View Full Version : TR2/3/3A Degreeing Cam for TR3

02-06-2008, 04:21 PM
OK, I got the crank in with the pistons and put the cam, which is original, but regound for more duration and lift. I lined up both gears etc. up to the original marks. I then used a degree wheel to check the cam timing against what the cam grinder said it should be. I used the timing at 0.050" lift method. Chart said at 0.050" intake lifter rise, cam should be at 9 deg. BTDC. I'm getting closer to 12 deg. BTDC. Same amount is out on the closing...about 3 deg. sooner.

Looking in the original TR3 service manual, it mentions that you can adjust the cam timing by 1/4 tooth intervals. There are 42 teeth on the cam gear. Now for the question...

Is that 42 tooth interval divided into 720 deg. (then divided by 4, resulting in slightly more then 4 deg.) or a 360 deg interval (resulting in slightly more then 2 deg.)???

Am I all out of whack here, or should I try to get this cam timing closer??

02-06-2008, 04:44 PM
I'm not sure how you correct your problem. I'd just try the procedure and see what happens when you recheck your valve timing.

If you don't get it "perfect," a slightly advanced cam (what you have) will favor low-end torque at the expense of high-rev performance. A retarded cam will do the opposite. Most street drivers will benefit from a slight advance. (Source: Iskenderian Cams)

02-06-2008, 04:54 PM
Well, if that's the case, I might just leave it. If there is in fact a 4+ degree difference when flipping around the cam gear, that would mean that the cam timing would wind up being retarded a little more then one deg.

Maybe I'll check a different cylinder and see what happens. I was working from #1, and only the intake valve.

02-06-2008, 04:57 PM
My recollection is that 1/4 tooth is about 4 degrees; and that slightly fast (less than 4 degrees) is better than slow, because the chain will stretch slightly in operation. So I think you're already right on the money.

However if you do want to try to get closer than that, several sources sell timing gears modified to either take offset buttons, or have a vernier adjustment.

02-06-2008, 05:23 PM
Real good guys. Since the general consensis is slightly advanced on the valve timing is fine, I'll just leave things as they are. I'm not about to get too high tech here with adjustable cam gears or offset bushings.

Randall, since you verified the 4 deg 1/4 tooth difference, I'll stay where I'm at.

Actually, when you think about it, having the ability to change the cam timing as little as 4 deg. by flipping around the gear was quite clever of Triumph. /bcforum/images/%%GRAEMLIN_URL%%/cheers.gif

02-07-2008, 08:45 AM
I build race motorcycle engines and degree cams all the time.

The main concern on advancing the cam is valve to piston clearance. Make sure it has plenty (at least .085 on exhaust and .075 on intake) By advancing the cam you are opening the intake and exhaust valves earlier and therefore it closes the gap between the piston and valve.
You could slot the bolt holes. It will allow you to set the cam anywhere you want it to be.
Where did the cam grinding company recommend it to be set?

02-07-2008, 11:31 AM
TRactor motors are a "nail head" non-interference design, so no worries about the valve/piston clearance.