Cam/Crankshaft Timing

[Lessen]

Ok, so I just got my little four-stroke dissembled so I can send out my crank to be modded and then have everything cryo-treated. Well, I had been paying pretty close attention to how everything was going together but just as the cam dropped out of it's sleeve in the block I realized that it's pretty darn important to know how that thing was in there relative to the crankshaft. I got the easy part figured out... which end goes in first. But as far as it's rotation, I could use a bit of guidance. It makes sense to me to reinstall it between the exhaust and intake stroke since I'll be able to see quite easily the timing.

So the cam turns one time per every 2 revolutions of the crankshaft. The cams on the camshaft are a little more than 90 degree offset. So for every quarter turn of a cam, the piston either goes up or down completely. I'm theorizing that the exhaust cam should be at peak contact with the pushrod somewhere around when the piston is halfway up.. I can see that there is a small mark on the side of the camgear. I'm sure the factory uses that as a guide when they are assembling them. I suppose it would be best to contact O.S. on this.. Not something I can guess and check. If I guess wrong I'll be buying a brand new valvetrain.. or engine.

 

[beason]

I've never built an engine but from what I've heard you build the engine with the 1 cylinder on TDC. then all the marks on the crank, cam, distributor, and everything should line up and match.

what kind of engine is this?

 

[Lessen]

It's an O.S. FS26S-C version II.

 

[Rolex]

I'm pretty sure Beason is right. Look closely for the marks. At TDC, you are at the top of the compression stroke, and all valves should be closed.

 

[Lessen]

there is definitely a mark on the the camshaft (actually on the side of the camgear) but I don't see anything that corresponds to it.

 

[Racer 1966]

Beason pretty much nailed for you. On automotive engines, there is a mark on the cam gear as noted and also on the crank gear, usually the cam gear mark is at 6 o'clock and the crank at 12 o'clock with the #1 piston at TDC. Then you drop in the distributor with it ready to fire the #1 plug. I have seen a couple of engines that the cam mark lined up to a mark on the housing also. There has to be something to time it to somewhere.

 

[Lessen]

I'll have to take a closer look, Thanks guys!

 

[Lessen]

Found the answer in that 4-stroke thread over at RCTech. With piston at TDC the mark on the cam should align with the lifters.

 

[Rolex]

Well, that's certainly some uplifting news.

 

[Lessen]

Well, that's certainly some uplifting news.

Thank goodness you only get one birthday per year. :nono: