Combat cam in a MK3

I am doing another MK3 and would like to install a combat cam that I have. Has anyone had any experience doing this? I thought if I advance it by 1 or 2 degrees it might give it a better power spread, I will spend a bit of time on the head and will up the compression to just under 10 to 1. How much do the pistons have to be fly cut etc. does anyone have any drawings?

Mr.Sparks said:

I am doing another MK3 and would like to install a combat cam that I have. Has anyone had any experience doing this? I thought if I advance it by 1 or 2 degrees it might give it a better power spread, I will spend a bit of time on the head and will up the compression to just under 10 to 1. How much do the pistons have to be fly cut etc. does anyone have any drawings?

Click to expand...

If you use an original Combat cam you will have to grind or turn grooves in to the bearing surface of the cam as there the oil grooves were in the bearing bushes instead of the cam itself. You probably won't have to fly cut the pistons but only measuring in situ will tell :!:

Nah Just groove the end bushes or fit the narrower bush pair on L cam end that forms a small oil reservoir and some oil grooving to assist the spreading layer. Not a bad id to groove the case walls and slightly funnel the tiny oil feed holes to assist their splash & drain down oil supply to bushes. I think 2S is all around best there is and upping the displacement tends to mellow cam profile so comes on cam earlier, which is a good thing as takes almost red line to feel its 3rd piston power kick in on my 2 750 Combats so may out pull em to top out yet still retain the more impression initial torque hit of 850's. Should be a tad easier to start too with slightly less effective CR at starting rpms. Retard timing till no back fire sense, likely close to Combat's 28' zone.

Mr.Sparks said:

I am doing another MK3 and would like to install a combat cam that I have. Has anyone had any experience doing this? I thought if I advance it by 1 or 2 degrees it might give it a better power spread, I will spend a bit of time on the head and will up the compression to just under 10 to 1. How much do the pistons have to be fly cut etc. does anyone have any drawings?

Click to expand...

Between decking the head and using a combat cam you will need valve reliefs. About .060 to .080 deep.

I usually set up the mill on a 750 piston and then put the 850 piston in the vise.
Doing it that way depends on what kind of piston vice you have. Mine locates on the wrist pin bore so that makes it easy.

I also have a fly cutter that fits in place of a valve. Using that and a hand drill I have cut many pistons in the engine. Jim

comnoz said:

I also have a fly cutter that fits in place of a valve. Using that and a hand drill I have cut many pistons in the engine. Jim

Click to expand...

The fly cutter fits into the valve guide? Hmmm.

I have been running a Combat cam (SS or 2S as some say) in my 850 MK2 for over 34 years now with stock 40th over size pistons, still running orginal valves but have a bit of head work done, I did not buy a Combat cam but got my orginal cam built up and grounded to the Combat specs, this cam works so well in the 850 engine, I still run my orginal lifters and push rods and my head has been shaved and I have clocked up a lot of miles on this motor, its a hot motor and is still very reliable has never let me down and I can still ride it in top gear at 40 mph around town before dropping it back to 3rd.
My orginal workshop manual had a high performace section at the back for the Norton motors and mine was build for the stage1 performance for the road which recomend the Combat, SS, 2S cam profile.

Ashley

ashman said:

(SS or 2S as some say)

Click to expand...

The Combat cam is a 2S. Not the same thing as an SS.

The SS cam was the grind used in the SS dommies Atlas etc from the early 1960s,
and was then the stock grind used in Commandos.

Until the Combat with its 2S cam came along - and disappeared.

I've run a 3S in my 850 with stock cam timing, a skimmed head, standard valves and no valve reliefs for many years. The motor lost a little low down torque until I fitted a Tri-Spark. From memory the factory offered the 2S as upgrade and I'm sure there was no mention of replacing the pistons.

Dave

It just depends on the particular head and how much is skimmed and the cam timing. I know I have run the 2s cam on an 850 with no valve reliefs but it is very close.
When I do a 2s cam on a fresh build I cut the valve pockets just to avoid having to tear it down and cut them after it's together and I find it doesn't quite clear. Jim

Thanks gents, based on all the info I'll run a 60 thou fly cut. I managed to find some blue prints for the pistons and will check the cc of the chambers to compare with the cylinder volume to get the CR right. What ever I take off the head will come off the pushrods.