Camshaft sprocket timing

[Alanretired]

Advice needed. After refitting the camshaft sprocket I checked the valve timing using the Brown Book timing check procedure 12.65.47 which involves rotating the engine until valves 11 and 12 are on rock and have equal gaps. To get equal gaps I needed to be at 5 degrees after TDC. I did rotate the engine to eliminate backlash. The camshaft sprocket is marked with two centre punch marks approx 20 mm apart and the crank sprocket is marked with one punch mark. Before the partial engine rebuild the car run ok with the sprockets aligned using the marks but I can't understand why the timing check shows the 5 degree difference.

 

Regards Alan

[Tom Fremont]

Hi Alan,

 

It's a standard camshaft, right? And you've double-checked the lash setting for the (2) valves?

 

If the chain and sprockets have high mileage on them it's possible that 5 degrees have accrued in combined wear / stretch. I've been told that it's good practice when setting up with all new parts to shoot for ~ 3 degrees BTDC to compensate for it.

 

I've also found that the sprocket holes can allow as much as 6 degrees ( at the crankshaft ) between the back and front thereof. You might see if loosening the bolts and retightening with the sprocket as far counter clockwise as it will go on the bolts gets you back to TDC. Failing that, you can turn the sprocket 90 degrees ( employ the other pair of holes ) and approach the sweet spot from the other direction, backing the sprocket CW against the bolts before tightening. Keep in mind that properly torqued bolts will never allow the sprocket to slip ( short of the camshaft seizing in the block ).

 

Cheers,

Tom

Edited February 15, 2013 by Tom Fremont

[Alanretired]

Tom

Thanks very much for the info, I will try your suggestions tomorrow and let you know how I got on.

Regards

Alan

[Tom Fremont]

Hi again Alan,

 

I think I had it backwards on the rotation; see edited version above.

 

Tom

[john.r.davies]

The cam sprocket has 42 teeth, so they are sopaced about 8.5 degrees apart, which puts your 5 degree error weel insode a one tooth error in setting. That's why it's good to chek the timing by another method.

 

And rmember that Triumph provided a fine tuning method for timing in the four holes in the sprocket.

Use the other two for a half-tooth adjustment (4 degrees)

Turn it over to get a quarter-tooth (2 degrees)

and thenuse the other two holes for three quarters (6 degrees)

 

It's all in't Brown Book!

John

[Tom Fremont]

Hi John,

 

That's correct about the 8.5 degrees, but that's at the camshaft, not the crank where it would be X2.

 

Hence 1/2 tooth difference will give ~ 9 degrees at the crank.

 

Except perhaps for the very rare single-row sprockets ( which may never have been available on the TR6 ) this advice doesn't apply:

 

 

Turn it over to get a quarter-tooth (2 degrees)

and thenuse the other two holes for three quarters (6 degrees

 

...because you can't flip the duplex ones due to their offset.

[Alanretired]

John

Thanks for your note. I should have mentioned that my car is a 1974 Tr6 which uses a twin tooth sprocket which cannot be turned over - has an offset centre. The brown book for this car does not unfortunately give any advice on cam timing adjustment other than align the marks.

Regards

Alan

[john.r.davies]

Of course! Doh!

Thank you for the correction. The flogging will terminate shortly.

 

But the point about one tooth, plus an old chain, may be relevant.

 

If it were me, I'd trust the "Equal lift" method, rather than any sprocket makrs.

John