Distributor shaft end play

[littleben]

It seems like every v8 Chevy I have seen, including new brand name performance ones, have a lot of end play or slack in the distributor shaft, like .150 or more. I've heard of some guys shimming these down to .010 or so, but the distributors seem to be made on purpose with a lot of vertical play or clearance. Why? Is it better to tighten them up or leave them as is?

[mag2555]

If you are still running points it matters more that you get the clearance down to .030", but if you are at .050" with a HEI type and not looking to rev over 7k that sould be ok also.
If the clearance is too tight when the motor heats up the two gears ( cam and distributor) will chew each other up and the loading of the distributor on the oil pump shaft will drive the main oil pump gear into the pumps cover plate, chew that up and drop oil pressure and oil volume. And also not to mention all the metal grit that will get onto the cylinder walls and damage them and the rings!

[Schurkey]

Shimming the shaft to eliminate end-play will effectively make the shaft longer. Clamp it at the top, maybe now there's no free-play and the oil pump binds up. Situation is worse with decked blocks, decked heads, and planed intake manifolds. Manufacturer has to assure that distributor will fit in a "worst-case" engine. On an Oldsmobile, where the helical angle pulls the shaft downward, shimming the shaft results in the thrust pad cast into the block being scored and worn.

I'd be surprised if a Chevy wet-sump engine gained anything in terms of ignition timing consistency, because the oil pump drag should push the gear all the way up to the thrust washer via the helical angle of the distributor and cam gears; and if you don't have oil pressure there's more important things to consider than some timing variation.

Still, lots of folks shim the distributor end play.

[raceman14]

Vertical dizzy shaft play will cause problems on acceleration and deceleration when the gear is jerked around on the camshaft. If you have slack in your timing chain coupled with thrust ( fore and aft of the cam ) coupled with the timing changes from the shaft running up and down it can be as much as 15-20RWHP on a 450HP GM crate engine. On other engines I have seen both more and less and have made dizzy changes with the same exact dizzy and on a regular basis I see 5-15RWHP.

People wonder how their flat tappets go bad and they don't even realize how important cam stability is to lifter rotation. I would bet 75% of the problems relate to that and not improper lubrication. Timing chain QC is almost non-existent as well as distributor gear size, alignment and positioning on the cam when installed. All of that contributes to how the cam rotates. I have used the same process for 25 years and have had only 2 cams go bad in that time and both were on cores that were not hardened properly thru-out. Both of them happened back to back on the same engine. Needless to say I do not run that brand any more. I went back to the COMP cores I was running and have never had that problem again.

Back to Dizzy vertical clearance;
To me it is worth it, I set cam thrust to .001" dizzy vertical slack .005" or less and TC slack ( tight as possible ). You can buy chains from Manley & CV that are .001"-.030" tight, and you can usually find chains if you measure them on a standard set of gears to be +.010" to -.005.

[novadude]

dizzy vertical slack .005" or less and TC slack ( tight as possible ).

Is that a misprint? 0.005"?

[raceman14]

Nova,
On my own personal stuff I run as close .001"-.002" is that still too much for you or too tight?

I have been running them that way for 25 years with no problems. I have almost no cam gear to dizzy gear wear and the timing stays rock solid on the chassis dyno from 3500-9500.

I know folks think they will bind up but there is plenty of oil and as long as you use the hardened shims to take up the slack you should be fine. Kits available from BLP. I think they are the same shims as the ones used in the timing cover set-up.

[novadude]

Nova,
On my own personal stuff I run as close .001"-.002" is that still too much for you or too tight?

I have been running them that way for 25 years with no problems. I have almost no cam gear to dizzy gear wear and the timing stays rock solid on the chassis dyno from 3500-9500.

I know folks think they will bind up but there is plenty of oil and as long as you use the hardened shims to take up the slack you should be fine. Kits available from BLP. I think they are the same shims as the ones used in the timing cover set-up.

What about thermal growth of the aluminum housing, steel shafts?

I guess I'm just not brave enough to try running it that tight...

[af2]

Nova,
On my own personal stuff I run as close .001"-.002" is that still too much for you or too tight?

I have been running them that way for 25 years with no problems. I have almost no cam gear to dizzy gear wear and the timing stays rock solid on the chassis dyno from 3500-9500.

