A question was posed to me that got me wondering. When checking the piston position BTDC with the head off, should it be done with or without the gasket? I've naturally always done it without the gasket. BUT if you do it with the head on, either through the spark plug hole(OHV) or timing plug (SV)....It would then include the gasket.....Hum?:thinking_face: Ron
Before top dead center.
So, the this is the total height the piston moves too before top dead center.
Top dead center is as high as it goes. It's not going any higher.
The number doesn't matter.
What you want is the measurement before.
Head off, is easy and allows the use of a dti.
If you can't reach the piston at right angles though another opening, you're going to have fun.
Sorry though the plug hole on an M20 is only going to get you the top of the valve.
There is a handy little hole in the M20 head.
For a quick check I've used a set screw (threaded to the head) with a couple of nuts on it to lock.
Rotating the crank by hand I find the point where the piston just touches the set screw.
Using the two nuts I lock the position and remove the set screw.
Now measure carefully and move the nuts by the amount you want before TDC
Make sure the piston is away from TDC and fit your set screw.
When you carefully rotate the crank again, it will stop at your points opening point.
Don't forget to rotate the crank in the running direction.
Put the setscrew in your tool bag, it could save you on the side of the road.
Yes of course DOH! I wasn't thinking logically. Wherever the piston is at TDC, you measure down to the top of the piston with a Vernier and then in the case of an M20, add 7/16" to that. I've only been doing it that way for 40 odd years:blush: Ron
