Valve Clearance

Last summer I re-timed the cam on my 1972 Combat. The bike still has the 2S cam, but I advanced the cam timing about 3 degrees to improve the midrange power. The factory valve clearances for a Combat engine are .008" intake and .010" exhaust. And .006" intake and .008 exhaust for the standard engine. What valve clearances should I use on a Combat engine with the cam timing advanced?

Stephen Hill said:

Last summer I re-timed the cam on my 1972 Combat. The bike still has the 2S cam, but I advanced the cam timing about 3 degrees to improve the midrange power. The factory valve clearances for a Combat engine are .008" intake and .010" exhaust. And .006" intake and .008 exhaust for the standard engine. What valve clearances should I use on a Combat engine with the cam timing advanced?

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I fail to see how advancing cam timing would affect valve lash requirements.

Advancing the cam timing can change the valve to piston clearance. A way to check is to roll-up a piece of kids' plasticine (play dough) and put it where the valves approach the piston and then rotate the crank so the valves imprint on the plasticine. Then section the plasticine with razor blade - you can see how much clearance you have.

acotrel said:

Advancing the cam timing can change the valve to piston clearance. A way to check is to roll-up a piece of kids' plasticine (play dough) and put it where the valves approach the piston and then rotate the crank so the valves imprint on the plasticine. Then section the plasticine with razor blade - you can see how much clearance you have.

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Al - the question was regarding the rocker / valve stem clearance, not valve / piston clearance.

The extra clearance you quote for the combat engine may be a temperature related thing?
Can't really see another reason.
I agree with @Dan1950 that advancing the cam, especially by so little, would have no bearing on tappet clearance.
Cheers

I was thinking that the increased valve clearances on the 2S cam would also affect the cam timing. Hence the question.

Without knowing your cam - but assuming 0.240" lift over, say, 60 degrees, then an extra 0.002" clearance would delay the lift by 0.5 degree.
If the lift is more than that the delay would be even smaller.
Not worth thinking about!
Cheers

Edit: the formula I was using was:
Clearance divided by full lift times opening duration.
It's a linear approximation but will give a ballpark figure.

Stephen Hill said:

Last summer I re-timed the cam on my 1972 Combat. The bike still has the 2S cam, but I advanced the cam timing about 3 degrees to improve the midrange power. The factory valve clearances for a Combat engine are .008" intake and .010" exhaust. And .006" intake and .008 exhaust for the standard engine. What valve clearances should I use on a Combat engine with the cam timing advanced?

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The same tappet clearances for that cam , it has more to do with the flanks and ramps etc ground on the cam from the manufacturer. Cheers .

WotIdidinmine , was Have the CAM , If we agree , the same . As I had been told it was a 3S , it was a bit trickey to get 2S timing . Stuff that for a joke . Find Overlap , Throw 2 & 1 , if that was it , on the chain / teeth ,
To get 3 deg early . Rather than middle of overlap at TDC . Easy . Set at TDC & middle , then reset to how . AS it was a THREE ESSE , it was SET AT 0.016 . Which was darn good .


But it had W&S springs ( Norvil ) at the 0.040 clearances here & there . Plus Mk II amals & BSA Auto Adv. So sat at 500 rpm while you went & made a coffe . And drank it .
Thwang Twang Twang Twang . Would watltz on the side stand some days . So youd have to cut it while you drank it , or take other measures . Maybe it picked p a few hundred , warm .
From 4-800 to 5 000 .
So , their you are . No Rattles . I believe it got a Newtons Cradle effect , throwing the followers , with the Prog Rate Valve Springs , the THWAng WAS THE VALVE THROWN OPEN .oops .
So Neutral loading , less friction . blah blah . But Yr supposed to keep it under 3200 for some time. To Bed In . So fancy follower lube , might be a good idea . Pre or in oil .
Ran That Cklearance all the time the cam was in it . about 8000 MILES , stutter at 3200 , on the go past 3500 , as in its on all twelve cylinders , then. as it were .
So your eyeballs were on stalks . or thrown back . 23 T gave a ' sweet spot ' of 80 , touring . in top .

Essentially a " TWO STAGE " effect , corse throttle below three o.k. , past 3500 is hyper mode . Some restraint reqd. tickly in the wet . Then Agian , throwing it into a corner 1/3 faster than usual fastest ,
and throttle steering it thru , drifting . Or the balk out , front drifting , speedo on 60 leant over coming out of it , on a off camber , id think at 20 / 25 up out to 40 maybe . so it has some grunt .
Dont take liberties tho if your brains at half mast . Or drink . They camber steer . Id leave Dew Cati 900s etc alone . As their frame etc is twice as stiff , and they had those new fangle Metzlers or perillis .
AND you wanted to watch the car in front moved off , a length or half , at the lights , before getting under way . Or youd be in & out on the clutch and weaving , unless you went over , around , or through it .
Seen a Hondas red 250 / 500 or two go over or up a VW . So , a count of TWO , before ' follow ing ' ( we think ) or you could tap it . 21 or 22 might not be as bad . A swine to jet .
No Battery , mayve been current / rpm related , as only 2 MC & 6 Volt . where you want real coils . Think t was 220 / 240 notta lotta differance , but 230 + Id think , safer .

I was thinking that the increased valve clearances on the 2S cam would also affect the cam timing. Hence the question.

