Head Pistons Knees and Toes!

staticmoves · Sep 16, 2013

Question Ladies and Gents

-I have an early 73 750 engine with and RH6 Head.

-It has high compression pistons in it. ( Had, engine is now completly torn down and getting close to reassembly).

- upon reassembling the engine I am going to use a 71 standard 30mm port head, ( RH6 head needs new valve seats from what I can see, and needs some thread repairs. )

- The RH6 head will be installed at a later date once repaired.

- I will leave the combat cam in the machine, ( correct me if wrong, but I believe the 73 750 RH6 has the combat cam.)

- Anyone see any issues with this setup, or incompatability issues, (ie high compression pistons, or combat cam with this head.)

Cheers..................... 8)

L.A.B. · Sep 16, 2013

staticmoves said:
I will leave the combat cam in the machine, ( correct me if wrong, but I believe the 73 750 RH6 has the combat cam.)

Why do you think it has a Combat camshaft?

ILLF8ED · Sep 16, 2013

My '73 750 (220627) came with a standard camshaft and RH6 head. For a while I ran Powermax pistons 10.5:1 compression. I would not do that with today's gas/ethanol. No longer have this machine, replaced it with '72 750 combat in '97.

Hobot · Sep 16, 2013

Some may have configured the '73 to Combat spec as so easy to do by just 2S cam and 10 CR or more. The 2S cam over lap lowers the effective ratio so likely can run 87 octane like me with full timing 28-29' and fast rise AAU ignition and what ever extra CR going w/o the cylinder base gasket-plate. The most responsive powerful combo I've had used so far was a 28.5~30mm standard factory kit non milled head, FlameRing gasket with 2S cam, so why not yours too. I've surveyed a handful of forums over a decade to find detonation is rather rare occurrence but some of the most common that did were the 1st attempts to detune Combats by just switching to std cam w/o lowering the CR. Tje Combat is as fine a low down grunter as any Commando but it takes a rather immature excitable impulsive childish character to experience its real power band joys.

staticmoves · Sep 16, 2013

L.A.B. said:
staticmoves said:

I will leave the combat cam in the machine, ( correct me if wrong, but I believe the 73 750 RH6 has the combat cam.)

Click to expand...

Why do you think it has a Combat camshaft?

I for some reason thought it was a 2s cam in it. Unscrolled.

I was not positive as the only marking on it looks like a T or an r.

So the 73 RH6 engines had an unscrolled standard camshaft?

Cheers......

RennieK · Sep 16, 2013

Combat cam is marked with 2 S's like this "SS" (although they are often not real neatly stamped)

Pic is too contrasty. It's hard to tell what cam you have there. Can you post a better shot without using flash? looks to be a cam from another maker other than Norton.

Cams often are marked on the side of a lobe as well since it is a nice flat area.

Hobot · Sep 16, 2013

Combat Bomb Saga basically shot Norton in their financial feet that just took 5 more years to sink them, so they stopped issuing 2S cams and 10 CR by '73 model year. Only real way to know what cam profile is to measure it. Easiest is send to Jim Comstock to put in his CNC machine to spit out the graph or numbers. Otherwise an degree wheel and dial gauage needed. IIRC the 2S and later had the bushes modified by a dual set at DS end for oil pocket and groove in the TS to catch some droll for lube. Good cam core can be welded hard surface and profiled to suit.

staticmoves · Sep 16, 2013

RennieK said:
Combat cam is marked with 2 S's like this "SS" (although they are often not real neatly stamped)

Pic is too contrasty. It's hard to tell what cam you have there. Can you post a better shot without using flash? looks to be a cam from another maker other than Norton.

Cams often are marked on the side of a lobe as well since it is a nice flat area.

