OzBeast

G'day all,

Car:
1975 C3 convertible. Third owner, great condition, mostly stock though engine has had some changes through PO to bring back some ponies. 1978 block, Edlebrock Performer RPM heads, alu intake manifold.

BLUF:
My existing distributor has been running fine (as far as I could tell), but I had to pull it for an intake manifold gasket change. I notice a lot of end play, measures at 0.079". There is a small shim installed already.
A bit of reading told me that might be excessive, so I ordered a shim kit and a whole new distributor from Jegs (because $90 so why not, good to have spare cap and coil anyway).
The new distributor end play measures at 0.045" out of the box. It already has a pretty large shim installed.
I have a Moroso shim kit with two 0.010" shims, two 0.020" shims and one 0.053" shim.

Research:
I've found several discussions on the issue and most say you want around 0.010 endplay (give or take 0.005) but this thread in the C1/C2 forum was particularly useful - and also confusing. It implies that perhaps my 0.079 endplay might be just fine. Spark scatter is a possible side effect, but maybe minimal?
https://www.corvetteforum.com/forums...-pictures.html

Question:
Which one would you put back in, and how would you adjust it (if at all)?

1) Should I shim my existing distributor with say 0.053" + 0.010" shims (taking the distributor it to ~0.016" end play) ?
2) Should I shim my new distributor with 0.020"+ 0.010" shims and take it to ~0.015" ?
3) Should I not worry and just put the old distributor back in?

4) I've read about checking for clearance against the oil pump drive with gasket in place, which I might try with both to see how much there is currently. I also have a Moroso housing shim kit for the top of the distributor if need be, so I could in theory reduce endplay right down and raise the housing to prevent the shaft pushing in the oil pump drive (if need be).

Thanks in advance!

Pictures:

3X2

My first instinct is to go with reshimmed new one just because you already have it and it is new. I'd contact Lars here on the forum for his distributor timing and vacuum advance guides.

stingr69

Keep old one if it was performing properly and shim as required. Return the new one. Don't over think this one.

OzBeast

Thanks 3X2, since I wrote the post I decided to shim the old dizzy and see how it fits. I got it to 0.021", which is good enough for me I think. I'm leaning toward using the old dizzy because it still seems ok - and because I won't have to find top dead center to re-install it properly

Is there a link to Lars's guides?

Interestingly, shimming the old distributor caused it to bottom out on the cam gear and/or oil pump drive shaft - significantly, so I need to drop in about 0.090" of plastic housing shim just to get enough clearance (the 0.060" shim wasn't enough which really surprised me). It's possible with a change of intake manifold gasket and a good re-torquing that the manifold is sitting a little lower, but it seems the 0.079" end play that was in the existing unmodified dizzy was actually probably for the best as it was most likely the only thing avoiding binding.

Also interesting is that when I dropped the new dizzy in (no gasket) and checked clearance (by taking off the rotor and pulling up on the top plate only) it was also bottoming out - even with the 0.045" end play that it came with.

stingr69: agreed, thanks. I have to overthink things because there is so much I don't know!!! I'll keep the new one, hard to get chevy parts here in Australia, so nothing lost there.

As usual I've learnt a lot just by forcing myself to post my question here in a semi-literate way - hopefully this helps another bubba like myself at some point.

3X2

Email Lars at V8FastCars@msn.com Make sure when you assemble the distributor the dimple on the gear faces the same direction as the rotor contact.

OzBeast

Peterbuilt

No! You have to remove the pin.
And the rotor can only go on one way also.

OzBeast

Wow, just realised the dizzy I pulled off had the gear on 180 degrees out... that's evidently how it's been running since I've owned it.

You can see from the first photo that the new distributor has the rotor contact on the dimple side, but the old one is on the non-dimple side. Additionally the old rotor pin aligns with a tooth on the rotor contact side instead of a valley, so it's half a tooth or 13.85 degrees off. Now I wonder how much my timing must have been off before, especially as I only set idle timing previously (thanks for the Lars advice, he sent me the guides).

Ok, someone work through my logic here:
If I correct the previous error of the distributor gear being 180 out, and flip the gear, I'll have to spin my distributor housing about 13 degrees counter-clockwise from the position I marked when I first pulled the distributor in order to get the rotor firing at the correct point on start up?

Ah yes I see now, thanks.

Peterbuilt

Ok, someone work through my logic here:
If I correct the previous error of the distributor gear being 180 out and flip the gear, I'll have to spin my distributor housing about 13 degrees counter-clockwise from the position I marked when I first pulled the distributor in order to get the rotor firing at the correct point on startup?[/QUOTE]

If you line up the distributor where it was before and if you have not rotated the engine and if the rotor is in the same position it was it will fire up at the old mark, as you will be close enough.
I'm not sure about the 13.85 degrees being OUT on the advance or retard side.

Here's more information about timing since your into that area:
https://www.corvetteforum.com/forums...long-post.html

OzBeast

bobs77vet

You are over thinking this.....flip the gear use a timing light to set timing and you are done . If the vacuum advance is hitting things,move the wires over one position.

stingr69

Agreed.

The hardest thing now will be getting the oil pump driveshaft to align with the distributor as it drops in. If the distributor is not 100% down touching the intake manifold, you are not aligned with oil pump driveshaft. Not to scare you too much but....you can bump the engine over a few times to get it to align and the distributor will drop in all the way. Alternately you can use a long screwdriver to adjust the oil pump drive shaft from the top to help get it aligned.

Wish I was close enough to help you - this takes maybe 5 minutes if you have ever done it before.

CanadaGrant

Sometimes the gear is installed 180 out so that you can get more advance without the vacuum advance can hitting any of the distributor shielding braces on certain older models or to get the tach drive straight at the hole in the firewall so there is less curve in the cable. Sometimes it was just a mistake. It doesn't really matter but you can change it back easily if you want or just leave it. When you install the distributor you just have to turn the oil pump drive shaft with a long screw driver to the same direction or orientation as the key in the bottom of the distributor shaft at the point where they engage. Sometimes it takes a couple of tries but isn't hard and only takes a few mins.

OzBeast

Flipped it, seated it, housing needed to be slightly rotated to match the rotor marks - but it was insignificant so you're right I was over thinking it. Will fill er up and set timing today.

Thanks guys, actually I've gotten really good at dropping in distributors through all this (checking for clearance on the oil pump drive). Long screw driver was the way to go, and rotating the rotor and main shaft anti-clockwise as you extract the distributor is also a neat way of leaving the oil pump drive in the right spot for reinstallation later. I didn't want to bump the motor over because I couldn't be **** finding top dead center and all that entails.

Grant, those reasons makes sense why sometimes the gears are intentionally backwards, good explanation thanks.

Like so many mechanical things, this has been daunting at first and then quite easy and straight forward after thinking through it and giving it a go.

OzBeast

Update: Fired right up as if it had never been apart, thanks all for your help and input.

dmaxx3500

also ,are you getting full contact at the dist gear and cam gear?

OzBeast

I assume so, but I'm not sure I know how to check.

The way I think I checked was to shim out the dist gear down to 21 thou, which should lengthen the dist length to max.
Then I checked it wasn't bottoming out on the oil pump drive by dropping it in (with no gasket) removing the rotor and pulling up on the top plate to check for a few thou movement. With no shim it was definitely bottoming out, with 0.060" shim it was still bottoming out, with the 0.100" shim (actually measures as 0.092" with callipers) it has just a little play.

So in theory the gear is as low as it can get without bottoming out, and the gear shouldn't be moving much as I've reduced endplay down to 21 thou... does this automatically imply correct dist/cam gear mesh?