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The gearbox in my 1990 NA6 is leaking transmission fluid out of the front and rear seals, crunches into second, is difficult to put into 5th, sometimes won't go into 1st when sitting at the lights... The mechanic has suggested that I replace it.

I have three options and would like to know people's experiences / suggestions on which would be the best choice.

1) Replace existing 5sp with a second hand unit from a "low kilometer" car.
2) Put in second hand 6sp from an NB also with "low kilometers".
3) Rebuild existing gearbox with new seals, and changing whatever internals are damaged.

Does anyone have any experience with second hand gearboxes, how can I guarantee I get a good box and not one that has been thrashed? It makes sense that all the crappiest MX-5 get scrapped in Japan and those are the ones that end up here as parts cars, hence a gearbox from one of these cars is questionable.

To counter that a friend of mine had the gearbox in his MR2 rebuilt (by a gearbox workshop) and it is still not much better than it was before the rebuild.

Does anyone know of a reputable transmission specialist in Melbourne?

While the gearbox is being sorted I will also get a new clutch, and do the rear main seal on the engine.

I did search and found these useful:
viewtopic.php?f=29&t=40830&hilit=transmission+change

Buy 5 speed for <200 dollars install. 6 speeds are too expensive to justify for most people. They will take 2nd gear 8000rpm clutch drops and you can drift them till your hearts content of <200whp, but nothing else about them helps unless you just really like them.

Dann

ill put in my 2c worth.

1)this will be the cheapest solution but like you said there's no way in guarantee it will be a perfect box, best thing if possible is to try it out before you purchase.
2) I am currently in the process of putting a 6speed in and my reasoning for this is because its a shorter gear ration i.e better torque range for our weak little engines and i got it cheap <$1k
3) I wouldn't bother rebuilding something back to standard; your better off doing something crazy with it if you went this route but it will definitely cost alot of your time and money.

You have an old car. I would be using a standard 5 speed ( the 6 spd want do much for your) and probably a used box would be ok. There are transmission specialists around. If you rang say the MX5 factory or went to see them you would be able to get some info and guidance. You can also get info about transmission specialists by checking other auto businesses in your area eg talk to Bursons who sell to most businesses. Most of these things come down to $$. How much do you want to spend or what is your budget on a car with a resale sub $10K- your call but helped along with quotes.

If you were doing the work I would say rebuild. However, if you just want a standard box and are paying others, get a second hand. Labour will get pricey for a rebuild and swap and parts. To give you an idea a rebuild usually costs a few grand from us. Most of that is labour. Would recommend a second hand 5 speed.

Yeah, as Pam mentions GB rebuild is going to cost you over $2k unless you can do it yourself - think someone on here mentioned they'd paid about $2.3k last year.

ave wrote:how can I guarantee I get a good box and not one that has been thrashed?

Very occasionally it's possible to get a GB from a car that's about to be dismantled, so you can test drive.

Thanks to all who have replied. I've decided to go with the second hand 5 speed, will get clutch and rear main seal done at the same time. I'm getting a workshop to do all the work, so lets see how much of a hole it blows in my pocket.

I'll post up my experiences once it is all done, may be helpful to others.

I had a 2nd hand NA8 5 spd put in mine after having the clutch swapped. Much better than the worn NA6 5 spd that was in it.

G

It has been around a week since the transmission and clutch were changed, so I've got a little bit of feedback.

For those interested in the practical details here they are:
Second hand NA 5 speed with 80k kms ~$450
New standard clutch ~$300
Labour ~$500
I also had a new clutch slave cylinder installed, swapped tail shafts, and the engine rear main seal replaced.

So all in all it was an expensive outing, obviously if you source the parts yourself and do the work yourself you can do better.

I got Allmaz in Clayton to do the work, thus far I've had no issues with them and would go back.

The second hand transmission is much easier to shift than my old one, but getting into second gear when cold can be difficult. I know others have the same issue. Once the gearbox has warmed up a little everything becomes very smooth.

One thing I had done wrong with my old gearbox was to fill the turret with transmission fluid. When I bought my car the turret was full of gunky black liquid, so I thought I was doing the right thing by replacing it... Turns out all that is necessary is some grease for the linkages and that is it. There are no parts in the turret that are constantly moving and the turret is apparently sealed off from the rest of the gearbox to prevent oil getting in.

[quote="ave"
The second hand transmission is much easier to shift than my old one, but getting into second gear when cold can be difficult. I know others have the same issue. Once the gearbox has warmed up a little everything becomes very smooth.
[/quote]

I had this with my 1990 NA6 for 16 years - then I replaced the Gearbox oil with Redline Shockproof - not cheap but advised that it's good for 200K kms (really?).

Since putting in Redline - no problems getting into second when cold and gear changes generally are now smooth as butter - recommended.

Ralph

nothing wrong with those prices. The g/box was also cheap. The gearchange thing is overstated. If you have an injury and you mask it you are asking for possibly doing more damage. The reason why 2 nd gear baulks is because that is almost a design certainty and partly why people prefer autos. If you rush changes esp 1-2 you will eventually end up with synchro damage. Fix is take it easy- whats the rush- when the gearbox warms up it improves. Its about having some mechanical empathy. No Redline in my car- does not need it.

What oil do you use in your gearbox Mr Morlock? Genuinely interested to know for my future reference.

ave wrote:One thing I had done wrong with my old gearbox was to fill the turret with transmission fluid. When I bought my car the turret was full of gunky black liquid, so I thought I was doing the right thing by replacing it... Turns out all that is necessary is some grease for the linkages and that is it. There are no parts in the turret that are constantly moving and the turret is apparently sealed off from the rest of the gearbox to prevent oil getting in.

You are opening up a can of worms.

Mr Morlock wrote:nothing wrong with those prices. The g/box was also cheap. The gearchange thing is overstated. If you have an injury and you mask it you are asking for possibly doing more damage. The reason why 2 nd gear baulks is because that is almost a design certainty and partly why people prefer autos. If you rush changes esp 1-2 you will eventually end up with synchro damage. Fix is take it easy- whats the rush- when the gearbox warms up it improves. Its about having some mechanical empathy. No Redline in my car- does not need it.

Actually you are totally right on this. When I first started having this 1st to 2nd issue I found that I was shifting wrong by trying to go straight from 1st into 2nd. I found that by going into neutral and then slowly feeling my way into second it is much easier.

I've tried Redline, in my opinion it is not necessary for our cars, the regular mineral oil is just fine.

I found this TSB a while ago and have noticed that its very rarely if ever referred to so I'll post it up incase others are interested in giving this a try. I'd be curious to know the cost of sourcing the necessary parts and labour but have never asked.

http://www.miata.net/garage/tsb/s004_92.html

"Hard shifting into second gear before the vehicle has had sufficient time to warm up may be caused by insufficient clearance between 2nd gear synchronizer ring and the 1-2 clutch hub. Design changes have been made to the 2nd gear synchronizer ring and clutch hub sleeve to correct this problem since April 1, 1991."

Someone mentioned earlier this year or late last that their MX5 5 speed GB rebuild was about $2300 - not sure if that included removal & installation or if they simply took the GB to a transmission specialist.