[bignate]

I am putting together a budget 455 using parts I have laying around. ‘75 block bored .060”, RPM I-beams and cast flat tops, ‘70 #64 heads with longer stainless valves, 041 cam, ‘70 Q-jet intake, and Holley 6213 800cfm spreadbore. it will be going in my Judge for a while so I can drive it. It has a close ratio 4-speed and 3.90 gears.

I have the original distributor from the RAIII that was in my Judge. However it is in poor shape. The shaft is scored badly and the housing will need to be bushed.

I also have a distributor out of a 1972 LeMans Sport with the 400 4 bbl. This distributor only has 30,000 miles on it so it’s in really nice shape.

Obviously the advance mechanism is different between the two. I have included a picture, the ‘72 unit is on the left while the RAIII unit is on the right. The RAIII has both lighter springs and lighter weights than the ‘72.

Does anybody know the difference in the curve between these two units? Also could I just switch the weights and springs from the RAIII unit to the ‘72?

Thanks, Nate

[gtospieg]

You can switch parts if you want to but it probably isn't necessary. Why not use the 72 distributor and see how it runs as is. Then if needed you can mess with the springs to change your advance curve.

[steve25]

Well with the torque of a 455 itself, it’s fast cylinder pressure rise, no less the 3.90 gears you will not want or need the fast mechanical advance of the RA set up!

Cliff is the one you want to help you out and if he does not respond by morning then pm me your email address and I will send you great info by Jim Hand and what he used in his 455 .

By the way ,do you have no choice but to go .060” over on the motor, as it does not help out in the compression dept with those 64 casting heads.

With the compression of those heads if your 455 block does not have valve chamfers in the top of the bores then I would grind them in!

Here’s a typical high performance advance curve from a 60s 400 motor which might be the very one in your RA distributor.

You say your building from parts you have around , but with 3.90 gears you really want some type of forged rod in the motor!

[dataway]

Bear in mind the shape of the curve is determined by the weight cam plate welded to the top of the shaft, the spring and weights will alter what rpm range the curve is active, but the actual curve itself is control by the shape of the plate that engages the tangs on the weights.

There should be a number stamped into the plate top or bottom. I'm not savy exactly what it means or how it relates to the weight plate numbers. Somewhere there is a Delco book I think that lists all this info.

You can see the different curves represented by the different shapes of this cam plate.

[4zpeed]

Here is some info from Jim, it's a HEI unit but same info applies for the mech. adv.

You can play with that 72 dizzy to see what you like, I wouldn't alter the RAIII unit.

https://pontiacstreetperformance.com/psp/distcurve.html



Frank

[Kenth]

Yours RAIII distributor has modified weights and weak aftermarket springs, and who knows the advance curve on such?
I would NOT use it on a bored out 455 with cast pistons.

Your 1972 distributor (number?) can be made work properly, but it needs at least a 1/4" bushing on the advance stop pin.

There is no need to alter the original 1970 455 timing curve if you want the engine to survive todays fuels and you´ll need to come close to a centrifugal curve that starts with 0°-3°@ 1000 rpm, 0°-4°@1250 rpm and 14°-18°@4400 rpm, add to that 9°-10° initial and 16°-20° from the vacuum advance (ported source).

My advice is based on 20+ years personal experience with my 462, FWIW

[bignate]

Originally Posted by Kenth

Yours RAIII distributor has modified weights and weak aftermarket springs

I did some digging last night looking at pictures of other RAIII units for sale and have also come to that same conclusion. Given that and the fact that it is in such poor shape it will be relegated to sitting in a box on a shelf until I can get it properly restored with correct parts.

Now for the ‘72 I ordered a brass bushing for the advance stop pin and took the plastic bushing off the RAIII vacuum can and put it on the ‘72 vacuum can. I’ll clean and re-lube it and reassemble with original weights/springs and good service parts. And I’m pretty sure one of the engine rebuilders/machine shops around here still have a Sun distributor machine, so I’ll have them test it for me.

One other question, what is the spec for endplay of the shaft on one of these distributors?

Thanks, Nate

[bignate]

Originally Posted by steve25

By the way ,do you have no choice but to go .060” over on the motor, as it does not help out in the compression dept with those 64 casting heads.


You say your building from parts you have around , but with 3.90 gears you really want some type of forged rod in the motor!

It’s .040 now and has some light scuffs in bore. I have a set of old RPM 5140 I beams I bought from Ace years ago when Pacific was still in business.

Nate

[DiamondJim]

I would run the 72 if it’s in good shape. Just set the total to the sweet spot and see how it performs. With the torque of the 468 the timing curve doesn’t have to be perfect.

[Cliff R]

The scored shaft and worn bushing take that unit out of the equation. Even if you install a new bushing the damage to the shaft where it runs in it can cause some issues.

I'd run the later distributor and use a timing light and your engine to see what it's doing.

A running engine is a distributor machine, all you need is a tachometer, and a marked balancer or dial back timing light........Cliff

[DiamondJim]

Cliff he said the 72 was in “really nice shape” ?

[DiamondJim]

If your balancer is the original just buy the peel and stick on timing tape off amazon and set your total.

[Kenth]

Setting the total is as simple as removing the springs in distributor while setting the timing, and this is done at idle speed using a dial back timing lamp.
In your case i would not have more than 30°, i´d rather be in the 26°-28° range.
If the initial gets too low with spring back, a larger bushing is needed.

"End play" (up and down) on a factory pontiac distributor is around .060".
There is NO need to alter this as the helical gears on distributor and camshaft WILL push the gear against the wear shim at ALL times the engine is running.
There is NO shaft wandering up and down as long as the engine is running, and certainly not when letting the foot off of the pedal.
(It´s not like the engine rotation changes direction at retardation.)

FWIW