So how the heck do I undo the transmission fill plug on my Miata?

Tried to change the gearbox oil on the Miata last week and wasn't able to undo the fill plug. Fortunately I remembered to check the fill plug before undoing the drain plug.

Haven't run into this problem on a Miata before- the fill plug so tight that the jaws of the 14mm wrench actually open and slide off a bit. And I'm not built like Arnold either.

Anyway, any good suggestions trying to undo the plug? Better tool/socket is probably the first place to look, but I don't seem to be able to find a socket that goes onto a 14mm male square plug. It's definitely not 1/2" as a 13mm wrench does not fit.

Other than that, heat? Replacement transmission? New car?

Did you try a big berkeleying crescent wrench?

In reply to BoxheadTim :

Drive it & get it nice & hot. Hit it with Freeze-Off, try tightening it a bit, hit it with more Freeze-Off, then try removing it. 

This might be a good place to try the heat and candle method.

  There are a zillion of these out there, is there a special tool?  I bet there is by now....

Vice grips! 

Maybe apply light heat with a propane torch to the transmission case around the plug and then hit the plug with Freeze Out.

Aluminum alloy generally has a higher thermal expansion coefficient than steel so that would work in your favor.

Hopefully the previous owner did not strip it into the case and all the threads come out with the plug.  That would be a Heli-coil problem probably with the transmission on a bench.

jharry3 said:

Maybe apply light heat with a propane torch to the transmission case around the plug and then hit the plug with Freeze Out.

Aluminum alloy generally has a higher thermal expansion coefficient than steel so that would work in your favor.

Hopefully the previous owner did not strip it into the case and all the threads come out with the plug.  That would be a Heli-coil problem probably with the transmission on a bench.

I'm not sure that heating alumimum is the best course of action, its already soft and I think the heat would make it softer, resulting in stripping the threads.  

Would a VW-style triple square socket work on there?

I assume the head is square so use a good quality crescent wrench with a section of pipe over the handle fo extra leverage. As someone else said try to tighten first to see if you can get it to move then back and forth until it loosens. I have had good luck with ATF and acetone mixed and sprayed on frozen bolts. I would be a bit careful with heat as aluminum softens at relatively low temps. When you do get it out look at plug carefully to see if the threads are embedded with aluminum. Clean plug with wire brush and it would not hurt to chase threads in the case with a tap. Use a bit of anti-seize on plug and then just snug plug. 

rothwem said:

jharry3 said:

Maybe apply light heat with a propane torch to the transmission case around the plug and then hit the plug with Freeze Out.

Aluminum alloy generally has a higher thermal expansion coefficient than steel so that would work in your favor.

Hopefully the previous owner did not strip it into the case and all the threads come out with the plug.  That would be a Heli-coil problem probably with the transmission on a bench.

I'm not sure that heating aluminum is the best course of action, its already soft and I think the heat would make it softer, resulting in stripping the threads.  

Would a VW-style triple square socket work on there?

Point taken but you need to get over 650 degrees F to soften aluminum  -  http://www.tinmantech.com/education/articles/aluminum-alloys-annealing.php

If it was mine I would heat it and check the temp with an infrared thermometer to keep it under 300 degrees locally.  You got to figure a transmission is getting to over 200 degrees in track conditions.    

14mm is awfully close to 9/16" (0.551" vs 0.562"). Home Depot lists it. So does Amazon Call it a "four point socket".

One of the guys here said he used a 12 point socket on one.

Keith Tanner said:

14mm is awfully close to 9/16" (0.551" vs 0.562"). Home Depot lists it. So does Amazon Call it a "four point socket".

One of the guys here said he used a 12 point socket on one.

12 point socket is how I've done it too.

Sounds like another specialty tool FM should develop and sell...

Thanks for the feedback. My long/needle nose vise grips didn't get enough purchase on the head. Haven't tried my "normal" vise grips but given that the head of the bolt is already a little mangled I'm a bit concerned about hitting it with anything adjustable.

I'll probably try the 12 point socket approach, but that might have to wait until my back improves and after Miatas at Laguna Seca.

I'm not sure that the plug is frozen in place, although it also doesn't look like it's been opened for a while and bears traces of having been tightened by Ook the Mechanical Gorilla the last time someone changed the gearbox oil.

If you're having to wait until next week, just order that socket from Amazon.

M16ta said:

Sounds like another specialty tool FM should develop and sell...

Why? It's just a regular square-head plug. There are plenty of existing tools that do the job. 12-point socket, open-end wrench, crescent wrench. There's no need for a special tool, because it really isn't a special fastener. 

A 14mm 8 point 'double square' socket would be good for this, but they appear to be rare.

Had a similar problem with the engine oil drain plug on my Miata. Ordered a new drain plug, actually got a set of 5 from RockAuto. Warmed the engine up and hit it with Freeze-off, waited a few minutes and it came right off easily. I had mangled the head with vice grips trying to get it off so new one went in. Found out that some id10t mechanic (most likely me since I've been the only one doing oil changes on the car since I bought it) lost the washer on the plug and didn't notice. 

I used a pipe wrench after breaking my open ended 14mm on it. Worked well. 

Are these transmissions able to be filled by removing the shifter and pouring the fluid in that way? thats always a good backup if you're able to. Also a pipe wrech can work pretty well on square plugs

In reply to edizzle89 :

They aren't unfortunately, the shifter sits in its own, separate oil bath.

I've got one of these and it sounds like it would be the ticket for your problem. I've used one plus a dead blow to free  o2 sensors that looked like they hadn't moved in 175k miles.

https://www.sears.com/craftsman-extreme-grip-10-inch-adjustable-wrench/p-00913272000P 

One of the universal sockets that grips everything might work too.

https://www.sears.com/craftsman-universal-14mm-socket-3-8inch-drive/p-00924981000P?plpSellerId=Sears&prdNo=4&blockNo=4&blockType=G4

NickD said:

Why? It's just a regular square-head plug. There are plenty of existing tools that do the job. 12-point socket, open-end wrench, crescent wrench. There's no need for a special tool, because it really isn't a special fastener. 

Marketing?  Filling a need that may/may not exist?  Add on purchase?  One stop shopping?  FM sells cam seal installer tools, cam gear ninja tool, crank holding tool, etc...  Do you have to have those tools to complete any of those jobs?  Nope.  Is it nice to have a tool that is designed specifically for the job (that you may use only a few times), but you know for sure the tool will do the job?  Absolutely.

Tilt the rear as high as you can and put the oil in thru the Vehicle Speed Sensor hole?

cmcgregor said:

I used a pipe wrench after breaking my open ended 14mm on it. Worked well. 

This.