Distributor interchangeability

[bigjones]

The latest VB catalog shows the same 45D unit (8-2070) as a direct replacement for all MGB's, MG Midget (948 through 1500) and TR 2,3,4.

This seems strange as surely the vacuum advance and centrifugal weights would change with engine size, etc.

They do mention, for the MGB, which had 9 (!!) different versions of the vacuuum advance, that you may need to purchase the correct advance, although you will be OK 5 out of 9 times. (Difficult to make out what they talking about so I may be wrong on this.)

This 45D is a Lucas unit and comes with the vacuum advance and (push-in) cap, all for $89.95, on sale. A good deal I think but can it really be used for all these different cars. I know it would physically fit but would it be suitable?

Cheers!

 

[TR6oldtimer]

I doubt it. As you pointed out, minimally, the centrifugal advance and weights would vary from car to car. Years ago I bought a TRF rebuilt distributor. Looked the same on the outside, but the mechanical advance was four degrees more then stock. I tried it on the car, then back into the box it went.


In my opinion, you are better to deal with firms like Advanced Distributors who know the system and stand behind their work. (which by the way is excellent)

 

[TR3driver]

They do mention, for the MGB, which had 9 (!!) different versions of the vacuuum advance, that you may need to purchase the correct advance, although you will be OK 5 out of 9 times.

Wow, maybe they are getting better. Used to be, they didn't even know that much about what they were selling!

It's fairly easy to come up with a curve that will work for multiple engines. Making it the optimum curve is a totally different question. But no way is the original curve still optimum for your engine and fuel today anyway, since our fuel is so drastically different than it was. Actually, they weren't even optimum originally, as they had to cover a wide range of possible conditions, including ambient temperature & humidity, different fuel formulations, carbon buildup in the combustion chamber, altitude, etc.

 

[rikiwho123]

back in the early 70s i needed a dist. for my GT6 there were no catalogs you went to a junk yard as i did ; i found a TR6 dist. bought it ;took it home pulled it apart and used the working parts from my GT6 ; and it ran better then ever; so you never know try a couple of junk ones before dumping alot of money.

 

[bigjones]

Thanks for all the comments.

I only posted for theoretical discussion - my only practical interest is in carrying a spare distributor. The TR3 has a spare (untried) DM2 in the trunk and I tried it out yesterday on the 1500 Midget. It didn't like it - bogged down at the higher revs but perhaps this is due to some other issue(s) with the MG.

Not a wasted experience though. This DM2 has been in the trunk for ages (got it off Ebay as a spare - looked fine inside) When I first tried it in the MG it wouldn't even fire up. Got to look at the points - totally roasted. Ha! Replaced those and then it was OK, got me to the grocery store and back but something ain't right, as noted above.

Cheers!

 

[TR3driver]

Just as an experiment, I once installed the dizzy from a MG Midget (year, etc unknown) on my TR3A. Ran about the same as the one I was using before, so the Midget dizzy ($10 at the local junkyard) became my onboard spare (in the center of the spare tire so it didn't waste trunk space). Never had a need for it myself, but I wished I had it along when a fellow club member's TR4A died on the way home from VTR 2000.

 

[Andrew Mace]

Similar to Randall's experience, I once took a distributor from a Herald 1200 and installed it in a friend's MGA 1500 after his distributor seized. Worked just fine! Not to say it was necessarily optimal, but no significant problems!

 

[bigjones]

...but I wished I had it along when a fellow club member's TR4A died on the way home from VTR 2000...

Exactly. Plonking in known-to-be-good dizzie can eliminate a bunch of possibilities when it comes to roadside analysis. (Ensure you have the right cap (and leads) for the spare - the TR3 cap, for example, doesn't fit the 1500 dizzie)

OK, so you can get home on another dizzie but not to be recommended for everyday use. Fair enough.

Cheers!

PS. Put in the regular dizzie for the Midget and the car runs great - so it definitely does not like the DM2 I put in as an experiment.

 

[TR3driver]

Plonking in known-to-be-good dizzie can eliminate a bunch of possibilities when it comes to roadside analysis.

Of course, sometimes that strategy can backfire. Many years ago, I bought a TR3A that had basically been driven until it wouldn't run any more, then pushed into a field and left. (Former owner had moved away, I bought the car through his sister.)

First thing I did was to swap in the "known good" battery, plugs, cap & wires from my Dad's TR3A, just to get it started so I could drive it home. We spent the better part of 3 days out there, standing in snow to our ankles, trying to get that thing to run. Even almost had to walk home, because we ran down the battery in my Oldsmobile from supplying the juice to crank the TR over and over and over. (Fortunately for us, shaking the battery from the Olds produced just enough juice to turn over the engine once, which was enough for the warm engine to light off.) Eventually, we gave up, and towed the TR to my buddy's house on a rope. After dark, no lights, no heater (of course) and I about froze stiff piloting the TR.

Next day, it finally dawned on me what the problem was : Dad's TR had the distributor drive gear installed wrong, and someone had compensated by moving the plug wires around! By installing the cap & wires as a unit, we had made the timing off by 180 degrees!

Knocked the spiders out of the original cap and popped it on; the engine fired on the first crank!

"Good judgement comes from experience. Experience comes from bad judgement."
:cowboy:

 

[bigjones]

Ha! Good story there Randall!
It's never easy - otherwise everybody would be doing it.
Cheers!