UN1 Gearbox Issue


Evening all

Ok got an issue with my box, finally got the car on wheels on the ramps I purchased, tried on the weekend to roll them off the ramps and nothing, box is locked right up. I can roll to the rear about 6" maybe 8" at best then it locks bang right up, Chris Cole thought maybe 2 gears had been left selected (it seems its done to torque a bolt up) but I removed the rear casing tonight and its as below

So all neutrals line up, we kind of thought it would not be 2 gears as I could select all gears which I would not be able to do if it was not aligned as below.

Anyone any next thoughts? Don't want to have to pull the box unless I have to as that would be the third time and the car has never been on the road yet!!


I did once have a similar problem with the bolts that hold the bell housing onto the box that were interfering with the diff - not all of them are the same length. The other thing to check is that your driveshaft cv bolts aren't too long and catch the side case of the box too.

Chris Kouba

Not an expert on this end but what Paul mentions is fairly important. Double selected gears would likely not allow any rotation- should be locked solid since it's trying to turn two mechanically linked systems at two different rates through the same shafts. Unfortunately, it's likely to be something less obvious, but hopefully less catastrophic, to remedy.
Hi, I agree with Chris. Double selected gears would totally block the gearbox. Im my opinion, something is wrong with the differential.

I would do the following test :

Having the car with rear wheels not touching ground, try to turn them by hand. Do they turn freely separetely ?
Also just to mention that, according to your pic, your slots for the finger are badly worn . Better to fix that and you will greatly improve your shifting precision and stop the damages you make to the brass forks inside your gearbox.

So basically, to summarize, your gearbox needs urgent care.


Ian Anderson

Lifetime Supporter
My thought was further outboard.
CV bolts snagging on something in the hub/bearing carrier?

Possibly a wheel balancing weight snagging on the brake caliper?

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Thanks all top tips, noted re selector forks, this was a fully recon box so not great if they are worn it was not cheap to supply rebuilt that's for sure, will check bolts and snagging later today, all much apprciated
Hi Shaun, I had a similar problem with nothing turning, turns out I had trashed the bearings that the gearbox shaft went into. Easy job to replace but a bugger to get the lid bearings out.

Good luck.



There was a spectacular moment for Dave Parker when we used to hill climb and sprint in the GTD 40 car Club. At Wiscombe he was full on it when the 1/2 inch (or so) pin in the differential and planetary gears moved and locked his gearbox solid in an instant. Unfortunately he was doing about 50 mph at the time and full on the gas. The gearbox casing snapped in two ending his day.

If you car is rolling about 6 to 8 inches then if this pin has dislodged in your diff it may be that is allowing some roll and then fouling. Only a thought rather than a definite diagnosis. Hope you resolve it soon.


Quick update, nothing jamming, bolts all ok, no rotation when wheels off the ground, so box out, probably not bad idea as the half shafts were leaking oil, when rebuilt there were no o rings so I replaced those and the seals but probably not correctly but when you spend £3k on a gearbox with rebuild you sort of don't expect to have to redo that work again!! Anyone know of a good UN1 gearbox specialist in the UK? Ken at Classic Car Gearboxes has been mentioned


Yep it came from Chris I know he is the guru on these boxes but I have had nothing but schoolboy errors on this one a proper Friday afternoon as they say so lost a bit of confidence...Cost a packet as well !!

Mike Pass

I have a copy of the UN1 manual. Drop me a PM if you would like a copy. It may help in locating the issue if you know what is inside.
Very surprised to hear it was a gearbox built by Chris. I know he was ill at a moment, so maybe it was not its best time.

I worked with him years ago as we made specific 3rd gear sets for th eUN1 and other projects like this. He has been always very precise and accurate in his job.

Anyway, UN1/UN5 gearboxes are very simple. We used (and abused) them a lot in my autocross club (and also in some GTs) and then I used to rebuild or transform them (from simple ehancement to sequential shifting) and they rarely break.

From my experience, when you start to have issue with a gearbox, better to open it than waiting for a miracle.

Specially, I repeat : your finger slots are worn and you must fix this. In your setting, the selector finger doesn't find its right place and does not apply an axial straight pressure to the shaft according their rounded shapes (so you have a mix of radial and axial effort), thus it impacts the brass forks and as they are against metal, they are eaten, thus end play, bad shifting, then mcuh effort to engage gear, then more effort on forks then they get more eaten and so and so....

The worn slots is a common issue when you get these boxes out of the donor car with high mileage. To rebuild the slots, I uded to bring metal with a TIG/MIG to restore the square shape they must have and temper them after, then they can start a new life as if they were new.

We are all too old for the autocross, then I retired from this and all the the UN1 spares will be sold to a rebuilder here in France. Meanwhile if you need parts, before they leave, send me a msg, I surely have what you may need to fix the default. Also, I don't know what is your final ratio, but I may have 2 or 3 sets of 3.44 crownwheel if you are not already equiped and in case of any interest.



Hi Stephane
Indeed I was as surprised his reputation is the guru of all things UN1 not casting aspersions just relating my issues with this box, this was now almost 3 years ago as well (its been a while in the build!!).
My thoughts as well it needs to be out and checked, no point in blindly hoping it will be ok once the car is done, easier to remove now which I will later today.
Noted re the slots, I was not fully sure what good ones look like but mine seem rounded off, top tip re repair
And thanks for the kind offer of part supply, I know the feeling of getting a little old sometimes, its easier to get down on the floor but seems harder these days to get back up!!

Example of shafts. (Forget the rust, these are just shafts for waste). On right, a new one, on left a used one. You can see that one side of the slot of the used shaft is shaped. This comes from the lack of control of an H pattern in the UN1 conception for shifting. As the finger is only cable driven, it can move in diagonal and then shift after shift eats the slot. This gives bad shifting as the finger is not at the right place.

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Thankyou I honestly thought thats the way they were made, so I guess the fix is to weld up and re-MI’ll the slot

many thanks for the info