'63-67 SB Timing Tabs (convert 6" cover to 8")

[Wayne M.]

Thread started about 4 days ago on 8" covers and correct tabs.

https://www.forums.ncrs.org/showthre...63820&uid=2757

Looking at the reproduction catalogs, there's a fair amount of variance on what tabs to use. Below is a timing cover for a 6-inch balancer; bought at a swap meet, probably not Corvette but that shouldn't make a difference. Don't know the year, but maybe '63 thru '67, for 6-inch balancers.

My question, as it relates to 1965's, is can the tab from this piece be removed and relocated outboard to accomodate an 8-inch balancer. I understand that (maybe) the outward-facing weld tab (same as this one) is correct for 8" balancers, to at least sometime in the 1965 MY ? [after that, the tab faced inward]. I figure any removal marks would be hidden under the tab and by the balancer. Probably could JB-weld it on at the new location.

Any thoughts ?

[Joel F.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

Wayne,

I recently did just this and it was actually quite easy. I used a Dremel with a carbide burr to grind out the spot welds from the outside. I over-did it though and gouged the cover a bit. The key is to grind out most of the weld from the tab side, but be careful not to gouge up the cover too much. Go slow, grind the welds a little, and use a flat tipped screwdriver or chisel to keep working it the tab up. Not using the chisel is what got me into trouble as I was expecting the tab to just fall off once I ground the welds. It never did just fall off. Paint, rust, or something else kept it all togethether until I gave it a couple of whacks with hammer and chisel. Once it came off I realized it could have been done a lot more easily had I tried chiselling it sooner.

Since my application has the tab pointing inboard, I probably could re-use it and have the grind marks not visible, however I am using a repro tab anyway as the tab's configuration is a little different that what I need. The grind marks on the cover were easily hidden by JB Weld.

Joel

[Richard S.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

i think your main problem will be timing although tdc will be tdc the degree marks on a 6 inch tab will be closer together than a 8 inch.draw a 6 in. balancer and a 8 in. balancer and draw out the degree lines and youll find them farther apart on the 8 in. not much but there is a difference

[Michael H.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

Thread started about 4 days ago on 8" covers and correct tabs.

https://www.forums.ncrs.org/showthre...63820&uid=2757

Any thoughts ?

Yes, actually quite a few thoughts. Since the real story is now close to being out, so to speak, I'll explain about the different tabs for 62-64, and some info for 65 and later. I've kept this a secret for decades but this information should be the start of a LOT of activity and, hopefully, some corrections made to the reproduction tabs and their applications.

First came the 62 to early 64 cover/timing tab for the 8" balancer. It's tab installation direction was covered in your previous thread. As I described there, the part and part number for the cover was unchanged from 62 through early 64 production.
There was a problem with the timing tab used on all of these early covers. It was discovered that the timing tab was incorrect. It was the same tab that was used on the cover for engines with a 6" balancer!
What's the difference? Well, there's a problem with the spacing of the marks on the plate. The distance between marks/degrees.

If you do the math, you will find that the distance between degrees on the circumference of a 6" circle is different than that on an 8" circle. It may not be much different but when you add up the difference in 10 degrees, it's substantial.

Here are the numbers.
The circumference of a 6" circle is 18.849", which when divided by 360, the # of degrees in a circle, comes out to .052" per degree.
For an 8" circle, the circ is 25.132". The distance between degrees is .069".

That may not sound like much but the difference in ten degrees is about 2 degrees. So, if you set the timing on a 62 to early 64 at the factory spec of 12*, your actual timing will be roughly 9 or 10*.
This is part of the reason why 62 to early 64's always seemed to run stronger with the timing set around 14 or 15*.

Between the "0" and "A" marks on the tab, there are 5 marks, or 10*. (two deg per mark) That distance on the plate designed for a 6" balancer is a bit over 1/2". The same 10* on a correct plate for an 8" balancer is nearing 3/4".

The problem was discovered and corrected some time in the early part of the 64 model year. I don't know when the actual part arrived on the Flint engine line.

The new corrected plate dimensions required more length and a new design which also required that the mounting tab be folded in the opposite direction.

If you happen to have an early and later plate, hold the two together to see the difference in space between the zero and 10* lines.

[Michael H.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

I have a few pic's of original early and later style timing plates that Jorjorian sent a long time ago. I'll ask if I can post the pic's when I talk to him later today.

