Lucas 25D distributors

C

Cutlass

Jedi Warrior

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• Mar 19, 2006

• #1

I just discovered that my 66 BJ8 distributor is held together by a generous, and ugly, patch of JB Weld. The local helpful parts store guy fished around in his batch of old diszies, and found a 25D. It's Lucas part no. 41378B, which differs from my no. 40966 in two ways that I can see: 1) The 41378B has an o-ring on the shaft housing and 2) the 40966 vacuum advance has a threaded nipple, as opposed to a plain nipple on the 41378B. My question is whether there are other, non apparent differences that will cause me problems? I can change out the vaccuum advance, and the o-ring will disappear into the block. Am I headed in the right direction?

 

dklawson

Yoda

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• Mar 19, 2006

• #2

Yes, you are headed in the right direction but don't stop with swapping the vacuum canister.

The database compiled by Marcel Chichak only lists the 41378A dizzy (for the Mk2 Triumph 2000) which has only 9 degrees of advance. Your 40966 should have about 18 degrees of advance. If the 41378B has a similar advance curve to the "A" version, putting it in your car would probably make the car feel anemic.

I suggest you disassemble both units, carefully, and separately. You should be able to move all of your old distributor's components to the new housing without issue. Obviously this would be the time to replace any worn components you find and to make sure your car's advance weights are free.

I wouldn't worry too much about the o-ring on the shank. It seems Lucas put them on some models and not others... even when they were installed in the same blocks. I would go to the trouble of moving everything else to the new housing... and re-stamping/marking the donor housing for your advance (40966).

 

OP

C

Cutlass

Jedi Warrior

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• Mar 19, 2006

• #3

That makes a lot of sense, and as I have been patiently waiting for folks like you to lend an ear, I did figure out that swapping all the old innards would be the route to go. Now re-stamping the housing will be a bit more problematic, due to my complete lack of any stamping dies. I'll check around. Thanks.

 

kennypinkerton

Yoda

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• Mar 19, 2006

• #4

Are the timing advance weights and springs different between 25d models? That might make a difference too.

Sorry, I've been the negative thinker all of today /ubbthreads/images/graemlins/smirk.gif

 

Keoke

Great Pumpkin

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• Mar 19, 2006

• #5

Well Cutlass Dough's input seems about spot on.However, I would not shift the Shaft and action plates between the two units. Shift the Weights and springs after giving the replacement unit a good cleaning up and lub.Use the contact breaker plate and the Vacuum advance units from you original dizzy and you should be good to go.-Fwiw---Keoke

 

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piman

Darth Vader

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• Mar 20, 2006

• #6

Hello Cutlass,

a slight conflict of opinion! However, if you do not replace the rotating cam section you will be stuck with the limited advance of the 41378.
For general information, it is possible to modify the advance span of these distributors either by grinding the beak on the cam assembly or sleeving the stop. This will give more and less advance respectively. To modify the advance curve needs different strength springs.

Alec

 

dklawson

Yoda

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• Mar 20, 2006

• #7

As others have said after my first post, don't stop with just the weights, move the cam (the part the rotor goes on) also along with the bob weights, springs, and vacuum canister. That will move all the unique advance components to your donor housing. (If you do this there is no need to grind the stops on the advance cam... this avoids confusion later also. The advance cam will be stamped with a number indicating its degrees of advance. Grinding the cam stops and leaving the old number in place is very misleading).

As for the number on the housing, use a Dremel to gently smooth over the existing numbers on the donor housing. If nothing else, use a vibrating engraver to write the Healey number in or below that spot.

 

OP

C

Cutlass

Jedi Warrior

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• Mar 20, 2006

• #8

Pinman, now that is something to ponder. It seems logical that the rotating cam, while divided into six equal segments, might just be fixed to the shaft in relation to the drive dog a little differently from one model to the other. I guess Keoke may be concerned about switching shafts, because of the mismatch between used bushings. Seems to me that concern might be allayed by having the 41378 re-bushed. Is this sounding better?

 

dklawson

Yoda

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• Mar 20, 2006

• #9

NO. The cam is NOT fixed to the main shaft with the drive dog. The cam is a separate part free (within limits) to move on the main shaft. That's how your centrifugal advance works.

Under the breaker plate the six-lobed cam will have a hook shaped arm with a kind of contact pad on it. The end of the hooked arm acts as a stop to prevent further advance. The six-lobed portion of the cam in integral with the hooked arm stop. Look at the exploded pictures in your manual.

To see what I'm talking about (assuming your manual isn't handy) take a look at the following PDF:
https://www.telusplanet.net/%7Echichm/tech/lucastuning.pdf
This document focuses on 4-cylinder Lucas dizzys but the construction is the same for your 6-cylinder one.

Your best bet is to not muck around. Remove the cam, springs, bob weights and vacuum canister from your Healey dizzy and put them in the donor Triumph one.

 

Keoke

Great Pumpkin

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• Mar 20, 2006

• #10

Yes Keoke, is worried about how one worn shaft is going to fit in another worn shaft's bearing. Keoke is also aware that you can do considerable damage to the shaft's bearing when you pull a shaft out incorrectly. Dough's last sentence is absolutely correct and there is no conflict of information here.---Keoke

 

OP

C

Cutlass

Jedi Warrior

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• Mar 20, 2006

• #11

Thanks wise ones. I'll start giving it a go tonight. I understand about pulling a worn steel shaft through a worn bronze bushing. My experience with these problems tells me that the biggest pain is getting those 40 year old, maddingly small, screws to exit their home without tremendous knitting of brow and gnashing of teeth. I'll let you know how it goes.

