Brought my !989 corvette to have the fluid changed in the transmission to my local mechanic. He couldn't break the fill plug loose and would not apply heat, or any other method for fear of damaging the aluminum case. I tried doing it myself on my back, couldn't budge it, I even heated it, no dice. I believe the plug is steel and has reacted w/ the aluminum case transmission causing the problem. It takes a 17mm Torque head, the plug is inset with very few threads showing to grab onto, also it's a convertible, and has the X brace for stability which leaves little room to work. Anyone have the remedy to get this plug out without breaking the housing to the transmission?

 

You are not going to break the case. Get a longer wrench and put a little sack behind it.


Ideally this is the type of tool you should be using. Snap-on CPT110A

 

Or an M17 Metric hex 'short/long' arm key and a M17 'longer' box/combination wrench. The more common M17 Metric hex allows you to support it and leverage with the box/combination.

 

Heat won't hurt the case unless you're using oxy-acetylene.

 

I would even try adding some penetrating oil to assist breaking the Plug Free. First you heat it up with a HOT AIR GUN and then you apply the Penetrating oil so it "draws" the oil into the joint. Have a Fire Extinguisher handy just in case and to be safe.

The problem is you have Steel bolt going into an Aluminum case and that can lead to galling the threads. When you re-assemble it use a bit of the graphite Anti-Seize on the threads and it won't get stuck twice. Our C4 has made me an expert at removing stuck fasteners. A lot of folks here use a mixture of ATF and something as their penetrating oil. I always thought the KROIL Oil was the best but the data shows I was wrong and that the ATF concoction works better than KROIL and that is really saying something. Kroil Oil is great on Steel into Aluminum and I use it all the time. If the ATF concoction can be mixed at home I would look it up and try it if you would like to.

You might have to heat the plug and surrounding case and then oil it a few times before it comes right off. The more heat cycles you do it the better it works. Also try tightening it before removing it might help, sometimes it helps break it free. I wish they had used aluminum plugs in places like the drain holes. Then with the Snap On tool remove the plug and drain the oil out of the 6 speed. Just be sure to use anti-seize on the plug when you put it back.

 

Not the main topic but just curious; How did you end up with a 6spd gearbox in your '89 convertible? I thought only the Z51 coupes had that transmission option in that year? Did you or someone else swap it in?

 

I believe you've 'thought/assumed' wrong! Dependent upon production date it might have a 3.33 GT7 or if early likely the 3.54 GH0. That might be the more interesting question for the OP - production date? Axle ratio verification. An '89 M6 would require only KC4, B4P and V01 I believe.

 

That's cool - I'm glad to be corrected on this. The dealer fleet manager told me when I ordered mine that they were so backlogged on deliveries of the 6 speed that mine would never show up. I'm glad that more than the Z51 cars had that option.

 

you could order the z51 suspension only if you ordered the coupe with the manual zf 6-speed. then you could order the fx3 adjustable shocks and leather sports seats. when fx3 was checked, gm replaced the z51 springs and sway bars with z52 springs and sway bars. then in the 91 model year the z07 package was offered which left the z51 springs and sway bars in place. i had my 89 6-speed coupe with fx3 suspension from 1990 thru summer 2020. i had also replaced the z52 springs and sway bars with the z51 springs and sway bars.

 

The Doug Nash 4-speed manual trans was used from 1984-1988. From 1989-1996, all C4 manual transmissions were the beloved ZF S6-40 (6-speed).

 

The other thing you can try is wax. Heat the case/plug with a heat gun or propane then melt candle wax onto the thread. I've gotten some things loose with that old mechanics trick that I would have sworn were not coming out in one piece. I'd remove the X frame for better access. I once had a 4+3 with the plug so frozen I ended up drilling and tapping the plug for 3/8" pipe plug. Worked fine, but I later got the plug out when I pulled the trans for something else.

 

The plug is beginning to strip, it will not take anymore stress. It is a terrible design and will probably have to be drilled out, unless someone has a magical way to remove these things. Thank you for your reply however. I'm just disheartened with this whole thing.

 

My fill plug was mostly stripped. Here's what worked for me..

I JB Weld'd a similar size head bolt into the plug
then used a deep socket with breaker bar with slow and steady force and it broke free. small miracle.

Let the JB Weld set for a couple days or a week to really set
Remove the X brace if that is in the way, need full access and a clear shot.
I also hit it with PB Blaster every day for a week leading up to the big moment.
no heat.
and ya know it don't come easy..

 

I used a heat gun and got mine out without too much trouble (fill plug popped loose easily but drain took a bit more leverage), not sure the heat did much. I think those having problems likely someone was in there before and over tightened.

all else fails maybe drill and tap the plug itself.

regardless love seeing all of the ZF6s out there being maintained and used!

 

Sometimes ive found you can break something loose by first trying to tighten it.

 

You can get it. Get a picture of how the plug looks. What tool you are using.


If I can get this out with no welding or heat or other magical tricks, you can too.

 

WOW. I thought mine was chewed up, how in the hell?

 

A good way to get frozen pipe plugs out is to heat them and the cool them with water, It causes the plug to shrink slightly and loosen it's death grip.

 

First, make sure you're using the right tool - 17mm male hex. Make sure there is no debris in the plug so that you can get the hex key socket all the way in and fully seated. Heat can be helpful. What I do in these situations is apply as much heat as you can with a quality heat gun (I prefer not to use flame on a plastic car). Then immediately shoot the plug with cool penetrating oil. The idea is that as the cool penetrating oil hits the hot air void in the threads, the air will quickly cool & contract, wicking the penetrating oil in during the process. It also helps if you can get a bit of a tilt on the car so that the penetrating oil can flow downhill. Repeat this a few times, and try with a long breaker bar. With the proper tool properly seated, you're not going to damage anything. If you still can't get it, add vibration (impact gun) to the mix. Start small and have patience, though, is the plug is a fairly soft material, and full blast with an impact gun will distort it. Good luck - you'll get there, just use patience.

 

Low, and I mean really low car and running over curbs on the track cutting corners.