- My model of MX-5 is: __1997 NA 1.6 90 BHP
- I’m based near: __Glasgow
- I’m looking for technical help or recommendations on: __Removing a stuck gearbox filler plug.
I’ve tried to change my gearbox oil tonight and low and behold the filler plug is stuck solid.
I’ve soaked it in penetrating fluid, tired a hammer on the end of the 14mm wrench with no luck.
I’ve a few tools coming tomorrow which I hope will aid in my plight including a draper wrench extension.
I’m concerned I strip it trying to get it out.
I was also thinking of getting an impact wrench with a universal joint to get access to it. However, I’m not sure even a compact one will get in there.
I was also going to try hitting it lightly with a normal hammer.
Any other tips anyone can suggest?
Thanks
Would an impact wrench be overkill and cause more issues than it solves?
Are these the iron plug type with the square head. Use a square socket on them and possibly a longer bar to undo. Previous owner may have overtighten it, doesn’t need much force. I used to put a couple of winds of PTFE tape on the threads to seal.
1 Like
Might be worth trying it after a run while the gearbox is still warm or applying a bit of heat to the area with a heat gun.
4 Likes
I’d actually taken it for a quick run to warm the oil to drain it easier. I thought this might’ve helped with the removal but no such luck.
Yes, 14mm square head.
16mm socket seems to fit better. I just didn’t want to try it to scared I’d round it.
If I get it out, would you recommend the PTFE tape when putting it back in?
I don’t really think that impact will help remove this type of filler plug, it’s not mating against a surface like the drain plug is. If you can get a tool on it at about the 4 o’clock position, I’d try using a jack to rotate the tool gently upwards. Ensure that you have a good fitting tool. From memory a 16mm (?) double hex socket fits if you can’t find anything better.
Edit: Was typing as you replied, you have confirmed my 16mm socket then…
3 Likes
Workshop manual says to apply sealant to the threads before refitting but PTFE tape will do.
1 Like
Sealant? Really? Wow.
Would the sealant be blue locktite or the likes?
Just silicone sealant.
For no particular reason really, here’s a nice photo of a clean plug, one with sealant on it and one with PTFE tape.
1 Like
I’ve found the jack and spanner trick video on youtube.
I might give this a go.
How to loosen a stuck Transmission Fluid / Oil Drain Plug (youtube.com)
2 Likes
And here is a chap @martin demonstrating a method of filling the gearbox from above by using a funnel and length of hose… 
1 Like
I’ve bought a 500ml pump type thing to avoid doing that
Can you overfill a Mark 1 gearbox?
When I first had mine it was so notchy from 2nd to 3rd I took the plug out to check the level and got a faceful of oil - it was spewing out everywhere.
Managed to get the plug back in and did nothing more to it. Had 10 years of trouble free gear changing after that.
I think it would be hard to overfill. Happy to be corrected.
When you open the fill plug some oil should come out then stop that way you know it’s at the correct level.
Then stick your finger in the hole and check the level that way.
Well that’s what I’ve been reading online and watching on youtube.
GOOD NEWS!!! I’ve got the plug moving.
A combi of everything basically
Penetrating oil plenty of it. Left over night
Hit the plug this morning a number times gently with a hammer
Used a laser wrench extension bar. Nice piece of kit actually on a 14mm wrench
https://amzn.eu/d/2ZeVQPS
Heat gun for a few mins then back to the extension bar and that was it.
A bit or relief as I had pictured having to pay to get it sorted.
When it goes back in i’ll wrap it with PTFE
2 Likes
They can get over filled if they have been jacked up at the back, this gives extra room but the level will be out as it should be checked on the level.
Could I use copper grease when putting it back?
So even though I read your post which said fill it on the level… I had the front of the car raised whilst filling it.
It then dawned on me I seemed to be putting more than 2 litres into the gear box.
I then jacked up the back of the car and of course a load of the trans fluid came out 
1 Like
Normally it’s just a matter of leverage. If something doesn’t break loose with a 3/8 ratchet and space is too tight for a breaker bar normally an extension with a 14mm deep socket slips over the handle of the ratchet nicely doubling your effort 
1 Like
I took a break from doing mine as it too is currently so tight and the weather hasn’t been good enough since anyway.
I had previously tried a 16mm socket as suggested since I didn’t have a 14mm, but like roadster1998 also didn’t try too hard in case of rounding it.
Bought a 14mm in the end, tried to extend its’ length with vice grips.
This gave the leverage, and they gripped the spanner perfectly so as not to move about, and I thought I’d succeeded as eventually the whole lot freely moved.
However the spanner must have been slipping round the square as it hadn’t loosened at all.
I’m reaching the point of wondering if it’s better to try vice grips on the plug, and if it gets too wrecked but does come off, buy a new plug.
I couldn’t find anywhere that sold hollow square sockets of 14mm size which is why I ended up with an open spanner, do you know of anywhere?
