Camshaft timing (I know Again!)

[jpmf]

Spent a few hours getting the engine back together and it was the dreaded fitting of new timing chain/sprockets and tensioner to my standard cam.

I set nos.11 and 12 valves to 3.15mm and started the process as per the brown bible.

The closest I can get is about one degree on the pulley, while maintaining the same valve clearance, or 15 thou difference if I bring the pulley to TDC.

Is this an acceptable tolerance for the cam as I have used up all my options of twisting and turning of the sprockets that we are now on first name terms ;-)

I set TDC with the head off and as the engine is now reasonably tight I cannot get a 'rock' at TDC, so my 1 degree could easily be there or in the 47 year old pulley.

[ed_h]

Without an adjustable cam sprocket you may not be able to get the cam timing dead nuts on.  The stock  cam sprocket only has 42 different positions it can be in relative to the crank sorocket, making the possible settings 8.6 degrees apart.  Flipping the cam sprocket over allows intermediate settings, giving a 4.3 degree granularity to the settings.  This is close enough for most purposes.  many say that advanced cam timing is better than retarded timing.

Ed

[Tom Fremont]

12 minutes ago, ed_h said:

Without an adjustable cam sprocket you may not be able to get the cam timing dead nuts on.  The stock  cam sprocket only has 42 different positions it can be in relative to the crank sorocket, making the possible settings 8.6 degrees apart.  Flipping the cam sprocket over allows intermediate settings, giving a 4.3 degree granularity to the settings.  This is close enough for most purposes.  many say that advanced cam timing is better than retarded timing.

Ed

The TR6 sprocket is offset so cannot be flipped. However, the alternate holes are 1/2 tooth from the others. 

I have found sufficient clearance in the mounting holes to get my cams exactly where I want them every time with standard sprockets. They will keep their position forever if properly installed, i.e. oil the threads of the bolts, degrease the mating faces on the cam end and sprocket centre and torque to the maximum value. The bolts never go into shear in service; if they did they would fail absolutely. The torque is transmitted entirely by friction between the two.

Chain/sprocket wear can ultimately retard the valve timing by as much as 6 degrees ( crankshaft ). I aim for 3 degrees advanced to compensate for wear and so far have hit it. 

My driver has 95K miles on its most recent cam; I did change the chain 80K miles back when the bottom end had 50K; it has 130K now. I reckon it's about 3 degrees retarded and runs more smoothly than ever, however it gets about 1 mpg less than it used to. I haven't noticed a performance change but that could be boiling frog syndrome.

 

Tom

[ed_h]

3 hours ago, Tom Fremont said:

The TR6 sprocket is offset so cannot be flipped. However, the alternate holes are 1/2 tooth from the others. 

 

Of course you're right, Tom.  I mis-remembered how the intermediate settings were achieved. 

Getting +/- 2 degrees from the bolt clearance would require the holes in the sprocket to be around 0.040" larger than the bolts. That sounds reasonable. (I swagged the size of the bolt circle.)

Ed

[TriumphV8]

First of all you must find true TDC and mark it.

I do that always and mark on the flywheel.

 

I am not confident in the pulley.

Does not make sense to fight for the last degree if the rubber on the pulles lets it move.

 

Advance the cam will add low end, retard adds top end performance.

+/- 1 degree from my view is fully okay and might be better than works tolerances.

As mentioned above the chain stretches and so I am friend of cam advance.

[jpmf]

Thank you gents, I will go with a small bit of advance and if Triumph built them with the same degree of accuracy then I'm happy with that.

Got my TDC with the head off and then fitted the pulley, which was in fairness on the mark.

So it will all get bolted up later.......

[Tom Fremont]

15 hours ago, ed_h said:

 

Getting +/- 2 degrees from the bolt clearance would require the holes in the sprocket to be around 0.040" larger than the bolts. That sounds reasonable. (I swagged the size of the bolt circle.)

Ed

What value did your SWAG generate? I'm thinking 1-1/4" or less. I suppose you factored in the 2:1 sprocket ratio too.

 

Tom