 

dklawson

Yoda

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• Mar 20, 2006

• #12

As Keoke said, if the donor dizzy has a shaft and bushings in good shape you may only need to move the top end bits from one housing to the other and be done with it. Good luck and let us know what you find.

 

OP

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Cutlass

Jedi Warrior

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• Mar 20, 2006

• #13

Just as you guys predicted, the Triumph weights are 9 degrees, and the Healey's are 18 degrees. Should be no problem from here. The article Doug posted was great. However, I did notice that the primary spring on the 40966 was not in tension, as the article suggested. It looks a bit worn. Moss shows new advance springs as "n/a." The condition of the springs clearly should affect the advance curve. Is there a source for new, correct springs?

 

6

64 BJ8

Freshman Member

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• Mar 20, 2006

• #14

Hello, Cutlass

I'm not sure about the swap; here's why. To be interchangeable, the distributor driving gear total lengths (in the housing-measured from the bottom of the shaft housing to the tip of the driving dog) must be identical(ie., same depth), otherwise the distributor driving gear will not mesh with the camshaft driving gear- in other words, the dizzy will not turn!

To complicate matters, on the BJ8, the original distributor drive dog has a small coupling (see official BJ8 parts book page MA 12-part no.AEC 243) called "distributor to gear coupling". This small part (which is NLA)is a press fit on to the drive dog end and is essential for all BJ8 dizzys to function, as it adds the exact length necessary to correctly mesh with the camshaft gear. You may try fitting the BJ8 coupling to the 25 D distributor driving dog (if it fits the machined slotted end), and then measure the total length to see if the measurements are identical. If they are'nt, I'd say the 25D dizzy is not useable on the BJ8. Hope this helps & good luck!!

 

OP

C

Cutlass

Jedi Warrior

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• Mar 20, 2006

• #15

Thanks, 64 BJ8. From the housing down, the two distributors are identical, and the 41378 fits perfectly, engaging the cam driving dog just fine. Indeed, as I have been disassembling both, the only differences I've found are in the weights and the vacuum unit. The differences between the cams must be there, but are not readily apparent to the eye. My parts book doesn't have the part you mentioned.

 

Keoke

Great Pumpkin

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• Mar 20, 2006

• #16

/ubbthreads/images/graemlins/savewave.gif
Your parts book does not have that part cuz that part does not exist.---Keoke- /ubbthreads/images/graemlins/nonono.gif- /ubbthreads/images/graemlins/grin.gif

OH an while im in a sarsky mood the primary spring is not in tension when the dizzy is at rest.- /ubbthreads/images/graemlins/nonod.gif-Keoke--AZ /ubbthreads/images/graemlins/cowboy.gif

 

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piman

Darth Vader

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• Mar 21, 2006

• #17

Hello Cutlass,

it is quite common to see the initial advance spring as 'loose' and gives advance to the engine from low revs, the stronger second spring controls the advance curve at a slower rate, i.e it needs more revs to get the bob weights to pull against it to full advanc.

Alec

 

dklawson

Yoda

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• Mar 21, 2006

• #18

Cutlass... the cams are different, you said so in your 5:23 post
"Triumph weights are 9 degrees, and the Healey's are 18 degrees".
The cam is the part with the 6-lobes and the hooked foot. It will be stamped with 9 or 18. The bob weights (the ying and yang, teardrop shaped bits) are probably the same but I'd switch them as a set anyway.

As for springs, they are as you noted, NLA. The only source I know of for any Lucas dizzy springs is Cambridge MotorSport in the U.K. They have a spring set primarily aimed at those custom tuning the advance in TRs and MGAs. The springs will work for you but it will be a hit and miss thing. You'd be better off simply moving your springs from the Healey dizzy to the Triumph one... at least for now. If the car doesn't feel right, then and only then, start tweaking the length of the springs. Don't introduce too many variables at once when making a major component change.

 

OP

C

Cutlass

Jedi Warrior

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• Mar 21, 2006

• #19

The suggestion to change variables only when necessary is quite sound. After all, the engine was running quite well before disassembly, and I'm only changing the distributor because it has two holes in the case patched poorly with JB Weld, and the rim where the adjusting ring seats is crushed. About primary spring tension, I guess I was a bit confused by the statement is Marcel Chichak's article referenced above that stated "Because the primary spring is in control of the lower end of the advance curve, it must be in tension under static conditions." To reconcile Keoke's and Pinman's comments with that article, I would have to assume that "static" does not mean "at rest." "Static" here must mean under idle conditions, under 800 rpm. A loose spring at rest would quickly come under tension at 2-800 rpm, without necessarily elongating immediately. In any event, thanks again to all. Learning about these things is almost as much fun as working on the cars.

 

tahoe healey

Yoda

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• Mar 21, 2006

• #20

Were these holes drilled? They might need to be open. I'm probably wrong but I think I remember there being some at the bottom of the bowl. It has been a long time since I had the dizzy out.