I know folks think they will bind up but there is plenty of oil and as long as you use the hardened shims to take up the slack you should be fine. Kits available from BLP. I think they are the same shims as the ones used in the timing cover set-up.

What about thermal growth of the aluminum housing, steel shafts?

I guess I'm just not brave enough to try running it that tight...

Yep??????? Same here! The gear will be where it is when the thing fires with oil pressure.

[MadBill]

The thermal expansion coefficient for steel is 7 ppm and for aluminum is 12, so going from 70° to 200°, the differential expansion between the two is around six and half ten thousanths per inch of length. A Chev distributor housing is ~ 9" long so the clearance will tighten by ~0.006" over that temperature range. A drag engine distributor housing might not reach more than perhaps 140°, in which case the end play would shrink by ~ 0.003".

[novadude]

The thermal expansion coefficient for steel is 7 ppm and for aluminum is 12, so going from 70° to 200°, the differential expansion between the two is around six and half ten thousanths per inch of length. A Chev distributor housing is ~ 9" long so the clearance will tighten by ~0.006" over that temperature range. A drag engine distributor housing might not reach more than perhaps 140°, in which case the end play would shrink by ~ 0.003".

I was too lazy to do the math, but I knew the in/in coeff of thermal expansion for aluminum was greater than steel. Not sure how it wouldn't lock up with only 0.001"-0.002" endplay.

[raceman14]

Bill,
Sounds like good numbers, and I don't doubt the expansion and contraction of the materials to be fact.

All I can say is I have done 100's of distributors like this on the chasssis dyno and at the track and I have run ARCA races 3 hours long and have never had any wear or breakage problems, but exactly the opposite. I have some bronze gears that I have used for 2000+ laps which is 4 re-freshes and they look like brand new. I just always figured it was from the gear not slopping around on a cam that was slapping back and forth in the block.

Just throwing out there what I run and as usual it is way different from anybody else out there but it works for me so I'll keep on with it.

I also run bronze gears on crate engines with no problems and other people have big issues with them wearing out???

I am usually not one that goes against physics and thermodynamics but in this case there must be something else at play helping the system to survive under pedal mashed thru the floorboard....WFOT.

As always I am up for discussion on it to learn why???

I am guessing the stack of shims has something to do with absorbing and desorbing shock and expansion. They won't survive lock-up but I have had about 25% of them things back after 1yr and they look like brand new.

[littleben]

I guess what I am wondering is why did GM make it with that much clearence, I think there must be a reason.

[340king]

On some engines it is absolutely critical to get the clearance spot on. One of those is an AMC 360. I had one come to my shop that was eating cam gears like candy. Ten mile tops it would be ate up and so far out of time it wouldn't run. It was amazing. I got it as a basket case, so it took a little time to figure out. The previous owner had replaced the factory distributor with a rebuilt. The rebuilt had something like 0.100" slack in it. I had to shim it down to less than 0.015" for the gear to live. The gears are so small on the AMC that even a slight mis-alignment killed the cam gear.

So it can be a critical issue depending on the engine.

I started running the timing chain tensioner from Mopar on all my stuff now. It eliminates a lot of cam rock/timing issues due to spring pressure oscillation. That better timing is worth the added friction and then some.

[Kevin Johnson]

I guess what I am wondering is why did GM make it with that much clearence, I think there must be a reason.

When I looked into distributor gear issues with the Mopar slant six I learned that there have been issues with incorrect manufacturing and remanufacturing, including clearance and positioning, of distributor gears at least extending into the 1960s. It was an industry wide problem with respect to the big three. Ford Racing calls it out in their catalog tech tips.

Part of the investigation involved buying an NOS factory authorized rebuilt distributor on Ebay that did not conform to the factory specified rebuild specs (it was rebuilt with zero end play).

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[Powertrip]

I started running the timing chain tensioner from Mopar on all my stuff now. It eliminates a lot of cam rock/timing issues due to spring pressure oscillation.

Have you fit that tensioner to anything other than a small block Mopar?

Slightly off topic, on big block Mopars, the only time I've seen unsteady timing is when the timing chain has too much slack. I wonder if that is because the distributer is in front, and the drive gear is not part of the distributor shaft?