Service Bulletin for 4+3 Overdrive

[Miles F.]

Good day.

I own an early 1986 coupe with the 4+3 transmission. It has 81,000 miles.

As an aside, I think it is really cool to have a four speed with a kick down passing gear.

The previous owner was unsure when the carrier bearing, and thrust washer were replaced to correct the factory problems.

I had no idea when the fluid had been changed and remember reaeding that the fluid should be changed every 10k. So I bought a filter and a gasket and took it down to my Corvette expert down at the local GM dealership.

Maybe he is not such an expert because after the fluids were changed I drove the car home, the next morning there was a Vladez size puddle on the garage floor.

So I looked it up on the internet and there is a service bulletin out that says you are to use RTV sealant instead of the gasket.

We tried the RTV sealant and the leak is better, but over about two weeks I loose a quart of fluid.

I hate leaks.

Although I have not been able to find it there is a service bulletin outlining GM solution to the problem.

If you have one or even know where I can find one I would really appreciate it. Thank you.

[Jim T.]

Re: Service Bulletin for 4+3 Overdrive

Was it leaking before you decided to take it to the mechanic?

Some leaks with a new pan gasket occur because the mechanic does not use a torque wrench to tighten the pan bolts.

Best luck I have had with valve cover/gaskets not leaking/seeping was to use a torque wrench on the bolts. Only inch pounds are required.

[Peter M.]

Re: Service Bulletin for 4+3 Overdrive

Hey Miles:

This is a common problem when having the Doug Nash 4+3 transmission serviced by folks who do not know what they are doing and do not have access to the GM service manual or simply do not take time to read it. YES, THERE ARE GM TECHNICIANS WHO DO NOT KNOW MUCH ABOUT THESE 23 YEAR OLD CORVETTES. The bottom line is that the OD unit in these transmissions will leak if not serviced properly. Believe me I know. I once spent close to 6 months and over $2000 to have this simple problem properly repaired.

What I learned from this tragedy is that you must follow the instructions in the GM service manual precisely. The brass filter can be purchased from Napa or other parts houses under non-Corvette part numbers for just a few dollars. AND the 5/8" ID grommet must be the correct ID and OD size and of the proper heat resistant material or it will desolve under hot conditions and further your transmission problems. Moreover, you can not use a gasket on the OD unit pan; only Right Stuff RTV will work. To be 100% effective, the RTV must be applied to a totally clean and dry surface. What's more, the pan can not be over tightened or tightened out of sequence or the pan will be warped. If the pan is warped and not 100% straight and flat, leaks will persist. Finding a new flat pan to replace a warped pan will be very difficult.

Over the years I have assembled a best practices 4+3 service instruction guide documenting what I have learned about this topic. I have attached a copy for your review and use. I have deleted the photos from the WORD file due to Forum size limitations. However, if you send me your email address, I'll send you the file complete with photos and illustrations.

Good luck with the restoration of your 86 Corvette