No markings on the side of the cam lobes

rvich · Sep 16, 2013

The cam is out of the engine? Why not measure it and see if the lobes are the same height or different heights. That would be a starting point. I don't know how long the lobes are on any of the cams but I think the standard cam has the same lift for both exhaust and inlet whereas they are not the same for the Combat. Get the measurements, I am sure somebody has a cam that they could measure to compare.

Russ

Ron L · Sep 16, 2013

Courtesy of Dyno Dave Comeau.

staticmoves · Sep 16, 2013

rvich said:
The cam is out of the engine? Why not measure it and see if the lobes are the same height or different heights. That would be a starting point. I don't know how long the lobes are on any of the cams but I think the standard cam has the same lift for both exhaust and inlet whereas they are not the same for the Combat. Get the measurements, I am sure somebody has a cam that they could measure to compare.

Russ

I measure 1.216" both inlet and exhaust give or take 0.001" to 0.002"
This is the lobe not the lift.

Hobot · Sep 16, 2013

One needs the chart for crank degrees turned vs cam lift as the graphics are too crude to read directly. Rocker ratio is 1.1:1 if dial set on valve stem or rocker end. I did it with head on which makes a few .000ths' and degree or so error prone but still good enough to resolve which factory cam it is.

David Comeau wrote:

> Actually IMHO thats the only way to do it
> doing with the head on is possible but for me it is not desireable.
> You probably will end up doing a complete profile like I have done on my
> web site
> Dave
> heres my data
>
> Commando 06-3536
> Duration E=262 I=266
> Lobe center=105
> EHXAUST
> DEGREE LIFT
> 79 0.010
> 128 0.050
> 144 0.100
> 158 0.150
> 174 0.200
> 192 0.250
> 214 0.300
> 235 0.330
> 261 0.341
> 281 0.330
> 304 0.300
> 327 0.250
> 345 0.200
> 360 0.150
> 375 0.100
> 390 0.050
> 413 0.010
>
> INTAKE
> DEGREE LIFT
> 291 0.010
> 335 0.050
> 348 0.100
> 361 0.150
> 375 0.200
> 389 0.250
> 406 0.300
> 428 0.350
> 452 0.380
> 469 0.387
> 485 0.380
> 509 0.350
> 530 0.300
> 547 0.250
> 561 0.200
> 574 0.150
> 587 0.100
> 601 0.050
> 625 0.010

beng · Sep 17, 2013

Hello Static,

Your cam checks out at the same dimension as standard nos commando cam I have here. If the Combat and the standard cam are ground on the same base circle then putting a micrometer on the lobe top to bottom should show a larger dimension.

So with the addition of the standard Commando head, you are going to end up with what is basically a standard Commando engine with raised compression. Usually the only thing you might have to do in that case is to retard the timing a degree or two from it's standard setting.

I would not waste your time messing with any factory 32mm Commando head, their ports are proven inferior to the standard smaller ports, good riddance to bad rubbish.

A standard Commando with it's compression bumped up a point is probably the best all around combo to be had below 6000 rpm. You will have great torque and your engine should last a long, long time if it is put together right, and it should turn easy quarter mile speeds close to 100mph, fast enough right?

rvich · Sep 17, 2013

The base circle for the standard cam is 0.885 and the Combat is 0.906.

Beng, I take from your comment that you would use the earlier RH1 head before you would use the RH6?

Some reading here re heads and carbs: 750-roadster-which-carburator-t14848.html

Russ

staticmoves · Sep 17, 2013

rvich said:
The base circle for the standard cam is 0.885 and the Combat is 0.906.

Beng, I take from your comment that you would use the earlier RH1 head before you would use the RH6?

Some reading here re heads and carbs: 750-roadster-which-carburator-t14848.html

Russ

Thank You.
Good read.

