TR2/3/3A OD switches

I am adding an OD to a TR3A tranny. Does anyone have a template showing where to drill/tap the holes for the two OD isolator switches in the tranny top cover? Do you drill in the exact center of the two bosses on the tranny top cover? The is an early A type OD with the large accumulator.
Thnaks, Chuck

I've heard conflicting reports, which leads me to believe the cover casting isn't necessarily all that accurate. Nelson got away with using the center of the bosses, but then had to machine the top of the boss a bit.
https://www.buckeyetriumphs.org/technical/Gearbox/ODSwitches/ODSwitches.htm

Here's a factory drawing (from TR4 workshop manual, which should be the same as TR3)

Thanks Randall. I guessed you would come through!! Chuck

The ones on my TR4 (also type A) appear to be centered on the bosses:

I had mine drilled on center along with the dipstick. No issues.

On my TR3 I had mine drilled per the drawing that Randall posted. The bosses were not quite accurate to the drawing. But the drawing put them in the correct placement.
Regards, Bob

They are actually not centered on the boss, they need to be centered on the shaft that carries the fork. They also need to be positioned such that they engage the fork properly. You need to start from the under side of the cover and drill a small hole that is in the correct position then back from the top to make it bigger.

Any idea why two of the measurements are different, although very slight, between the TR3 service bulletin and the diagram in the TR4 manual? (#'s 1 and 3)
Regards, Bob

TuffTR250 said:

Any idea why two of the measurements are different, although very slight, between the TR3 service bulletin and the diagram in the TR4 manual? (#'s 1 and 3)
Regards, Bob

Click to expand...

Just a quick thought with no real back up info, syncro on 2nd, 3rd, and 4th on the TR3 box vs all four forward speeds syncro on the TR4 box slightly altering positional requirements?

Could be, I guess. But I've been running a TR3 top cover, with original OD switches, on top of a TR6 box for a long time. Seems to work fine.

TR3driver said:

Could be, I guess. But I've been running a TR3 top cover, with original OD switches, on top of a TR6 box for a long time. Seems to work fine.

Click to expand...

Well, so much for the earlier BAT ....

Well, measurement 1 (2.22" vs 2.25") would only affect when the switch closes when going into 2nd gear; and would be compensated for when selecting the shim pack to go under the switch. A small change in measurement 3 isn't going to change anything at all, since the shift rail is much wider than the tip of the switch plunger.

So I suspect there is no systematic difference, just two different sets of measurements (using different reference points). As I mentioned before, there seems to be a fair amount of variation in the castings, so one (or both) sets of measurements may have been affected by, say, the bolt holes not being in exactly the same place.

BTW, I should have said "original OD switch holes" above. The switches themselves have been replaced multiple times.