Cam sprocket alignment

[stebbs]

I understand where the line on the sprockets should be pointing at TDC. My issue is that I had a broken camshaft and am not sure how to position the sprocket on the camshaft. There are 3 holes on each sprocket and now I'm second guessing myself which hole should be used to position the sprocket properly. As I have had both sprockets off to fit the chain, I made the classing mistake of not taking enough pictures. Don't want to bust up my valves or break another camshaft so does anyone have a picture of the camshaft position while at TDC? That way I can approximately figure out which hole to use and then line it up correctly.

[74Twincam]

I think it depends-
The dowel pins only fit the center hole in each sprocket (you can verify this with a quick measurement). So long as you didn’t mix up the intake and exhaust sprockets (hopefully they are/were marked), it should be straightforward.
However, do you have offset dowels to locate the cam sprockets for correct cam timing? With a new timing chain and cam I’d be inclined to degree the cams again to be sure, as the scribed timing marks on the sprockets are a close reference point but not 100%. I’m not sure re-using the same dowels is best practice without checking first.

- BIll

[alan.barker]

It depends if the Cams are old stock or as original . Not reprofiled. If you are sue original stock no problem with straight Dowels.
If any doubt set up with Degree Wheel as Bill said.
Alan

[512BB]

I don't know if this picture is helpful to you. The engine is set to TDC. Your ignition timing will almost certainly need to be reset. Remember that you replace the distributor with the crank pulley set at 10 - 12 degrees on the compression stroke, with the rotor arm pointing to no.1 in the dissy cap.

Good luck with it.

Leslie

[Concrete-crusher]

If you check miles Wilkins book it shows where the cams are pointing at TDC, that gets you in the correct position, then use a degree wheel to get mop right on number 1 inlet then exhaust

It's much easier with adjustable sprockets and makes a huge difference

Steve

[Elan45]

If you do not degree the cams w/ a degree wheel, You need to make ceertain the sprockets should be marked IN for the intake cam and EX for the exhaust. The only difference between the two marked sprockets is about 1 tooth difference in the timing mark location on the edge of the sprockets.

Roger

[stebbs]

this picture is exactly what I was looking for. It certainly looks like the center hole is correct but I bought a used replacement cam and it is not quite fitting that hole. I will compare this picture with mine today and make sure it's the same. My worst fear is to gt it all back together only to damage the valves and head and who know what else because of misalignment. I'll take my time and hopefully I'll get there.
Thanks

[512BB]

Here is another picture that I took just after removing the cam cover, to see how accurate the marks on sprockets were. As you can see the marks lined up pretty well, so I knew how they had to go back. As seasoned twin cam tinkerers will know, sometimes the marks do not line up well at all, but moving the chain a tooth makes it worse.

When you have fitted the cams and sprockets and you are happy with the marks, and set the tension, rotate the whole assembly with a socket on the crank very slowly, obviously with the plugs out. If you have managed to get things wrong, so long as you are rotating carefully, a piston will come up and just touch a valve, and you will not be able to turn any further, but no damage will have been done, and you just back off the crank and investigate. As you can see in my previous picture, I place wooden dowls of equal length down the plug holes so that I can see at a glance where each piston is in the cycle.

Once you have reassembled and you are happy, I always turn the engine over a couple of times and observe the marks, just to be sure. Then you can go ahead and set the ignition timing.

Leslie

[2cams70]

I bought a used replacement cam and it is not quite fitting that hole.

If you bought a used cam anything is possible. Do you know if it has been reprofiled during it's history? If it has or the cam is non standard the alignment marks will only be a rough guide to setup and you should time the cams properly with offset dowels or vernier sprockets.

If the head has been skimmed during it's history this will also affect the cam timing and hence how the original timing marks line up.

[74Twincam]

As an added suggestion, once you’re finished setting everything, (whether you use a degree wheel or get lucky gambling without) I add a stripe of paint (white) to the true location on the sprockets to the horizontal surface of the head. The stock marks are close as stated but rarely even and parallel, the paint becomes my new true reference point for the next time maintenance is needed.