Flywheel marks

82vdub · Post by **82vdub** » Thu Mar 06, 2008 4:31 pm

If you look at the two posted pictures above your posted pictures, both those marks appear to be machined into the flywheel after it's manufactured, not during manufacture or else the edges would be more rough.

Post by **Fatmobile** » Fri Mar 07, 2008 3:13 am

The one with the paint looks like TDC to me.
I've seen flywheels with the TDC mark (almost allways with the teartdrop cast below it), that also had other marks to the left.
The other marks had numbers above them indicating timing advance.... an '84 flywheel.

Greenmachine · Post by **Greenmachine** » Fri Mar 07, 2008 11:41 am

Damn I was hoping for a consensus!

idhoho · Post by **idhoho** » Fri Mar 07, 2008 3:08 pm

Greenmachine wrote:Damn I was hoping for a consensus!

There are a couple of pics of the correct mark~ It has the little zero under the diamond shape. At least, that is the one that I have always used and had successful timing! I did have a p.u. years ago that the flywheel was not aligned properly and I had to make a new mark. Again, if you have any doubt find TDC with a copper wire in the #1 cylinder- poke the wire down the inj. port and feel for the piston at top of it's stroke. Then look at the IP to make sure it is at the mark.... Then you KNOW you have the right timing mark- whatever it may look like!

Greenmachine · Post by **Greenmachine** » Fri Mar 07, 2008 3:21 pm

I tried that test Idhoho, and normally I believe it would be fail-safe. However, both marks are so close, and both are in the TDC area (verified with the rod I used), but there is no appreciable height difference in the rod between the two marks.

The more I look at those pics, the more I think the two marks are identical.

Vincent Waldon · Post by **Vincent Waldon** » Fri Mar 07, 2008 3:26 pm

Unfortunately at the top of the stroke you can move the flywheel quite a bit and not get an appreciable amount of piston deflection, so if your mystery marks are close together the "wire down the injector hole" may not have enough resolution.

One work around is to extend the wire a bit and stop the piston an inch or so before TDC in either direction, scribing marks both times. TDC is exactly in the middle of these two marks, and you've avoided the dead zone right around TDC.

Easier said than done, of course!

Greenmachine · Post by **Greenmachine** » Fri Mar 07, 2008 4:19 pm

Vincent Waldon wrote:Unfortunately at the top of the stroke you can move the flywheel quite a bit and not get an appreciable amount of piston deflection, so if your mystery marks are close together the "wire down the injector hole" may not have enough resolution.

Absolutely, this is the problem.

Vincent Waldon wrote: One work around is to extend the wire a bit and stop the piston an inch or so before TDC in either direction, scribing marks both times. TDC is exactly in the middle of these two marks, and you've avoided the dead zone right around TDC.

Thanks for that gem of an idea Vincent, but I'm not sure what you mean about stopping and inch or so before TDC. Wouldn't I just find the zenith twice (or the highest point before it goes flat) by rotating the engine toward TDC from both directions and then find the center?

Vincent Waldon · Post by **Vincent Waldon** » Fri Mar 07, 2008 5:43 pm

You could try to find the zenith from either direction. but you are still at the top of the crank's throw so subject to alot of error.

However, if your stop is, say, an inch or two below the zenith then you are at a different part of the cranks throw and a long way from the dead zone... much more accurate.

So, you put the piston at BDC and then put your stop (wire etc) so that it's an inch or two into the cylinder. Rotate in one direction till the piston touchs the stop and scribe a mark on the flywheel. Rotate in the other direction... the piston will retreat and then come back up and touch the stop again... scribe a mark on the flywheel. TDC is now exactly in the middle of the two marks.

Clear as mud ???!!!

rallydiesel · Post by **rallydiesel** » Fri Mar 07, 2008 8:33 pm

I recently installed a new pressure plate and had all sorts of timing problems. I eventually I removed the oil pan and found TDC using a dial gauge and found the timing mark to now be off by 3/4". I marked the new position, retimed to that and everything is fine now.

Maybe the pressure plate was not right for this engine but it is something to keep in mind.

Post by **Fatmobile** » Sat Mar 08, 2008 12:13 am

I don't know if stopping the pistons an inch down would work as well as using a fat piece of wire and stopping it close to the top.
An "L" in the wire might stop the piston as it reaches it's apex, as the piston smashes the wire against the head, mark it on the flywheel,... then turn the crank the other way until the fat wire stops it again, TDC will be in the center of these 2 points.

82vdub · Post by **82vdub** » Mon Mar 10, 2008 10:05 am

I just thought of a better way of determining TDC for #1 cylinder. I'm not in a position where I can verify how the crank throws are for these engines, but it's likely that one of the other cylinders will be at BDC when #1 is at TDC. When the cylinder that's forced to BDC forces #1 to TDC (need help here with this one guys), forcing an adjacent cylinder to BDC will force the #1 cylinder to exactly TDC and tell you which mark (if either) is correct for your motor. To force an adjacent cylinder to max BDC, rig up a fitting and use compressed air to force the cylinder to BDC. This will find exact BDC (and thus TDC for #1) for that cylinder.