MKIV transmission for a MKIII

Hi..Sorry about the avatar guys, I have both a mga and a Vintage Racing MKIII spitfire..

anyhow, the transmission is going (cannot shift into fourth at high revs) on the MK III spitfire.. in my stock pile, I have a rebuild MKIV (1500) transmission...


I realize that the input shaft is different, but, if I get a new clutch to match, will the MK IV transmission bolt up to the backing plate of the III and fit with mounts, tail shaft etc. etc.

thank you for the assistance in response...

r

A Mk IV (4 synchro three rail) should bolt straight up to a Mark III. I think it will even use the same clutch. The 1500 trans is a single rail and even the early ones of those had the same pattern on the input shaft. At some point they changed.

Both later gearboxes are slightly longer than the early gearbox, but usually there's enough "give" so that it will fit without problem. (No problem at all if your propeller shaft is the sliding spline type!) Aside from sorting out clutch discs v. input shaft splines, you'll need to round up a u-joint flange for the front of the propeller shaft from a 1971 or later car (u-joints are the same), as you can't swap flanges on the back of the gearbox.

You say can't shift to 4th at high revs -could be the clutch disc coming apart & not releasing at high revs (had it happen once). Check when trans is out .

As I understand your post, you have a Mk3 engine that you want to use with a single rail gearbox. There are more steps involved than just bolting the gearbox up.

You can transplant the single rail box in where you originally have a 3-rail box. It will bolt up to the back plate but there are several more steps involved. To use the single rail box on the Mk3 engine you need a flywheel and all the clutch parts from 1500. You will not be able to use your 1300 friction disk or pressure plate... those have to be from the 1500. You will need to sleeve the bolt holes in the 1500 flywheel to work with the smaller bolts that work with the 1300 crank.

Andy said you can use the driveshaft. My 1300 had the "strap type" driveshaft and had to be replaced with a later driveshaft from a 1500 to work with the single rail box. Lastly... don't forget the speedometer cable. The 1300 will have a cable that screws onto the gearbox while the single rail box will have the speedometer cable that is clamped onto the gearbox.

Bob, I have done this a couple of times and I don't recall having to swap out the fly wheel. I changed the clutch plate to fit the splines of the new transmission, think it's 21 and the rest of the internals matched up. The big issue was the drive shaft and like others have said a flange to match will be required if you have a sliding one, but I cut the shaft on one and had it balanced which worked out a bit smoother, the other one may have had a issue but the individual was not willing to let go of the cash to have it done. We just ordered the new cable for the speedometer, the long single one, as the other is a split one.
The metal tunnel cover was a bit of a pain because of the extra length so it was bent out of the way for the duration of this exercise.

Wayne

The two flywheels are different. The 1300 flywheel is flat while on the 1500 the pressure plate bolts onto a raised ring. The single rail box does have an input shaft with more splines. That means you have to use the 1500 friction disk. Because the friction disk for the single rail box is bigger in diameter you have to use the bigger pressure plate of the 1500. The larger pressure plate dictates that you use the 1500 flywheel.

In a friendlier world there would be a small diameter 20 spline friction disk to allow use of the original flywheel and pressure plate with the single rail box. The only existing disks like that are for racing and they lack the "springs" on the center hub that you want on a street car.

Contact Ted Schumaker at TSI for more details about this if it sounds confusing.

dklawson said:

In a friendlier world there would be a small diameter 20 spline friction disk to allow use of the original flywheel and pressure plate with the single rail box. The only existing disks like that are for racing and they lack the "springs" on the center hub that you want on a street car. ...

Click to expand...

That friendlier world used to exist and may still, in the form of a driven disc from the home market Dolomite 1300 sedan. That plate has the 20 splines on the 6.5" diameter driven disc. Part number is GCP244 if available. Both Canley Classics and Rimmer Bros. list them as available, but they aren't cheap.

All, I think I see why there is a bit confusion. I'm pretty sure the swap I made was on a early 1500 and checking with the owners of both cars confirmed this. One had brought a wreck and we used the transmission and drive shaft, the other was a off Ebay transmission only and we had to make up the rest.
It's been 7 years and a lot of projects in between since the last one. I was surprised they both still had the cars and not had any trouble, other than a leaking clutch master.

Wayne

Some of that confusion might well lie in the fact that the US 1500, 1973-74, continued to use the same three-rail all-synchro, 10-spline input shaft gearbox of the MkIV, albeit with a larger clutch (7.25" v. 6.5" for the earlier cars). The 20-spline input shaft, single-rail gearbox came along in '75, using the same pressure plate but obviously a different friction disc.

Wayne, when I switched gearboxes in my car you were one of my sources for information. Unfortunately, all the steps I mentioned earlier were required to get a working combination of components for my car. Ted at TSI advised that I not use a racing clutch disk and suggested the least difficult fix was to fit the 1500 parts I mentioned earlier.

Doug, sorry to have made you have so much trouble. I was just lucky on both that they were a 1500 in the years that worked. I will stay out of these, kind of questions in the future , too much to remember at 72.

Thanks Doug, glad you were here to keep thing in prospective.

Wayne

Wayne... I am by no means suggesting you caused me trouble or problems. Far from it. You may not remember, but you even took the time to discuss this with me on the phone 3 or 4 years ago. I appreciate your telling me what you did.

My car required all the steps I described but that is not your fault, it just reflects the evolution of the Spitfire and the mix of parts over the years.