Hunting for TDC

[bosco659]

View attachment 183292 With winter upon us and Covid restrictions, what better to do than tear the bike apart. Since I purchased the bike I’ve always questioned the location / accuracy of the timing marks that were placed by the po. Anyway, an excuse to buy a rotor puller for the installed PMA. Bought a Motion Pro puller and was impressed by the quality of the tool. I pulled the rotor (hope I’m calling it by the correct name) and was pleased to see the key was still in the crank to positively locate the rotor on the shaft.
To properly find TDC I needed a timing degree wheel. Looked on line and decided (cheaped out) I should make something up on my own, since I would most likely never use it again. Looked in my toolbox and found an old 6” Staedtler Mars protractor that I would probably never use again (but know that now that I’ve hacked it up I’ll need it tomorrow . Decided to mount it to a CD and couldn’t find any new ones in the house, so poor Jethro Tull was sacrificed. A bit of Krazy glue, 3 rare earth magnets and my makeshift degree wheel was born. Will try it out tomorrow.
Maybe if I leave it on the rotor and shoot it with a laser I’ll be able to listen to Jethro on the road
We all like pics so here we go.

 

[Jim]

I heard tell you was thick as a brick. Guess not.
Well done.

 

[mike in idaho]

If,some day, you want a permanent scale on that rotor here is template to use:https://www.blocklayer.com/degree-wheel.aspx just plug in the hole size and diameter figures and hit print.

 

[bosco659]

If,some day, you want a permanent scale on that rotor here is template to use:https://www.blocklayer.com/degree-wheel.aspx just plug in the hole size and diameter figures and hit print.

That’s a great link to keep. May come in handy for the car this spring too. Thx.

 

[bosco659]

When I put this back together, the rotor nut torque appears to have a number of suggested torque specs. I believe the factory spec is 50 - 54 ft lbs but I was just reading last night on Hugh’s Hand Built instructions, to use 39 ft lbs. The other interesting point was in the instructions I read, it said to pitch the woodruff key for the rotor (gggGary advised me of this in another thread). I’m going to leave it in.

So should I stick with the factory spec of 50 - 54?

 

[5twins]

Can't say about the torque but why would it be suggested to pitch the key? Seems to me you want it there to keep the rotor from shifting position and throwing the timing marks off, your means of accurately checking it.

 

[bosco659]

Can't say about the torque but why would it be suggested to pitch the key? Seems to me you want it there to keep the rotor from shifting position and throwing the timing marks off, your means of accurately checking it.

Not sure why, didn’t make sense to me. They seemed to rely on the taper of the shaft to prevent rotation. I saw a calculation of the applied torque on the rotor head while generating 100% of the PMA’s output and the writer suggested that the rotor would never slip on the taper. For me, I like the positive location provided by the key, especially since I will be going to great lengths to find true TDC.

 

[Jim]

So should I stick with the factory spec of 50 - 54?

Manuals seem to be all over the place. My 78 manual calls for 29 ft-lbs on the rotor. Bearing in mind that's the end of the crankshaft.... wouldn't want to booger it up. I torque mine to 29.

 

[Jim]

 

[5twins]

I know the key was eliminated or modified on some PMA kits because they either didn't have a matching slot for it in their rotors, or had no slot at all.

 

[bosco659]

Thx. I think I’ll play it safe too and do the 29 ft lbs to be safe. I’ll add a bit of thread locker too. When I removed it it didn’t seem too tight.

Edit: the nut for the rotor is 19mm. That could suggest that it’s a 14mm shaft. The 14mm torque spec is 50-54. Just checked and the shaft is 12mm. Glad I asked and checked. Could have ruined the end of the crank!

 

[mike in idaho]

That's a tapered shaft, get too carried away with the torque figures and it will be EXTREMELY difficult to remove later.