XR75 Lower timing sprocket: Opinions wanted.

[Uncle_Gnome]

Hello,

I recently purchased a timing chain and upper & lower timing sprockets for my nephews 1974 XR75. The engine is really clean, with a relatively fresh top end and brand new valve job. So I think it is worth at least a little effort.

My questions are regarding the lower timing sprocket. In researching the site I have learned that it needs to be pressed off/on at a shop. On this crank the key-way lines up with one of the valleys on the sprocket. In another thread however someone questioned this and said they had seen them both ways.

In the past it seemed I could not get my valve timing exactly the way I though it should be. It wanted to go either too far one way or the other, as if it was out by a half tooth. How do I know that the sprocket has not slipped in the past? Which way is it supposed to go? Line up the key-way with a valley or a tooth?

And lastly, assuming it is located properly, how bad does it need to be before it should be replaced? I can see some wear such as you would see on a worn drive sprocket but it is not extreme by any means.

I have the crankshaft removed and the new sprocket on hand but I don't know which way to jump with this deal, have it pressed on unsure of the orientation or reassemble it as it is. Any input you guys can give me will help immensely.

[Old School Al]

This one slid off the top page....................anybody going to take a shot at it? Or should I make something up.................?

Old School Al

[Old School Al]

I thought it was pretty much standard for a tooth to be lined up with the key way on all models, not positive though. Don't remember this ever being addressed in any of the manuals either, may have been in service schools. What I think I'd do is slip the crank in the cases with the new cam chain. Then add the cyl with tensioners, head and cam with new sprocket. Want the gaskets in place also, or something the same thickness. Adj chain, and see how far off you are, and where you need to be. Keep in mind how ever far you move the lower sprocket will only move the top half as much. Reason being of course, the cam turns at half crankshaft speed. This would be the low tech way, high tech way with a degree wheel. I'd go low tech myself. I'm sure things vary a bit from from engine to engine as quality controll is a bit loose. Differences in distance in from camshaft center line to crankshaft center line would be the one that would affect mark alignment on the top sprocket. Or cam chain length differences. Just like a worn streched cam chain would, retarding cam timing!

We had this same problem on the Old School XR engine when we put it together, lower tooth off a bit. We used the megacycle adj top sprocket to take care of it. Went the low tech road on it also.

On wear of the sprocket, top always seemed to wear the worst. If you see much wear I'd replace it. Mixing new with worn is not good with chain and sprockets.

I told you I could make something up..................!!!

Hey...............where's our "XR75 guru" been hiding on this one? Anyone seen him???

Old School Al

[Uncle_Gnome]

Al,

Thanks for taking the time to put together that response.

I thought it was pretty much standard for a tooth to be lined up with the key way on all models, not positive though. Don't remember this ever being addressed in any of the manuals either,

Mine is definitely lined up with a valley. I have two manuals and neither mention this aspect.

So in a nutshell you suggest that I temporarily assemble the engine to determine how much to move the lower sprocket when it is finally replaced. Hell I just took the whole thing apart, getting pretty good at it by now.

We had this same problem on the Old School XR engine when we put it together, lower tooth off a bit. We used the megacycle adj top sprocket to take care of it. Went the low tech road on it also.

I take it that this is nothing more than a sprocket with another set of bolt holes slightly out of phase with the first.

On wear of the sprocket, top always seemed to wear the worst. If you see much wear I'd replace it. Mixing new with worn is not good with chain and sprockets.

Yes the top is worn more than the lower, the lower is not truly hideously worn, but it is noticeable.

Thanks again.

[klxd]

It seems to me that as long as you line up a tooth or a valley it will be ok since the cam sprocket has twice as many teeth.

EDIT: This is incorrect, I see it now. I should probably delete it so I don't look so dumb.

[Uncle_Gnome]

Klxd,

Thanks so much for that. Even though that is incorrect, it got me thinking and led me to figure it out for myself.

By letting the crank hang by the connecting rod (TDC) it showed me that the keyway is in the 12 O'clock poition. I then drew a vertical line on a piece of cardboard and assembled both sprockets to the chain and lined everything up on the line I drew.

The "O" on the top of the sprocket is positioned at a "valley". The only way that the "O" would align with the vertical line I drew was if the bottom sprocket was also positioned on the line at a "valley". No real surprise I suppose that the way it is now is the way it should be.

[klxd]

Thought about it some more. I still think I'm right. You say you have to line up a valley with the key. If you line up a tooth, that's equivalent to 1/2 a tooth's worth of advance, let's say. Since there are twice as many on the cam sprocket you should be able to retard the cam one tooth to compensate.

EDIT: Still wrong.

[Old School Al]

Thought about it some more. I still think I'm right. You say you have to line up a valley with the key. If you line up a tooth, that's equivalent to 1/2 a tooth's worth of advance, let's say. Since there are twice as many on the cam sprocket you should be able to retard the cam one tooth to compensate.

The top sprocket does have twice as many teeth as the bottom, but only turns half the speed due to having them. If we move the bottom sprocket a 1/2 tooth that would only equal 1/4 tooth movement at the top sprocket with a 2-1 ratio. (Camshaft turning half of crankshaft speed.) If we only move the bottom a 1/2 tooth, we can't make a 1/4 tooth adjustment at top, and this is where we have the problem I think.

Am I thinking this out right.........? I wish I had some parts in front of me!

Old School Al

[klxd]

Well, when you look at it that way I think you're right.

[dirtbkr188]

Well, here goes nothing...

As it stands right now, the cam sprocket is the constant; the least amount of adjustment you can do at the top is one tooth either way. Down below, the crank sprocket is the variable; it is pressed into place and can be moved a 1/4 or 1/2 tooth either way, to get it to align with how the chain falls into place around the crank ear, if that makes sense.

My suggestion, for what it is worth, would be to heat up the crank sprocket, and see if you can slide it into place on the crank without pressing it on. If so, assemble the complete engine like Old School Al said, warm up and slide on the sprocket, and set it where it needs to be. Let it cool down, and then break down the top end enough to install the cam chain in place.

[Old School Al]

I would tend to think U/G should really be able to figure it out with the sprocket that's on the crank now. Should be able to figure out how far off it is pretty darn close, and know where to position the new one.

On the heating the sprocket, it would be easy to over heat and affect hardness, as it would have to be HOT. Also as little mass as it has, I think it would tend to lock instantly. As fast as things would happen would be hard to align it I think, maybe lock before it was clear on also. I kind of think I tried it once years ago and it didn't work to well. Not sure though.

Old School Al

[Old School Al]

It seems to me that as long as you line up a tooth or a valley it will be ok since the cam sprocket has twice as many teeth.

EDIT: This is incorrect, I see it now. I should probably delete it so I don't look so dumb.

Nope, don't look dumb at all to me...........................you were just thinking things over out loud! Nothing wrong with that at all! It's been a interesting topic to discuss.

Old School Al