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YEP ;

Triumph Blokes , Dirt Track , went at it like that , with the clearances , to alter the tourque & pick up .

As a Turkey came back after a good time , after taking it for a " tweny minute spin " . Dusk , in fact .
Stated , " Ive riden 11 Commandos , Ive Ridden Rees Jones Dunstall , Ive ridden Blah , Ive Riden ( maybe it was thingos 72 PR ) THIS is the Fastest Commando Ive ever ridden "

There may be some benefit to ' pick up ' with a 2S ( Which is a Dminator RACE Grind ) set to .016 .
( Might be worth checking the Domi setting ) First P R s ran 2S , 3 S was Daytona ' Top End ' power cam . For super speedways . 155 from almost a Combat motor . ( Like a S V H.D. ! )
So isnt a lot , really . ( aktly thats wong . the STD Cam ' S ' was the ' S S ' domi Cam , if wot weretold is true . Theres a Thing . Compare profile to a 3 S maybe , wot runs SIXTEEN . dead On . )

A bit of a snot , in town , and accidently let it out to 115 into hatsfields , so can run away with you and be deceptive as to who / what is actually in charge . Steer with the kness & butt . Light Grip on bars .
Tight grip it'll weave , on uneven surfaces . Set Iso's at 2 to 5 , Max.

Stephen Hill said:

What valve clearances should I use on a Combat engine with the cam timing advanced?

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Same as before.

MAP Norton Commando 750 & 850cc 4S High Performance Cam

• The 4S cam provides a very similar grind to the famous SS or Combat cam with a little extra lash clearance
• Provides broad power band with great all around performance
• Makes good use of big Norton engines with strong mid range and increased top end
• Wide, smooth power band
• Fits all 750 and 850cc engines with slight modifications

Note:

• Be sure to check clearances on 750cc engines
• Cam is threaded with 1/4 x 26 TPI thread. Use of any other size may damage cam and void warranty

Specifications

• Duration: 276 degrees @ .040 - L.L. .388" - Cam lift at TDC 168" intake. 145" exhaust
• Timing: 36btc/64abc (in); 64bbc/32atc (ex) - lash .016" in. .016"ex
• Timing at .40" cam lift with stock lifter
• Lobe center 104 degrees (intake) 106 degrees (exhaust)

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That TOO , a 4S is 40 thou MORE , same lift graph , Therefore , whadus a fouresserun , isnt entirely irrelevant either .

Refaced ( Stoned ) lifters , check it dont top out in cam tunnel / barrel ( Wants 40 free , extra , plus off coilbound , plus to pistons )

Orrl Roite .

HA ! "

• Timing: 36btc/64abc (in); 64bbc/32atc (ex) - lash .016" in. .016"ex "

whadid itellya .

In my youth, I always thought valve timing was critical. The Triumph race cams I used were E3134 from the 1953 T100 factory race kit and we had details of the exhaust and inlet tracts. However later on, I was trying to get more torque from my short stroke Triumph motor and substantial variations in cam timings made very little difference. Whatever arrangement you end up with, you usually adjust the jetting to get the best out of it. The cam in my 850 motor is advanced 12 degrees, and with the 2 into1 exhaust seems to work well. But does not mean it could not be better

Shelby-Right said:

The same tappet clearances for that cam , it has more to do with the flanks and ramps etc ground on the cam from the manufacturer. Cheers .

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From the reading that I have done, this is my understanding as well. The clearance is to allow the proper initiation of the follower on the cam. Too much clearance and the cam follower misses the ramp and impacts the flank of the cam. Too little clearance and the cam follower continuously drags on the cam.

Interesting. This could explain why when I checked the valve clearance, the position of the engine seemed to matter more than I expected. For example, if the exhaust valve was "open", it was not easy to find a position where the intake valve could be adjusted. The "window" available for adjustment was quite narrow, if that makes any sense. Or put another way, the "base circle" was much smaller than I expected, measured in terms of cam rotation.

Stephen Hill said:

Interesting. This could explain why when I checked the valve clearance, the position of the engine seemed to matter more than I expected. For example, if the exhaust valve was "open", it was not easy to find a position where the intake valve could be adjusted. The "window" available for adjustment was quite narrow, if that makes any sense. Or put another way, the "base circle" was much smaller than I expected, measured in terms of cam rotation.

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Rotate the engine until both intake and exhaust valves are opened equally on one cylinder. This will put that cylinder on overlap. Now you can adjust the valves on the other cylinder. Rotate the engine 360° and adjust the valves on the remaining cylinder.

Dan1950 said:

Rotate the engine until both intake and exhaust valves are opened equally on one cylinder. This will put that cylinder on overlap. Now you can adjust the valves on the other cylinder. Rotate the engine 360° and adjust the valves on the remaining cylinder.

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I have to see how good I am at eyeballing the open equally part. Good tip

I think that the increased valve clearance on the combat engine had more to do with how hot it was going to get and the rpm it was to be run at than it did to the shape of the cam shaft, If advancing that cam puts you into a lower rpm power band and your intension is to run the engine more normally then I think you could put the clearance back down the .002" to the original spec.

Schwany said:

I have to see how good I am at eyeballing the open equally part. Good tip

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You don't have to be exact.

I just pull both plugs and use a drinking straw to find TDC - one cylinder will be on compression and have both valves closed so check/adjust that one.
Rotate 360 to the next TDC and do the other.
Cheers