shims in points type distributor 1970 350 c.i.

Does anyone know what shims should be used above the gear drive on my distributor shaft? I am having a lot of peculiar problems with erratic running and difficulty in timing the ignition. I noticed when
I took the distributor right out that the gear at the bottom of the shaft could move up and down about 1/4 of an inch. This does not seem correct, but could have been like this for years as I have always had problems like this, - and for years I changed the distributor to a HEI type, but - of course I lost the Tachometer. I want to go back to the old distributor.
If I shim it to give only a few thou clearance is this likely to cause any problems?
Thanks. George.
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George:
It's not a shim, it's a thrust washer (tanged), and there's only one.
If the shaft, as you say, is able to raise a whole quarter of an inch out of the camshaft gear, then you have some excessive wear either at the cam gear or the distributor gear, or both. Typically, there is a small amount of run-out, maybe a sixteenth of an inch to the eye.
If your shaft is able to raise a quarter of an inch the rotor could come too close to the electrodes on the cap. This could cause a variance of the air gap between the rotor electrode and the cap electrodes. It can also cause excessive wear on rotor contact, and will wreak havoc with the points follower and alignment in relationship to the distributor shaft cam.
Yes this will alter your dwell as well as your timing. Sound like the distributor is worn out.
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George,
The travel that is spoken about on the up and down should have no effect on the timing of the vehicle. If the timing is erratic I would be looking at a bent distributor shaft or loose bushings at the top of the distributor shaft. The distributor is pretty easy to rebuild and it is just a matter of taking it a part and replacing the bushings. The up and down travel should have some movement and not be preloaded. Understand that the distributor timing does vary due to the Chevrolet design of having the distributor at the back so you will never eliminate the torsion of the cam shaft from front to back. Now, with the vacuum plugged the timing should be steady. That is, unless your carburetor is adjusted up and out of the idle circuit. If it were me I would rebuild the distributor and put in a Petronix breakerless system in. It is under the cap so no one will know and you can always convert it back to points to sell the car. The nice part is you do not have to set points again. I did a conversion on a 62 Chevy 409 Impala and it works great.
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George,
Also, if you have a cable drive tach the cable could be dry or corroded and causing drag. Thus, causing your points to bounce. For every degree of dwell you get a degree of timing so this is a possibility.
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George wrote:

I think the clearance would be about .060 in a new distributor. Why not play it safe and have it rebuilt?
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I agree with Billy, about 1/16" end play is normal and will not affect idle quality.
Some Qs: Is this a base engine (300hp?) or 350hp/350 ? Is the vacuum can source ported or direct?
Joe 72 coupe
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George wrote:

