MTFCA Forum

So I was watching some videos concerning the shimming of the magneto and one of the things the guy said was to align the Main Shaft of the transmission and Crank Shaft. Seems like it can go on 180 degrees different. How is this alignment done and why is it important?

If the alignment was made when the engine was assembled and it has not been changed since, then I would agree that the alignment is important. You should mark the crankshaft and the flywheel and the transmission shaft when you take it apart so that you can get things back as they were before. If the alignment was not made at the time the engine was first assembled, it might not make any difference how you put it back because you don't know whether it was before. This alignment will keep the vibrations and wear on the bushings to a minimum and so it is a good thing to do when you rebuild the engine and transmission.
Norm

Sounds like a job for a Floating Transmission Shaft. Dan

“Aligning” the tailshaft is nothing more than indicating it for concentricity to the crank. If running eccentric to the crank centerline, the transmission bushing life will suffer and can also put additional strain on the rotating components. Much like the cam gear, there is no significance to which way it goes on the dowel pins. Simply check the tailshaft for concentricity, and try both mounting orientations to find the best one. I have seen dowel pins out .010”, and so feel this is a worthwhile check

Also, the floating transmission shaft does not correct for the potential eccentric issue at the mounting flange. It certainly will accommodate other types of misalignment tho

All other issues aside, if your engine is on a stand, tip it up nose down and put an indicator on the transmission tail shaft and rotate the crankshaft.
The result will tell you how deep you need to dig into things. A well built engine / transmission will have less than .005" TIR. The Tulsa club web site has some nice videos done by Mike Bender that go into pretty good detail about alignment issues and how to correct problems. I don't agree 100% with whats there, but its a great start.

I agree it needs to be checked but don't agree that using the tail shaft/drive plate is the way to go. I've found that just the brake drum and drive plate has it's own problems for run out.
Do each on own, crank on V blocks, shaft bolted on with it's dowels and use a dial indicator.
The brake drum and drive plate can be rotated each bolt to find the least run out, if you don't have a lathe, do this by rotating on a drive shaft in a vice or even on the shaft when the engine is assembled with a dial indicator, unless all parts have been resurfaced, it's very had to get a zero run-out.

Robert

If your question was placing the trans/flywheel on the crankshaft during magneto gap maneuvers then YES, mark one of the flywheel dowel pins , there are two of them, and strive to lower the same two pins onto the same holes on the crank flange.

Reason.....if you keep putting the flywheel assembly on the same place, you are keeping the gap spaces obtained in the same relation to the interface of the crank flange and flywheel, although only little off, at least you are setting the gap to the same relationship of the crank flange and the flywheel.

Just can't quite see, but there is a white line on the edge of the flange to i.d. the singular mating dowel hole.

Thanks everyone. Unfortunately I did not mark them when I took it apart. Even if I did I had to replace the flywheel, shaft, drums, etc. Everything was balanced and surfaced. I will mark them so as I continue to work the shims I'll put it on and off the same.

Robert, the transmission shaft runout can occur for different reasons. It is best to index the shaft to the crankshaft so that in any operation you are eliminating one variable. With the trans shaft bolted to the crankshaft, check the runout at the crankshaft end. If it is out there, the trans shaft is not concentric with the crankshaft. You may then be able to improve the situation by switching the shaft through 180 degrees and bolting it up again

If the runout is at the back end of the trans shaft, the faces on the crankshaft and trans shaft need to be trued to achieve the best results. Dan's floating transmission shaft will help with this.

Hope this helps.
Allan from down under.

I agree it needs to be checked but don't agree that using the tail shaft/drive plate is the way to go.
I did not mean to imply that checking the tail shaft is the only check to do- only that runout here is indicative of conditions that can and do originate in other area of the assembly. Nothing more, nothing less.

Floating shaft will NOT fix a crank flange that is not true. If the flange is out you need to start there. Dan

Thanks for all the info everyone.