[Timothy B.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

Michael,

I always thought the timing tab on my 300 HP car was not correct as it bent downward then in just a small amount. I have a cover off another early 63 300 HP motor that's bent the same way which is opposite of what your saying.

This tab is also square and not cut on an angle like the picture in the previous post. I will take a picture of what I believe to be correct and post.

[Timothy B.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

Pictures from my early 63 300 HP showing tab bent down. I wish I could get in there to show how it looks where it's bent under.

[Michael H.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

Pictures from my early 63 300 HP showing tab bent down. I wish I could get in there to show how it looks where it's bent under.

Tim, I think the tab for 62 to 65 with the small/6" damper is completely different than the one used for the 62-63 8" damper.

The tab used on the 62-63 cover for 8" is likely from an earlier era?

In the 63 printing of the parts book, the 3812994 timing chain cover for 250/300 HP engines is described as having a "double timing mark bracket". That's two tabs, one below the other. That design tab was never used with engines that had an 8" damper.

[Duke W.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

I discovered this problem with my '63 L-76 when I put timing tape on my balancer in the seventies, and it turned out that 10 on the tab was only eight on the tape.

You can also clearly see this error with a dial back timing light.

Back in the sixties it ran best with the intial timing set at about 16 - eyeballing beyond the end of the tab, but with the tab error the total WOT timing was probably only about 37. (The dist. max centrifugal is 24.)

If you ran the OE recommended 10 initial the total WOT advance was only 32, not 34.

This was back in the day when you diddled with the initial timing until the engine felt as strong as possible. No dial back timing lights back then and chassis dynos were few and far between.

Duke

[Michael H.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

I discovered this problem with my '63 L-76 when I put timing tape on my balancer in the seventies, and it turned out that 10 on the tab was only eight on the tape.

Yup, yer close. Ten degrees on the tab is only 7.5* actual. Twelve degrees on the tab is 9* actual.
That's exactly why a lot of guys with new 62-63's were setting timing at 16* on the plate but were actually at 12*.

If you knew this in the 70's, why haven't you been telling guys that ask about timing settings?

[Duke W.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

Few guys understand timing maps, and this minor timing error introduced by the tab is not signficant for road going engines - only racers, and I assume that, like me, most racers figured this out years ago.

The important thing for typical vintage Corvette owners to understand about "timing" is the spark advance requirement for idle, cruise, and WOT - otherwise known as the "timing map" or "spark advance map", and I've written thousands of words on this subject.

Duke

[Michael H.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

Wayne, here's a shot of the early (62-63) and later (64-65) timing chain covers with the two different design tabs. Both are originals from engines with 8" balancers.
The tab on the right is the early design. Notice that the distance between the "0" and the "A" is different by roughly 2.5*. The "A" is the 10* location. (each mark is 2*)
The part number change is shown in the GM parts change book as January 1964 but I suspect the change may have occured earlier than that.
The early cover is part number 3817174 and the 2nd design is a 3851549.

I wonder how many 63-65's have the wrong cover?

Scan thanks to Robert Jorjorian.

[Irby G.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

What would be the best way to move that tab back and line in up properly? Is there some measurements someone could take and post?
Aloha

[Michael H.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

What would be the best way to move that tab back and line in up properly? Is there some measurements someone could take and post?
Aloha

Irby,

The "0" on the tab is in the correct position so the only part of the rest of the marks that matters would be the one that you want to set the timing at/to.
I'll be gone for a few hours but I'll explain how to use the original when I return......

[Michael H.]

Re: '63-67 SB Timing Tabs (convert 6" cover to 8&quot

What would be the best way to move that tab back and line in up properly? Is there some measurements someone could take and post?
Aloha

Irby,

If you have a 63 with the weird short timing tab, the correct timing can be set easily with a piece of masking tape with a few lines drawn on it.
If you make two marks on the tape, one for zero and the other for the desired timing setting, you can apply that piece of tape to your existing timing tab.
Decide what timing setting you want and calculate the distance required between marks. Each degree on an 8" balancer is .069", so if you want a 10* initial setting, multiply the number of degrees times the .069 figure. Ten deg would be .690", or roughly 11/16". Twelve deg is roughly 13/16", and so on.
Place the tape on the existing timing tab with one of the marks aligning with the "0" line on the tab. Adjust timing so the mark on the balancer aligns with the other line on the tape.

Or.... you can estimate the difference, using the picture that I posted earlier, and wing it. Those old cars with the short tab usually ran best with the timing set JUST SLIGHTLY off the end of the timing plate.