurate, just check the inlet has more lift than exhaust. If it does, and as it’s a Combat, chances are it’s a 2S.

"...For a Combat the manual says .346" exhaust and .390" intake..."


"Technical data: Material EN 32 5 , Maximum lift inlet .390 in. (9.906 mm.) Maximum lift exhaust: .346 in.(8.788 mm.). Your lower numbers can be construed as cam wear after 35,000 miles but obviously keep checking as pulling the cam is a big job so be sure first. I'm impressed you got that kind of mileage out of a Combat SS radical highlift cam. You must have had a couple of valve jobs during that period of service. A nasty cam with a no nonsense sound ,super high lift..."
Torontonian

Unless the base circle of the cam that is fitted is different than the expected Combat cam, you should be using the looser .008" and .010" clearance. That is, if your bike actually has a Combat head fitted. The Combat heads were shaved, so the distance from cam to valve gear is shorter. A shorter pushrod length is called for. The pushrod to valve gear geometry was not changed for the Combats either, so the alignment is a bit substandard. You should be able to tell by looking at the fin spacing between head and barrel. If it looks about the same as the other fin spacing, you have a non-Combat head. If the spacing is narrower, you've probably got a Combat head.

"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 leftside are negatives (depressions in the surface)..."

see images and more info here:

Atlas Commando said:

Unless the base circle of the cam that is fitted is different than the expected Combat cam, you should be using the looser .008" and .010" clearance. That is, if your bike actually has a Combat head fitted. The Combat heads were shaved, so the distance from cam to valve gear is shorter. A shorter pushrod length is called for. The pushrod to valve gear geometry was not changed for the Combats either, so the alignment is a bit substandard. You should be able to tell by looking at the fin spacing between head and barrel. If it looks about the same as the other fin spacing, you have a non-Combat head. If the spacing is narrower, you've probably got a Combat head.

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The geometry observation is a good one - The first Combat head I serviced had tappet witness marks toward the edge of the valve tips which clearly showed the alignment was out. While shorter pushrods were definitely called for, they weren't fitted. The '72 parts book just lists one part number for all models.
I can't recall exactly how much I shortened the pushrods, but it was more than 1mm.

As already said, the only way to be sure is to measure valve lift; there's over a millimetre more lift on the Combat inlet vs exhaust, so a dial gauge shouldn't be necessary to visualise the difference.

I don't know why different valve clearances where specified for inlet and exhaust with the combat cam (2S) since both the inlet and exhaust ramps have identical 0.010" ramps and the 4S cam has 0.016" ramps with both valve clearances specified at 0.016". My combat motor has a 4S cam fitted but, if I put a combat cam in it I would use 0.010" clearances for both inlet and exhaust.

I thought the larger exhaust valve clearance was because the exhaust valves run hotter and the clearances close up with expansion of the components.

Thanks for the help. Measured valve lift (somewhat difficult to fixture) .365 exhaust, .435 intake. The intake comes out precisely as it should with 1.13 rocker ratio, .390 x 1.13 = .440. The exhaust should be .346 x 1.13 = .391, .026 greater than measured. I don't know why the intake is spot on and the exhaust isn't. Maybe the ratio is off or an I made an error in measuring. Error in measuring is more likely, as I said it was difficult to fixture the indicator perfectly in line with valve travel.

Measurements verify a 2S cam. I've seen clearance specs for .008" intake, .010" , and also .010" for intake and exhaust. Which setting do you like for a 2S cam?

I can't recall exactly how much I shortened the pushrods, but it was more than 1mm.

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Just read in another AN post, Combat pushrods at 0.050" shorter, and another was 0.060" shorter than standard which "brought the rockers back in line..."

We had a discussion here about that. You really need to know the target length and where it was measured (end caps on? overall length?)
Then you can measure yours from the same perspective and determine whether to leave them or make a change.

Reading of Dirt tracker TRIUMPH rider biasing clearances for tracks , by two thou. or so .
Altering the opening and closeing by this . So you could consider barometer & use -
Heavilly laden definately not tight . Ran the Bonevel with intakes just gripping & zorst just free on the feeler .

SO even ' feeler just gripped ' and ' feeler just free ' , bias it by a few theths thou. Ran my Combat Cam at . 016 but with clearanced W & S springs & hardware , rest stock . 8:1 measured comp.

3200 was where it stopped trundling . Slight ' megaphoneitis ' at 3200 in normal use . was said the MUTES gave a similar flat spot . THROW the clearances way out and see if they clatter .
Old blind men set em by sound , old deaf men by screwdiver stethascope & fingertip sensitivity on its end . a matching resonance in screwdivers required of course .

Agreed - usually called a '2S', my Combat cam is stamped 'SS'.
I set mine at 8 & 10 thou. Ran great, not noisy.
Combats are definitely a fun bike

B+Bogus said:

Agreed - usually called a '2S', my Combat cam is stamped 'SS'.
I set mine at 8 & 10 thou. Ran great, not noisy.
Combats are definitely a fun bike

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Right. I loved the Combat engine in my 72 Commando.
The nomination thing is a little tricky. The "standard" Commando cam grind was always the same as the ("Mk 2") Atlas but it really originated with the "SS" prounounced "ess-ess", "Sports Special" cam in the 650SS from the early '60s. So, really the standard cam was an "ess-ess" cam. When the Combat cam came along, it fell into the Thruxton "race shop's" coding for the performance cans and became called a "two ess", like the "three ess" and "four ess" cams.
What I don't know was how the profile was developed. It is a nasty little piece of work (yeah, on the road it can be a kick in the butt but that's all. The opening and closing ramps were wildly inaccurate, resulting in the valve train (lifters, pushrods, adjusters, rockers) being badly beaten up. The crude profile would cause valve float prior to 7K Rpm if the valve spring pressures weren't close to perfect. It just doesn't seem to me to be the work of the race shop (they were way more knowledgeable and experienced to come up with a cam grind like this) but, if it wasn't developed in the race shop, where?
There were not the modern computer design development tools that we have today, but there was really no excuse for such a bad cam going into a production engine (IMO). If a modern redesign could clean up this cam (like PW's reworking of the 4S cam into the PW3) to give minimum accelerations and decelerations during opening and closing and at all lift points, it might be useful but the '70s version was just a nightmare. And that doesn't take into account the insulating washer debacle and the poor arrangement of the pushrod lengths.
But, oh, that kick when the Rpms hit about 3500!