Reassembling 4 Strokes Cam Gears
 

Hi, I recently reassembled my first 4 stroke - a Saito 72. The engine is fine, runs as good or better than before.

My question is about engine reassembly, specifically, the alignment of the cam gear. The instructions are as follows:-

"Assemble the cam gear lining up the timing mark at the “6 o'clock” position. The crankshaft must be positioned at the “12 o'clock” or “top dead center”

How accurate can/must I be?

First, in getting the crankshaft to TDC and second, in setting the cam gear bench mark in the correct orientation. Is a visual alignment sufficient? Do I need to think about making/purchasing some tool? I tend to work slowly and can be pretty careful with these things but it occured to me that even with my best efforts, I might still be off. Is this something to worry about? Or perhaps the gear teeth do not permit the kind of resolution that makes all of this a real problem?

Thanks!

RE: Reassembling 4 Strokes Cam Gears
 

The gear teeth control the resolution. You must get the proper teeth to align.

You already have the proper tool at hand. Use a pushrod stuck into the intake lifter hole. It will engage the oil hole on the intake cam lobe and hold the cam at the 6 O-clock position

RE: Reassembling 4 Strokes Cam Gears
 

If you are off by one tooth, your engine will run like crap. A good double check is to see that the middle of the intake/exhaust overlap period happens right as the piston reaches top dead center. On almost all four stroke engines, the intake valve starts to open before the exhaust valve is completely closed. The short period where they are both simultaniously open is called the "overlap" and it is an exellent reference point for engine timing and if the timing is off by even one tooth, it will be obvious.
Remember, the piston reaches the top of it's stroke in the middle of the "overlap" period.

RE: Reassembling 4 Strokes Cam Gears
 

Thanks w8ye, that sounds like a more scientific method than simply eyeballing it! Will try the pushrod method to confirm that I've got the timing right.

BLE, thanks for answering my next question - what if I'm off?

Your explanations are a great help to me. Don't think I would dare dismantle my engines but for the great support this forum provides. Thanks!

RE: Reassembling 4 Strokes Cam Gears
 

This pushrod trick in the intake lifter hole will not work on the "Big Block" engines such as the 120, 150, 180, & 220 engines for they have "mushroomed bottomed" lifters that cannot be installed or removed with the cam in the holder.

On these engines you hold the cam gear in position with an Exacto blade jammed into the gear teeth on one side while you insert the cam assembly onto the crankcase.

I have just free handed the timing and then checked myself with the above mentioned "overlap" method.

RE: Reassembling 4 Strokes Cam Gears
 

In case you are worried that the "overlap" method of checking your engine's timing is not accurate enough, let me state that if you are off, you will be off by an entire tooth on the gear. I'm not sure about the tooth count on Saito gears but on a small motorcycle engine I worked on, the crank had 18 teeth and the cam had 36. On this engine, each tooth is 20 degrees of crank rotation, so if it is one tooth off, it will be 20 degrees off, easily discernable by the overlap check.
I have used the overlap method to check automobile engines for jumped timing several times and I always use it as a double check when assembling car, motorcycle, and model airplane engines.

RE: Reassembling 4 Strokes Cam Gears
 

Fish:

The Saito engines will run with the cam off one tooth either way, but when advanced the idle will be lousy, when retarded the top end rpm will be down. So, if your engine is running well after reassembly, you got it right.

Bill.

RE: Reassembling 4 Strokes Cam Gears
 

I pack the cam housing with petroleum jelly to hold the cam in place. When the engine heats up, it melts away. Seems to work reasonably well to get the timing correct.

RE: Reassembling 4 Strokes Cam Gears
 

Thanks guys. Great info and good tips. I really appreciate the info as I do like to check my work after I think I'm done.

My dad was trained as an engineer but I've got absolutely no background with these things. With the amount of reading I've been doing, it's sorta like going to school again; really wished they had taught me stuff like this instead of all the sillier stuff that's since gone unused!

Thanks again!