Oil pump / distributor gear replacement

[Tom W]

This follows on from my other thread in the upgrade forum about mappable ignition and wandering timing. Does anyone know if it’s possible to replace the oil pump/distributor drive gear with the engine in situ?

The workshop manual suggests yes, but is obviously written based on the engine being stripped and assembled out of the car. The process would appear to be:

Sump off (done that before, not fun)
Oil pump & off pipes off
Distributor off
Undo tab washer and nut securing drive gear
Tap distributor drive shaft up out of engine block
Fit replacement gear, ensuring it’s orientated so the distributor drive shaft slot is facing the right way. Engine needs to be at TDC on number 1 compression stroke at this point.
Tighten nut and tab washer
Refit oil pump & pipes
Refit sump
Refit distributor & time

Have I missed anything there? Looks like a lot of faff, particularly trying to get the gear and shaft lined up in the correct orientation; working under the car but needing to see what’s going on from above.

Thanks, Tom

[rswaffie]

Hi Tom,
I guess you need to weigh up the level of certainty you have that this will solve your problem against the work involved as described, versus time and cost to explore the other electronic ignition options you mentioned in the other post.
If the dizzy drive replacement can be done with engine in situ, then with an assistant to help with lining things up, it may be your cheapest potential solution. If fitting a crankshaft sensor based option is ostensibly easier, then it’s just down to the cost involved.

Hopefully there’s someone on the forum whose done a drive swap or can give you a definitive answer to help inform your decision.

[Tom W]

Very true.

At the moment, it’s only an assumption that changing the drive gear will cure the wandering timing. Without taking anything apart, I won’t know what’s worn. I also don’t have any measure of how much play is acceptable in an unworn distributor drive.

I think it’s most likely it’s the bronze driven gear, and not the steel driving gear. However, it could be the fit of the driven gear to the distributor drive shaft via the woodruff key. The driving gear definitely isn’t replaceable with the engine in the car, as I’m not getting into stripping the timing gear in situ.

I’m currently weighing up the options for a crank triggered ignition system. The car really isn’t bad, but as we’re not allowed to drive anywhere at the moment, time and thoughts turn to fixing the little niggles that would be otherwise left in the back burner.

[paulsco]

Hi Tom,

Please excuse my ignorance, but I am at a loss to see why you would want to remove the distributor drive unless it is damaged. You should be able to detect any movement in the drive by removing the distributor and checking the play via the slot in the drive.
To answer your question though, yes you can do all of this with the engine in the car and the gear is I think bronze.

I just thought… from distant memory, I’m not sure if the main bearing housing or the drive gear is in the way?.




Paul

[Tom W]

There is play in the distributor drive, at a guess about 2 degrees. It’s this I’d like to fix. I’m assuming 2 degrees is not normal.

[christopher storey]

The driven gear is indeed bronze. There is always some play in helical gears, but it tends to result in "lag" of the driven gear rather than in scatter . My own view would be that if you are getting scatter, it is more likely to be end float in the distributor shaft, thus allowing the driven gear to "float" in and out of mesh to a degree - and how you would cure that I don't know - or side play in the distributor shaft which is very common indeed, although with a new distributor you would not expect it . Have you tried an oscilloscope trace to see if the variations are common to all cylinders or just restricted to one or two? It is amazing what you can obtain from such a trace

[Tom W]

There’s a thrust washer between the driven gear and the housing. That could be worn. The workshop manual mentions a new gear being a cure for correcting end float, but it talks about insufficient end float, which I don’t quite understand.

Ive not tried an oscilloscope, I don’t have one. I did try timing off both 1 and 6 and the readings seem the same. For the most part, the timing it consistent, then occasionally it jumps 2 - 4 degrees.

The distributor is a 123. There’s no appreciable play in the shaft, and minimal end float. Side play shouldn’t affect the 123 anyway. I assume the fit of the drive dog to the drive shaft is good, as the play feels the same whether measured at the rotor arm, or with a screwdriver poked down to rotate the distributor drive.

[paulsco]

I would have thought that excessive end float would be vertical and not rotary??

You might even be able to check this on the distributor drive to some degree with a magnet.

Paul

[Tom W]

The factory figure for end float is 0.1 to 0.15mm. I expect to get 2 degrees of rotary play from excessive end float, the drive would be moving up and down significantly. Certainly enough to see with the naked eye. However the play definitely seems rotational. I did take a video. I’ll have to see if I can upload it to here.

[xke68]

Hi Tom
Did you ever get to the bottom of the distributor issue you were having? I also have a 123dizzy and am seeing a few degrees of timing jitter which I think is what you were also seeing? Trying to figure out where to start digging. My car runs reasonably well but idle is a little lumpy which may be the random jumps in the timing. Pretty sure my carbs are well tuned
Thanks
Andrew

[Allrand]

I just thought… from distant memory, I’m not sure if the main bearing housing or the drive gear is in the way?.

Yes the main bearing cap will have to be removed, and undoing the nut that holds the driven gear onto the shaft can be a real pig, you could even end up damaging the bronze gear more.

[Tom W]

I never did attempt to replace the drive gear. I made some more adjustments and maintenance on the carbs which improved the idle quality. I expect the timing is still able to fluctuate as nothing I did will have remove any of the free play. However, a smoother idle will give a more constant input into the distributor drive so it will be less inclined to float around.