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TR2/3/3A Positioning new cam sprocket

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bobhustead

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Following my thread about radiator removal to get into the timing cover, I now own a new cam sprocket and timing chain. The new sprocket is unmarked. The cam and crank are in the correct positions (aligned timing marks with #1 TDC on compression stroke). I want to mark the new sprocket off the old one. To do this, I have stacked the sprockets, put a snug spindle in the center holes, dropped 4 bolts in the holes and jumped 1 bolt hole at a time until the teeth on both sprockets are in perfect alignmment. I then pen marked the new sprocket at the mark on the old sprocket and when I test fit the new assembly visual alignment is correct and the cam sprocket bolts thread right in without moving the cam. The question arises because when I turn the new sprocket over and repeat the spindle and bolt insertion process, I find that there is also a place 3 teeth over where the sprocket tips seem to correspond exactly and the dry fit also aligns perfectly with the bolt holes in the cam. There are 42 teeth and 4 bolt holes on the sprocket and 2 bolt holes in the cam. I feel like, if I had not dropped trig (or had not been sitting next to Tammy in geometry), I could figure this out, but does anyone know if there indeed is a correct position on both sides of thie sprocket.
Thanks, Bob
 
Even teeth and even bolt holes means it would also line up 180 degrees off. But after the half, there are 21 teeth in between, and even bolt holes (2), so there are no other positions that would align. So, just 2 positions...and that is also why the cam has to also have a mark to prevent setting it 180 out
 
The holes are supposed to be drilled such that flipping the sprocket over produces a 1/4 tooth change in timing; while changing which pair of holes you use produces a 1/2 tooth change. I've read before of replacements not being drilled properly, so that the 1/4 tooth adjustment is not possible.

As long as the valve timing comes out close to right (within a couple of degrees), I wouldn't worry about it. But frankly, I also wouldn't trust that it was set correctly before. I would recheck using either the method in the book or something similar to the method Macy gives. (For example, using the "on rock" method to find TDC on the cam, but a piston stop to find TDC on the crank.)

As a side comment, both sprockets should be replaced as a set (along with the chain). If there was wear in the old chain, the sprocket teeth will have worn to match, giving a slightly longer distance between the teeth. That means all of the driving force will be concentrated on the first link to catch the sprocket, instead of being spread along multiple links; and cause the new chain to wear rapidly until it matches the old, worn sprocket. At least that's the theory as explained to me.
 
Thanks for the information. The existing marks were good enough for 3000 miles of good running since the rebuild. As for the crank gear, Randall, you are, of course correct. My initial decision not to change it was inertia coupled with rejection of the standard model of reality and substitution of my own. I ordered a new one from Moss just now, so I can get it buttoned up on Thurs.
Thanks, Bob
 
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