ILLF8ED · Sep 17, 2013

Combat cam on the left - standard cam on the right. Note the lobe shape difference...combat is quite fat.

staticmoves · Sep 17, 2013

So It looks like it could be a standard cam shaft.
can anyone make sense of the marking on the Shaft in between the lobes?
Looks like an r or L or part of a t

beng · Sep 17, 2013

rvich said:
The base circle for the standard cam is 0.885 and the Combat is 0.906. Beng, I take from your comment that you would use the earlier RH1 head before you would use the RH6?Russ

Okay, so the Combat should measure larger with a micrometer for sure then, taller lobe on a bigger base circle. This guy has a standard cam for sure.

The 32mm heads used on the Commando were ported poorly and by today's standards are ruined. About all they did for the Commando was let them use the phrase "32mm" in spec sheets for sales purposes.

The 32mm heads are fine for someone with an original bike to get down the road with, but they will get down the road better in some parameter with the small port head, and if they want to port a head the small port Commando head has fabulous potential for someone who knows what to do with it. You don't have to take my word for it either, all the information is here to be found for those who turn the right rocks over.

Matt Spencer · Sep 18, 2013

Is the one pictured the one your useing in it ? , Mr Static .

Throwing the virneer across the base , x 1/2 . subtracted from the toe to heel gets near correct ' lift ' .
Alternately throwing a dial guage on the cam or lifter gets true figures there , or at rocker screw ; Valve Lift ' .

just tripped over THIS , http://www.johnsoncams.com/jc_n_cams.html . so threw it on .

Heres a picture of ' hand polishing camshaft ' . Those lightly scoured tips ( lobes ) on yours could do with dressing smooth & square , if youre useing it .
( relitive terms ) . Old Magazines periodically had a picture of old Bruce or Trevor , makeing a new Cam for their dirt track BSA Single ( Gold Star even )
A single obviously wasnt critical on matching both cylinders valve profile . :?

Nevertheless , a steady hand & a good stone , CAN produce good results.

drivle on makeing camshafts . http://www.dragzine.com/features/shop-t ... t-is-made/

Home Made Lumpy Single Cam .

The POINT BEING the Cam / Lifters interface should be smooth matched polished surfaces , or refinements thereoff . Good valve springs arnt a bad idea .

Rohan · Sep 18, 2013

beng said:
The 32mm heads used on the Commando were ported poorly and by today's standards are ruined. About all they did for the Commando was let them use the phrase "32mm" in spec sheets for sales purposes.

The 32mm heads are fine for someone with an original bike to get down the road with, but they will get down the road better in some parameter with the small port head, and if they want to port a head the small port Commando head has fabulous potential for someone who knows what to do with it. You don't have to take my word for it either, all the information is here to be found for those who turn the right rocks over.

While this may be somewhat true for the 1973 model 750 Commandos, in case anyone is being mislead by the above, it should be pointed out that 850 Commandos used the 32 mm inlet ported heads in Commandos very successfully.
And with 52 fl/lbs of torque, it puts 850 Commandos as one of the torquiest motors ever to come from England.
Lovely strong top gear performance, from 15 mph to 115 mph - with the right gearbox sprocket.
Thats Vincent and Brough Superior territory, in the days of old..

The modern Rocket 3 with 2.3 litre motor somewhat eclipses it of course, in modern bikes.
But it is somewhat porkier...

And, if you were not satisfied with that level of performance from your 850, the factory published "Tuning Sheets #1 and #2" to give your 850 a Combat spec motor.
So equipped, the factory-built bike of Dave Rawlins clocked an official measured 143 mph 2 way speed in 1973 at the International Speed Meet at Elvington Airport.
On a otherwise fairly stock looking roadster.
Not bad for 32 mm ports ??
Because they don't work too well in just one model for just one year, don't write off 32 mm for everything ?!

Head Pistons Knees and Toes!

staticmoves

L.A.B.

Moderator

ILLF8ED

Hobot

staticmoves

RennieK

Hobot

staticmoves

rvich

Ron L

staticmoves

Hobot

beng

rvich

staticmoves

ILLF8ED

staticmoves

beng

Matt Spencer

Rohan

Similar threads