TR2/3/3A Differences in Distributors?

[karls59tr]

I have a couple of TR3 Distributors that I am taking apart. In terms of the vacuum unit fitted to a distributor what would be the difference between the vacuum units and would I be able to install one of these distributors in a TR4A? One is marked 40480 A which according to the Vic Brit catalogue is for a TR3. The other one is 40698A and is for the 3A. The 4A is 40795. Also what's the best way to get the driving dog pin out of the shaft? Can it be tapped out with an appropriate punch? thx Karl

 

[TR3driver]

The vacuum advance units should interchange physically; but each has a different advance curve and I don't know which one would be most appropriate for a 4A. The specs should be stamped on the body somewhere, as three numbers, like 7-14-10. The first number is the vacuum where the advance starts to move, in inches of mercury. Second number is vacuum where it reaches full advance. Third number is the amount of advance, in camshaft degrees (so double the number to get crankshaft degrees). According to my notes, a 40795 should have a 2-6-3 advance. A more aggressive vacuum advance might produce part-throttle pre-ignition (which would be a Bad Thing), but maybe not. (Depending somewhat on engine condition, modifications, fuel, even altitude etc.)

The pin is tapered (or at least was originally). Look carefully at the ends, you should be able to see the larger end. It should tap out from the other end. Probably a good idea to mark the relationship between the dog and shaft. It's only supposed to fit one way (the holes are slightly off center). but I have seen them forced together wrong (probably even did it myself once or twice).

 

[karls59tr]

So other than the vacuum advance all three of the distributors are the same? I could put a 40795 vacuum unit on one of the other distributors and put it in the 4A correct?

 

[TR3driver]

No, the centrifugal curves are different as well. Same caveat as before, might work OK, might not.



Pre-ignition at cruise particularly worries me. That is the most likely cause of this broken piston, and yet I never heard it over the usual wind noise and so on. First notice I got was a cloud of blue smoke chasing me down the freeway !

 

[TexasKnucklehead]

Also, be aware that the shaft can be bent easily. I bent one driving out the pin. One check for a bend is to check the point gap on each of the 4 lobes. They should all be the same.

The bottom plate where the weights attach can have different holes, changing the amount of centrifugal advance. I think there is a number stamped on it.

 

[karls59tr]

Also, be aware that the shaft can be bent easily. I bent one driving out the pin. One check for a bend is to check the point gap on each of the 4 lobes. They should all be the same.

The bottom plate where the weights attach can have different holes, changing the amount of centrifugal advance. I think there is a number stamped on it.

I noticed that the weights themselves have numbers. Would the weights be different from dist to dist to aid in the centrifigal advance or is it like you say determined by the location of the holes?

 

[TR3driver]

The listing I have shows different weights between 40480A and 40698. Not entirely clear, but it looks like the weights have different effective masses, so swapping weights would change the advance curve.

But usually, the springs determine the curve, and the lower arm determines maximum advance by how far the arm sticks out. The document at https://www.triumphowners.com/uploaded/34/111457_86lucas_dissy_tuning.pdf shows a selection of arms for comparison. Watch out though, I have seen arms that have been modified to allow more advance, without changing (or crossing out) the stamped number.

 

[karls59tr]

The listing I have shows different weights between 40480A and 40698. Not entirely clear, but it looks like the weights have different effective masses, so swapping weights would change the advance curve.

But usually, the springs determine the curve, and the lower arm determines maximum advance by how far the arm sticks out. The document at https://www.triumphowners.com/uploaded/34/111457_86lucas_dissy_tuning.pdf shows a selection of arms for comparison. Watch out though, I have seen arms that have been modified to allow
more advance, without changing (or crossing out) the stamped number.[/
Thanks for the info Randall. Seems there is a lot more to it than I thought. I'm just going to keep one of the TR3 distributors as a spare for the 3 and I'll use the existing dizzy in my 4A after I check it over to see if everything is functioning properly. Good to know that because of different advance curves you shouldn't swap one in that's not appropriate for the engine.
I did send my 3 dizzy to Advanced Distributor years ago. He requested the mods to the engine for the rebuild. I told him about the header and street cam I had on the 3. I guess he takes all that into consideration in terms of springs and advance curves because after I installed the dizzy the 3 has been running great ever since.

 

[charleyf]

So other than the vacuum advance all three of the distributors are the same? I could put a 40795 vacuum unit on one of the other distributors and put it in the 4A correct?

I found that the offset drive dog of the early 40482 unit was opposite that from the later units that fit on the TR4/4A's. If this is true on your units then you would need to reorient your spark plug wires on the distributor cap by 180 degrees.
Charley

 

[karls59tr]

I found that the offset drive dog of the early 40482 unit was opposite that from the later units that fit on the TR4/4A's. If this is true on your units then you would need to reorient your spark plug wires on the distributor cap by 180 degrees.
Charley

One more reason I'd better stick with rebuilding the stock 4A unit.:smile-new: