Mallory & Bosch Distributor Shaft End Play vs Temp

[Scott Novak]

Mallory Distributor Shaft End Play Versus Temperature

The distributor shaft needs a certain amount of clearance, between the distributor shaft housing and the drive dog, to compensate for the aluminum distributor shaft housing expanding at a faster rate than the steel distributor shaft, so it doesn't reduce the end play clearance to the point that it binds.

Mallory recommends a minimum of 0.020" end play clearance for a Chevy distributor, to compensate for the thermal expansion of the distributor shaft housing. The Chevy distributor has a 7.5" long distributor shaft housing, whereas the VW shaft housing is only 2.5" long, 1/3 as long as the Chevy. Logic would suggest that the clearance for a VW distributor would only need to be about 1/3 as much as the clearance for the Chevy distributor.

To be sure, I did an experiment to determine just how much thermal expansion really existed.

As the thrust washers that Mallory supplies often have burrs, and are dished or warped slightly, I ground my thrust washers flat so my clearance measurements with the feeler gauges would be more accurate. I also used two thrust washers instead of the one thrust washer that Mallory uses.

Because in actual use there would be an oil film on the thrust washers that would change the clearance slightly, I oiled them before doing the testing and measurements. The oil film thickness may also vary with temperature, so it was important to have the thrust washers oiled.

The distributor shaft housing that I used has an upper ball bearing and a lower bushing.

To measure this clearance accurately you need to use two feeler gauges, one on each side of the distributor shaft at the drive dog retaining pin. Measuring with dial indicators would be a PITA when the shaft housing was hot, and feeler gauges should be more than accurate enough for this type of measurement.

I used a K type thermocouple pushed inside the hole in the lower distributor shaft housing, so it pressed against both the shaft and shaft housing, and used nylon tie wraps to hold the thermocouple in place.

I used a Fluke model 54 II thermocouple thermometer for temperature measurements. The meter was in a room temperature of about 75° F.

Here are the results:

0.009" clearance @ -8.1° F
0.007" clearance @ 75.1° F
0.005" clearance @ 230° F

I'm not sure that if in actual use the distributor shaft housing will get much hotter than the 230° F that I used for my testing. I'll have to measure the distributor temperature on my engine this summer on a hot day.

Has anyone ever measured the operating temperature of the engine case near the hole for the distributor?

The new Mallory distributor that I have originally had an end play of 0.027" which seemed excessive. I'm going to use 0.007" clearance @ 72° F. That should still give me 0.005" clearance at @ 230° F, and a safety margin if the temperature got a little hotter.

Does anyone see a need for anymore than 0.007" room temperature clearance for the end play of the distributor drive dog of a Mallory distributor?

I don't know if these test results will apply to Bosch distributors, as they use a fiber thrust washer and I don't know it's thermal expansion coefficient.

The pin through the shaft of the of a Bosch distributor, that retains the distributor drive dog, is a press fit.

The the pin through the shaft of the of a Mallory distributor, that retains the distributor drive dog, is a loose fit and will actually fall out once you remove the retaining spring.

Would there be any advantage in using a slightly thicker pin that required a press fit in the Mallory distributor shaft?

Perhaps it would reduce spark scatter?

Or is the loose fit floating pin better for some reason?

Scott Novak

[Scott Novak]

Just aso make sure that if you change shims between drive dog and dizzy body....you heat test the assembly to abot 325 in the oven to make sure that you have enough slack. Ray

Ray,

I'm wondering if the temperature of the distributor shaft housing ever gets as high as 325° F.

Have you ever measured the temperature of the engine case near the distributor? I plan to measure this when it gets a bit hotter outside.

I figured it would was safe heat the distributor shaft housing to around 230° F, because engine oil temperature can get that high and I didn't figure the rest of the engine case would be much cooler. But I didn't want to heat the distributor any higher than necessary, as there are some plastic parts inside, as well as a sealed bearing that I don't want the grease to ooze out of.

Scottt Novak

[raygreenwood]

250F is what I have measured. I was just figuring 325 for worst case. Using 230-250 should be more than enough.

Tightening up the vertical or axial play in the dizzy does worlds of good for the accuracy of timing. Ray

[Scott Novak]

Ray,

Thanks for the engine case temperature measurement!

Rather than torturing my Mallory distributor at 350°F, I decided to use a German Bosch 009 instead. I removed everything except for the distributor shaft and the distributor body.

The shaft housing length of the Bosch 009 is 2-5/16" long. The Mallory shaft housing is 2-1/2" long, but the upper ball bearing is steel, and it's width of 5/16" needs to be subtracted from the 2-1/2", which results in 2-3/16" of aluminum shaft housing for the Mallory. That's only 1/8" difference between the Bosch and Mallory shaft housings.

I removed the fiber thrust washers from the Bosch distributor and used a stack of 12 steel Bosch thrust washers to adjust the end play. I oiled the thrust washers to simulate the actual engine condition.

Test Results:

0.010" Clearance @ 72.1°F
0.005" Clearance @ 354°F

The springiness of the stack of 12 washers probably makes my end play measurements less than accurate. But I think I'd be comfortable using an end play clearance of 0.007" on a Bosch distributor

I didn't know just how much effect the oil film between 12 thrust washer would have, so I did a second test run with only two thrust washers ground to size. As I use two thrust washer in my Mallory I thought this would be a more accurate test.

Test Results:

0.008" Clearance @ 75.2°F
0.005" Clearance @ 355°F

I'm not going to bother thermal testing using fiber thrust washers. I don't think that they are reliable, as I have seen several Bosch distributors where the fiber thrust washers have crumbled. If I every used a Bosch distributor again, I would replace the fiber thrust washers with steel thrust washers.

Looking at the results, I think I'm comfortable using an end play clearance of 0.007" for a Mallory distributor shaft. At worst case with an overheated engine, I should still have an end play between the distributor shaft housing and the drive dog of least 0.002" clearance.

Scott Novak

[Glenn]

Scott... try running a cast iron Bosch distributor. I've been rebuilding them for years and nave had any problem with thermal expansion. And as far as the fiber washers... when I build a new engine, I rebuild the Bosch 010 from my old engine that had 97,000 miles and the fiber washers and shims were in excellent shape.

Of the over 2000 cast iron distributors i've rebuild most of them still had the fiber washers when I took them apart but they were brittle. Remember these are over 40 years old and most likely should of been rebuilt years ago.

When Bosch switched to the aluminum bodies, it was a cost cutting reason and they simply don't hold up as well. I don't rebuild them simply because they are usually too bear up. The cams wear, bearings don't last and are just cheaply made.

As to the Mallory, i've not worked with them much, but I suggest not getting the YL models. I've tested a few for customers and found issues with the shafts and unstable dwell. I've only heard good things about the Unilite model, but never used one.

As to the larger caps... i've never had a problem with crossfire on the terminals. And that's with many miles of actual driving. Nothing scientific, but for the average street driven car, the stock cap is find. Now drag racing is a different story.

For the older cast iron there are two caps available. The 03019 is the standard replacement and is black plastic. I like the 03012 with is the light brown caps made in Spain. Their application is for early 70's BMW 2002s. But my real favorite are the old dark brown Bakelite caps.



All this esoteric stuff just isn't needed for a street engine.

Give me a cast iron distributor anyday.

Bosch 019 on a Sun 600 Distributor Tester.
http://youtube.com/watch?v=l34CmJgHjU8

Brand new Mallory YL
http://youtube.com/watch?v=t_EeEEPbtnA

[Scott Novak]

Glenn,

You can use the fiber washers if you want to, but quite frankly, knowing that they can get brittle and crumble gives me the willies. I'll use steel thrust washers as they give me the warm fuzzies.

Crossfiring isn't an issue with a Bosch distributor cap until you increase the fuel mixture density enough by raising the compression ratio and, or supercharging, and, or increasing the spark plug gap. Then the small Bosch cap is a problem. I have my doubts that a Bosch distributor cap could handle the 0.065" spark plugs gaps that I'm now using in my daily driver. When I install my new Mallory Comp 9000 distributor, I'm going to also increase my spark plug gaps to 0.085" and see if the performance improves any further.

All this esoteric stuff just isn't needed for a street engine.

If by esoteric you mean an adapative spark electronic ignition system, a high quality ignition coil, magnetic suppression ignition wires and a distributor cap that can handle large spark plug gaps, I'd have to disagree with you. My seat of the pants testing tells me that it's an improvement for the street. I hope to have some numbers later this summer to prove it. At present, I'm getting 28.7 MPG @ 70 MPH. Once I get the accelerometer hooked up I should also have some accelleration times.

I would imagine that the thermal expansion rates of cast iron and steel are fairly similar so I would think that the distributor shaft and shaft housing would expand together and maintain the end play clearance.

Keep in mind that as the magnesium engine case increases in temperature, it grows and moves the mounting surface for the distributor away from the distributor drive gear. Normally the aluminum distributor shaft housing expands moving towards the distributor drive gear and I suspect it comes close to negating the two expansions.

However, as the cast iron distributor shaft housing doesn't expand significantly, the distributor drive gear clearance will increase with temperature. With a cast iron distributor you really need to make sure that you have adequate distributor drive clearance for subzero weather, as that is when your clearance will be the smallest.

From what I've seen of the various Mallory advance systems, the only one I would use is the newest YH advance system. I don't like the YL advance system at all.

All of the newer Mallory VW distributors have been coming with an upper shaft ball bearing and a lower bushing. They are built really well. Their spring kits are also handy and make adjusting the advance curve simple. Setting the maximum advance is also easy.

But I'm not a big fan of the Unilite. I've looked at the Unilite module circuit and I cringe. I don't think that the design engineer intended it to be used without an active power filter. I'll have more to say about that later. Personally I prefer the Mallory Magnetic Breakerless Ignition (MBI). But only when it's used as a trigger for an electronic ignition system. And as Mallory doesn't make the MBI distributors for VWs, they only way to have one is to do some part swapping and modifications to an 8-Cylinder MBI distributor.

This one started out life as an AMC 8-cylinder distributor. It also has the larger 208M distributor cap modified for 4-cylinder use.



Scott Novak

[raygreenwood]

Actually...all of this esoteric stuff is definately needed on some street engines.
All of my work with the distributors and the shims have been on D-jet injected engines. Tightening up this measurement and getting rid of excessive vertical chatter cleaned up idle timing stability by an easy 3* each direction of the mark....and greatly cleaned up injection point stability. That allows much tighter tuning of the engine that the factory was ever capable of.

The .010 max (or slightly less) gap between dizzy body and drive dog is what I found to work as well.

The vertical or axial slop between dizzy body and drive dog....is not to be confused with the gap between drive dog and distributor drive gear. They are seperate issues. When one is changed the other should be checked and adjusted if necessary.

Another issue is this.....when vertical or axial shaft place is excessive, the bushings in the dizzy wear twice as fast. Also...on D-jet and some other newer distributors...there are a pair of very thin shims on the top section of the shaft right under the D-jet trigger point cam...between the cam and the body. You can't see them unless you disassemble to dizzy. If you keep running the crappy tolerance the factory gives you...those shims will no longer be there after 70k miles.....and the D-jet system will have trigger point "rattle"...and correspondingly bad running...forever....until you fix the chatter in the dizzy.

I'm looking into making a set of torlon bushings for my type 4 dizzy until I can move over to a Mallory with D-jet triggers installed. The Torlon runs good to close to 600F...is chemically inert....and more rigid than delrin. Itshould make great distributor bushings.

Oh...if you are looking for the better fuel interspersing with spark that comes from using larger and larger gaps......try the Bosch W7DTC's tripple electrode plugs. No these have nothing to do with thos platinum 4 type plugs. These have been around for 20 years and work very well fo what you are trying to do. Youwill need a Bosch "pin" type gap adjusting gauge to make sure all three electrodes are precisely gapped to the center. These plugs are better than most I have found. They take the place of increasing gaps quite well. Ray

[Glenn]

Scott... yes, after 40 years they can get brittle. But I ran one for 97,000 and it was fine. Honestly you have to be foolish to think and single part in a hi-po engine doesn't not need inspection and possibly replaced after 100k. Why do people have no problem with having carbs rebuild, but never think a distributor can have wear issues.

And again, i'm talking naturally aspirated street engines. Not drag race engines seeing 8000+ rpm and needing ever last bit of performance. And not turbo or NO2 engines that have extreme chamber pressures.

You need the right tool for the application. You don't need to use the same tools required to rebuild a commercial jet engine to rebuild a lawn mower engine.