9 replies to this topic

[colinu]

Folks,

Engine rebuild is half way through and I'm in the process of fitting vernier timing gear (Swiftune alloy/steel). I'm using a steel rule as a straight edge to see how much I need to shim the gears to get them aligned correctly. Normally the crank sprocket would sit low (as it did with my simplex gears) and would need shimmed out to make it align with the cam sprocket... but in this case the crank sprocket is sitting 15thou high.
Am I right in thinking it's impossible to shim the cam sprocket (depending which way you shimmed it you'd either end up with too much cam endfloat, or zero endfloat and a locked cam)? If so then is getting 15thou machined off the nose of the crank sprocket a sensible way to get correct alignment?

Cheers,
Colin.

[colinu]

Dropped the crank sprocket off at the machine shop this morning. The sprocket is damned hard - too hard for normal milling etc tools. They are going to do either a precision grind, or stick it in a lathe with a funky ceramic cutting tool!

[bmcecosse]

Most unusual problem. You are definitely lining up the teeth on the two sprockets ?

[colinu]

Yes, very unusual. To double check that I hadn't done something dumb during the rebuild I tried fitting the old simplex items... and they were as expected (i.e. crank sprocket needs slightly shimmed). It's almost as if the Swiftune sprocket is slightly out of tolerance.

Next question... when I set the cam timing should I offset by a couple of degrees to account for chain stretch during run in, or is a duplex chain unlikely to stretch much?

[Ethel]

It's not the oil pump pushing the cam across? Also, does where you're checking the alignment correspond the same to the actual sprocket teeth on both gears?

[dklawson]

Next question... when I set the cam timing should I offset by a couple of degrees to account for chain stretch during run in, or is a duplex chain unlikely to stretch much?

Most people on this board suggest doing just that... 1-2 degrees more cam angle to account for the initial chain stretch.

[colinu]

No it's not caused by the oil pump as it's not fitted yet. Checked it with camshaft at either end of endfloat - makes little to no difference. I've checked my alignment method (removed sprockets and measured to make sure the teeth are central between sprocket teeth faces - and they are).

Cam timing - so if cam is spec'd for 106deg do I set it to e.g. 105 or 107 to compensate for chain stretch?

[Ethel]

It depends what you're timing it off, btdc inlet or atdc exhaust. I don't think it really matters, unless it's a scatter cam, improving the timing of one pair of cylinders will most likely degrade the others on the siamese ported A Series.

[colinu]

Cam is an AC Dodds ACD-RS+... which should be 106deg ATDC exhaust

[colinu]

... But to get that cam working correctly I need to lower my compression ratio a fair bit (currently 11:1, needs to be 10:1). Will get a suitably thicker gasket, but off the shelf ones are 73mm diameter - my block is a 73.5mm 1380. Do I need to get a custom 73.5mm gadget made or will I be OK using a 73 and having a slight "overhang"?