Ok, here's a different, off-the-wall thought which might be less costly than a rebuild on the old distributor.
If the HEI is doing the job it might be worthwhile to keep it and convert to an electronic tach. (The HEI module should have a tach terminal.)
Don't know what's available in the U.K. but you might find a spedo/tach/clock gent who could fit the guts from a 78-79 GM V-8 tach into the case of your mechanical tach. Note, you'll also need a low cost 'tach filter' to keep the ignition hash out of the tach circuit.
While you have the distributor off, check to make sure the lube spiral is clean and the shaft/sleeve is getting oil. As the shaft wears, lube gets less efficient and it tends to accelerate wear.
-- pj
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Thanks for all the usefull comments. The history of this car is rather unique and it is very low mileage. It belonged to the Oil Minister in Abu Dhabi first and he had it flown to a lot of meetings round the world. When it got a bit old, he gave it to one of his aquaintances who sold it to me in the early 80s. it is a 350 c.i. / 350 hp. and is a convertible with a hardtop. I roared about in it for a few years putting a lot of black lines all over Abu Dhabi's roads and collected a lot of speeding tickets.
I shipped it to UK a few years later and it has hardly been used since. Petrol prices have deterred me a bit. I have a bit of spare time at present and thought that it would be nice to get it going again. I have a big workshop with everything I might need and I did box up a lot of spares to be sent home with it, - like 6 various small block engines! of various models and years. I can remember having a lot of trouble with this distributor and it may be that I had it all apart and did not put it back correctly. I have 3 or 4 HEI distributors too.
I never thought of it as any sort of collectors car and when I realised that aircon would be more of a problem than a benefit I dumped it. I will take the advice on board and see what seems to be the best plan. Today I have established that the TDC mark is correct but I am not sure how many degrees there are to a division. Also, however I adjust the gap my dwell meter always reads off scale. It has got so difficult to start and keep running that I have ordered the contact, condensor, and some new plugs and leads and I shall start from scratch when these arrive. It has a brand new Rochester Quadrajet which seems OK and I don't suspect much to be wrong with that. I connected the vac advance to a thin pipe coming out of the side of the carb on the right side looking forward. It has vacuum, but I am not sure that this is where it should go. I can easily connect it directly to the manifold.
It's an interesting exercise that keeps me in the dry so I will plod on and see how it goes.
Thanks again.
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New Query, the bits I ordered for the distibutor have not come yet so I am a bit stuck. What can I do about a few gouges and scratches in the paintwork. I was thinking of cleaning them out and the filling with epoxy resin mix and the touching up and grinding down the surface using etching primer and trying to get this smooth before applying the top coat. Will this work or is there another technique for fibreglass?
By the way it has done 35,411 miles, - hardly run in. The distributor does not seem to be worn at all and the play I wrote about is less than 1/4 inch. (but more than 1/8th. The coil has 6 V written on it so there must be a dropper resistor in the circuit somewhere. Anyone know where this is?
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wrote: By the way it has done 35,411 miles, - hardly run in. The distributor does not seem to be worn at all and the play I wrote about is less than 1/4 inch. (but more than 1/8th. The coil has 6 V written on it so there must be a dropper resistor in the circuit somewhere. Anyone know where this is?
GM did not use the so-called "ballast resistor" on their V8's with points and an internal alternator voltage regulator. Instead they use a resistor wire to drop the voltage down around 9 volts, and a by-pass wire off of the R terminal on the starter to the coil +. This allowed for 12 volts in the crank position, then would revert to the resistor wire in the run position.
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Thanks Billy,
I should be able to test that this is working OK. Maybe I shall get a 9 volt coil. I expect that the one labelled 6v is an incorrect one. I am still waiting for the spares I ordered. (Christmas/New Year delays, I expect). There is not much experience of Corvettes in this rural area of UK. It's getting really cold in my workshop now, so I may have to stop and organise a bit of heating!
Regards George.
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I should be able to test that this is working OK. Maybe I shall get a 9 volt coil. I expect that the one labelled 6v is an incorrect one. I am still waiting for the spares I ordered. (Christmas/New Year delays, I expect). There is not much experience of Corvettes in this rural area of UK. It's getting really cold in my workshop now, so I may have to stop and organise a bit of heating!
Regards George.
George:
I'd stay with a 12V coil and verify that the resistance wire is in-fact there. If you have a 6V coil with this resistance wire then your primary voltage will be diminished causing your secondary to be less. Your spark may not be as hot as the plugs are designed for. The original coil was a GM# 1115238 (or AC-Delco# D512). The AC-Delco D512 coil has been replaced by a "universal" coil part numbered as U505, or E552C. Both replacements accomodate the conventional high-performance ignition systems on the 350, and the 454. Also, these replacements cover models with the old resistor configuration as well as the newer resistance wire set-up. Hope this helps you out.
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Billy Ryman wrote:

pull more current and produce a hotter spark with 12v applied.
The 12v transition in 1953-55 was made to provide both improved cranking and a hotter spark to match up with higher compression ratios. Stayed pretty much that way with either a ballast resistor or resistive link until we got HEI.
The objective was to run the 6v coil at 6-8v during cruise but apply about 9 volts to it for cold weather starting. -- pj
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