Spitfire & GT6 Forum

Hi All,
Just a question/comment that I couldn’t find searching this forum. My gearbox was starting to sound a bit louder than usual so I decided to drain the oil and refill it. I had previously used Redline MTL in there and always felt it was a little on the light side. Being unable to find MT90 locally I decided to grab a quart of 20w/50 motorcycle oil along with another quart of MTL. I figured the motorcycle oil, being designed to work with a wet clutch, would be suitable for the gearbox. If it sucks I can drain it out and put MTL back in. To my delight the gearbox seems quite happy with the motorcycle oil. The gearbox is quieter and shifts beautifully.
Anyone else tried this?
Is there any grave concern running motorcycle oil in the gearbox?
Let me know what you all think.
Cheers,
Mike

You may get lots of input ----- Matters little at this point as you've got a "done deal".... Let us know how it turns out.

Personally, when I start to hear funny noises from a system, I try to find the cause rather than a band-ade.

Z

I've been running my 3-rail w/overdrive (72 Spit) on straight 30WT for over 10 years now. Volvo recommended ATF in their overdrive gearbox. I know they are overdrives, but the lube in the gearbox is shared by the gearbox. Personally, I would not run a multigrade oil in the gearbox.

Marty

IF I read online specs correctly, MTL is a 75/80, while MT90, which I have used in several cars, is a 75W90 and, therefore, arguably a bit more suitable for these gearboxes?

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http://www.fairpoint.net/~herald948/database/

I did raise the idea of motorcycle (wet clutch) oil in our gearboxes with Richard Widman (he owns a South American oil company and is very enthusiastic obout classic cars) We were discussing optimal lubricants for O/D transmissions.
He suggested the motorcycle oil might not be the best choice.
These (motorcycles) are not syncromesh transmissions and typically contain a fair amount of Molybdenum.

It could have been the MTL was shearing down over time, or your gearbox has sufficient clearances that a heavier oil (like MT 90) would be better.
However, see how it goes, and let us know.

The gearbox was rebuilt last year and anything not in-spec was replaced. There has always been a rattle sound coming from the gearbox when in 4th gear, that sounds like the 1/2 forks or synchro hub. I’ve taken the gearbox apart 3 times trying to find the true culprit to no avail. Cruising in 4th at low RPM with everything warm you could hear the rattle. Pushing in the clutch would stop the noise, but pulling it of gear with the clutch pedal out would not. Only way to stop it would be to clutch in and select second gear, which lead to my forks or hub conclusion. Anyway, the heavier oil seems to have diminished the sound. I found a couple gearboxes and a new selector so I will be rebuilding again this winter. I took the beast for some aggressive driving last night and the shifts felt good. I’m going in the hunt for some MT90 or heavier GL4 oil. Selection is pretty crappy in this neck of the woods, unless you’re buying oil for your tractor.
Thanks for the input. I’ll update when I remove the motorcycle oil and pump in some MT90.
Cheers,
Mike



Edited 1 time(s). Last edit at 2022-08-27 09:22 PM by Mike Harris MkIV.

Motor cycle oil has detergent in it and lacks the qualities you want for an old synchro transmission.
The detergent oil will foam.
ATF or straight 30 weight non detergent oil would be a better choice in a pinch. 80 or 85/90 gear lube marked SAFE FOR YELLOW METAL is your best choice.

Steve



Edited 2 time(s). Last edit at 2022-08-28 08:04 AM by Jediscuba.

I had some bad grinding when shifting into 2nd. When I was doing this year's spring tune-up, I put straight 30W Non-Detergent Pennzoil in the transmission (non-OD) - cleared it up.

Bob

Can you not get MT90 from any internet sources? It's what I am now using in the TR6 box I rebuild this past winter. It falls into the viscosity catagory that is recommended. I don't think you'll do any harm with your formula for the rest of the season but I'd consider seeking out MT90 after the rebuild. - Pete

The noise you describe sounds unusual (unless you have very sharp hearing)
Yes the shifter forks can wear, especially on the 1-2 rail that gets the most wear. I have even had a fork break.

Beware that modern GL oils are Gear oils that are intended for gear trains and the protection against gear wear.
They are developed to offer boundary lubrication under extreme pressure where the oil film would otherwise break down and metal on metal contact would occur.
In recent years vehicle manufacturers have designed gear trains that are smaller, run faster, run quite, handle more power and require fewer (if any) oil changes. This all requires a great deal from the Gear Lubes and their formulations have been adjusted to meet these demands (in layman's terms, they are VERY slippery) Even if they still display the old (obsolete category) GL4 or higher GL5 rating on the bottle.
This is why we now have MTL's or Manual Transmission lubes, as most GL's are no longer suitable.

What I am saying is to be very cautious if using GL rated lubricants in a synchromesh transmission.
Non detergent 30 weight might be a safer bet.

Or if you want thick, add some Lucas oil stabilizer !

Now I can't believe I wrote that last line, the stuff is CRAP. It is, almost entirely VERY heavy oil base stock with virtually no additives whatsoever. Perfect for used car salesmen to quite a clunky old engine, or maybe just what you need as a stop gap.

I’ve gone ahead and drained the motorcycle oil and dumped in the bottle of MTL I had. I’ll consider the motorcycle oil a “flush”. I like the idea of adding Lucas stabilizer to thicken things up a bit, I’ll decide after a few days of driving. I think the rattling sound is related to excessive play in the 1/2 synchro hub inner. It has a fair bit of “slop” which is mitigated when it’s in gear. Unfortunately this part is unobtanium. I have a replacement top cover and selector forks and will be getting a parts car this month. I also have a single rail gearbox I could swap in, but it’s requires a 20 spline clutch.
Thanks everyone!

Got mine on Amazon. $20/Qt. w/free shipping.

Right, it's not cheap but then again it'd not a fluid that gets drained and replaced at short intervals. - Pete

Www.soloperformance.com is my supplier for MTL and MT90. Based n in St. Louis and our SCCA sponsor.

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Larry K, 1972 Gt6, previous owns:
1964 Spitfire MK4 1980 1500
1973 MKIII GT6
Under care:
2 MINIS, 03,011, 2 Jag Sedans 57,58, 2 Land Rovers 97,011, Valiant V200 6 1964
1972 GT6 MKIII O/D, Retired 25 yr ASE technician. 58 yr. mechanic shop experience.