distributor looens up

[chevyart]

found out my mallory super comp 9000dustributor loosens up. engine builder told me to put on new hold down . my distributor loosened up last time upon deacceleration from 8000 rpms at end of quarter mile, and it seemed tight when i checked it. it loosened up again from just revving it up in pits., so i know there is some sort of problem' any advice is appreciated. is there a way to check to see if the distributor is bottoming out on oil pump shaft. i have the moroso shims to put under the housing(on top of gasket) if this will be necessary. any advice is appreciated. Art in NY

[chevyart]

PS forgot to say that when distributor loosens(or whatever it is doing) it definitely RETARDS the timing. also the emgine has a jessell belt(if that matters in this situation) the distributor moves enough in reatard position to make fire out of carb till i advance it back to 38 degrees. thanks Art

[BOOT]

Simple, gently pull up on the rotor if it goes up some it's not bottomed out. Still idk what is normal movement/play in that brand dist but all you have to do is pull it and measure what you have to compare to what it is installed.

Or you may be able to see the wear pattern on the dist gear, that could be checked with grease.

[BillK]

Pull the distributor out and then put it back in with no gasket under it. If it goes all the way down and hits the intake manifold then it is not bottoming out.

When it "moves" is the hold down bolt loose ? Any chance the bolt is bottoming out or the hold down you have is not formed properly and not really clamping the distributor ?

How about a picture of your setup.

Just thinking out loud

[ClassAct]

Make sure the lower bushing isn’t worn out.

[1980RS]

I had that same trouble with a couple of engines in the past. Turned out the metal hold downs GM used in the later year bend pretty easy, what I did was look for the old wire type GM used with the metal cover that went over the wire hold down. Took care of the problem, most of the time I see distributor movement has been with HV oil pumps in the past.

[rp930]

https://www.summitracing.com/parts/stf- ... /chevrolet

[miniv8]

If the distributor goes through the manifold, and the manifold has some play, and has even been shifted to alingn the ports better, the bore and seat for the manifold and block can be misaligned so the shaft binds when tightened down. It doesn't need much to close up the clearances in the shaft, that will only cause excessive friction at higher RPM.
When you check for the endplay not bottoming out, try to see if there is enough clearance all around the circumference of the shaft in the manifold.

[FC-Pilot]

I had issues a couple times with Mags moving on me while using crummy factory hold downs. I then switched to these as they are much thicker and with the bent sides it makes them much stronger. I also like the stud they come with as I can get plenty of thread engagement. https://www.competitionproducts.com/mo ... L_L0iUif7o

They are priced well and chrome so the street rod guys don’t even mind it.

Paul

[BOOT]

I'll add to the mount suggestions. I like the MSD one but it's overpriced, it's steel while others that look the same are aluminum. Never liked the GM one, as mentioned bent a few, I got prob 10 or so floating round my garage LOL I have an old chrome one that a friend was gonna toss that I used on my last HEI, works great but herd recent versions are like GM n bend easy.

[dannobee]

But what is the root cause of the distributor moving? Putting a badass hold down might cure the symptom but not the cause. Is the distributor worn at the lower part, where it goes into the block (NOT the upper/intake manifold half)? Are the distributor bearings/bushings worn out? Is there an issue with the oil pump? Oil too thick? Or simply a shimming issue? I'd hate to see you fix the distributor moving issue, only to hear you broke the cam at the last journal or apple cored the distributor drive gear. Or worse yet, stripped the oil pump drive.

In circle track racing I've seen some crazy stuff, but I've never had to use anything but the factory hold down and even then, never cranked the bolt down super hard, even with wet sump engines.

[af2]

Simple, gently pull up on the rotor if it goes up some it's not bottomed out. Still idk what is normal movement/play in that brand dist but all you have to do is pull it and measure what you have to compare to what it is installed.

Or you may be able to see the wear pattern on the dist gear, that could be checked with grease.

That!!! .050" is all you need, sounds like it wasn't tight to start with.

[Schurkey]

I don't understand what the oil pump has to do with the distributor housing moving around.

All the load from the oil pump is on the distributor shaft and gear, but not on the housing.

Seems to me that the only reason the housing moves around is because the shaft is applying torque to the housing. Some of that would be from the friction of the points, or the magnetic pull of the pickup/reluctor. Maybe a tiny bit from the rotating gear against the thrust washers, assuming that the distributor hasn't been shimmed too tightly. But all of those would be pretty minor.

Enough torque to turn the housing would have to come from friction between the shaft and the bushings in the housing. (i.e., misaligned or worn/scored bushings) or the distributor shaft doesn't have enough clearance 'cause someone shoved too many aftermarket shims between thrust washer and distributor gear. Am I missing something?

[BOOT]

But what is the root cause of the distributor moving? Putting a badass hold down might cure the symptom but not the cause. Is the distributor worn at the lower part, where it goes into the block (NOT the upper/intake manifold half)? Are the distributor bearings/bushings worn out? Is there an issue with the oil pump? Oil too thick? Or simply a shimming issue? I'd hate to see you fix the distributor moving issue, only to hear you broke the cam at the last journal or apple cored the distributor drive gear. Or worse yet, stripped the oil pump drive.

In circle track racing I've seen some crazy stuff, but I've never had to use anything but the factory hold down and even then, never cranked the bolt down super hard, even with wet sump engines.

Others commented on a hold down and I simply gave my opinion. Still might wanna let the guy respond before anyone tries to convince him to build a new engine.

[Tuner]

I don't understand what the oil pump has to do with the distributor housing moving around.

All the load from the oil pump is on the distributor shaft and gear, but not on the housing.

Seems to me that the only reason the housing moves around is because the shaft is applying torque to the housing. Some of that would be from the friction of the points, or the magnetic pull of the pickup/reluctor. Maybe a tiny bit from the rotating gear against the thrust washers, assuming that the distributor hasn't been shimmed too tightly. But all of those would be pretty minor.

Enough torque to turn the housing would have to come from friction between the shaft and the bushings in the housing. (i.e., misaligned or worn/scored bushings) or the distributor shaft doesn't have enough clearance 'cause someone shoved too many aftermarket shims between thrust washer and distributor gear. Am I missing something?

You are missing the oil pump load vs. cam/distributor gear angle is pushing the distributor up against the hold-down clamp and the friction between the distributor gear and thrust washer assembly is causing the housing to rotate in the retard direction.

A week ago a friend was high centered on one of those chrome plated die-cast hold down clamps that bend so easy they eventually get tight to the manifold under the bolt. In his case with the bolt tight as could be you could rotate the distributor by hand. An old stock GM hold down clamp solved his problem.

I don't understand why you guys are having trouble with the original Chevy clamp, they have always been OK with me, even the wire ones from waaaay back in the 283 days (as mentioned above ^^).

What I would like to know is why the distributors in short oval track SBC engines will advance a couple of degrees or so in a half-dozen Friday night race sessions. There was a time when I was helping several people with their cars and most of them would creep the timing ahead a tiny bit each week. Iron or aluminum housing, Delco or aftermarket, all would do it. I think it had something to do with heat cycles and the elevated heat and high RPM of oval racing, but ????