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MG MGB Technical - Driveshaft for Tranny Swap?

I'm getting ready to replace the original 3-sync transmission in my '67 GT with a 4-sync transmission from a later MGB. Both units have no overdrive. Will I need to replace the driveshaft as well? I haven't taken a close look at the existing shaft, but its picture in the shop manual appears to have a sliding section to accommodate slight changes in suspension geometry. If so, is it enough to make up any difference in the length of the transmission?

I should point out that my car has a later 18V engine, but the rear engine plate and flywheel are for the early transmission. I will be replacing these items as well.

Thanks. -G.
Glenn G

3 Syncro non O/D shaft is 29inches on a 67. 4 syncro shaft is 31&1/8th inches long
jim soutar

Check mgambg.com under technical articles. Scroll down to the transmission section and find the swap article. In that article there is a table showing drive shaft lengths and Moss #'s for various swap combos.

Per this table, 3-synch non-OD to 4-synch non-OD swaps can use the original drive shaft whether banjo or tube axle. However, I would double check the dimensions. My 65 has an 18V with earlier front/back plates. It is a banjo rear end.

There are lots of good tips in this technical article.

Let us know how this works out. I am interested in how you handled the transmission mount and fore-to-aft restraint. Also the tunnel cutting.

Bob
Robert McCoy

I look at this gearbox switching effort as a perfect time to do an overdrive up grade either 3 or 4 synchro gearboxes. A lot of work & money to replace gearboxes; and to get the same functionality; no overdrive. Look & ask around the MG web pages, Ebay or Craigs List for an overdrive gearbox.
Rich Boris

Your current driveshaft, which should be 31.125" long, will work with the 4 synchro transmission. I did this conversion 25 years ago and had to use the 30" long driveshaft because my car has a Banjo rear axle. All GTs came from the factory with a Tube axle that requires a slightly longer driveshaft than the Banjo axle. RAY
rjm RAY

2nd Ray: The GT Salisbury axle with 3-synch no-OD engine and gearbox is the same length as the 4-synch gearbox (OD or no OD, Salisbury axle, roadster and GT) on an 18V engine, at 31.125".
PaulH Solihull

It's not easy to measure a driveshaft, what with the u-joints flopping around and the sliding joint to consider. I finally stood the thing on end and tried to keep the flanges parallel. I came up with 30-3/8" with the sliding joint fully collapsed. I think that may be close enough, since the sliding joint can take up some difference. If it looks obviously wrong after I get it all back in the car, I'll find a longer shaft. -G.
Glenn G

It's the fitted length that is quoted, i.e. strictly speaking the distance between the gearbox and axle flanges. Propshafts also have a Flange to Flange Closed (FFC) measurement which is when it is fully compressed. My V8 which is 31.25" fitted has a FFC of 29.75". When fitted the sliding joint needs to be about midway between fully compressed and the point at which the overlap between the internal splines starts to reduce. The V8 gives 2" of movement, i.e. 1" either side of a notional mid-point.
PaulH Solihull

Three weeks and a day into this transmission swap, and I'm beginning to see the light at the end of the tunnel. The original driveshaft fit fine. Length in place is about 31-1/8" with the new gearbox installed. Last night I filed off 1/4" of aluminum from the upper starter fastener boss on the bell housing to make clearance for the tunnel.

I cut off the rear part of the tunnel cover and enlarged the hole in the tunnel for the gearshift lever. Not sure what this will look like when finished. If anyone can point me to photos of a good job, I'd like to see it. Also wondering if a tunnel cover from a later MG would be an easy fix? I'll probably be driving for a while with only duct tape covering the hole while I ponder this.

I read somewhere that I could use the original solenoid with the late model starter by wiring two terminals together on the starter. True? Which terminals?

I am looking forward to having a more dependable starter. This is the reason I traded an old Buick 215 V8 for the parts months ago. Then a noisy throwout bearing gave the final push to get the job done. -G.
Glenn G

Go here and look around. Unfortunately, I do not think their neat pre-made cover is available any more.

http://www.mgbmga.com/tech/index.html

It would be silly and counterproductive to include the original solenoid in the start circuit. People do it, but if not correct the drive won't engage, and it gains nothing.
You can either put both cables (and any secondary take-offs) on one terminal of the OE solenoid, so that it functions only as a connection point; or, reroute the battery cable (plus take-offs) to the new solenoid. In either case, reroute the WR wire to the new solenoid. BUT: It is strongly recommended that you install a start relay per later cars. In that case, the WR activates the relay, and the relay output goes to the trigger terminal on the new solenoid.

FRM
FR Millmore

Glenn, I'm glad the driveshaft worked out.

Regarding the starter are you using the old 3-synchro flywheel? I think that was your intention per your initial post.

If so, I don't think the later model pre-engaged starter will work. If I remember right from the archives the gears don't match properly. It is worth a double check.

You could go with a gear reduction starters. These are available to replace either the 4-synchro flywheel or the 3-synchro flywheel starters. These are compact and do not wear the ring gears like the 3-synchro ones. But you're talking hundreds of dollars.

Out of curiosity how did you handle for/aft (rebound) control for the new setup? On the 3-synchro that was handled by the old strut rod.

I envy you. I'm still struggling with a 3-syncho with a bad 2nd baulk ring.

Bob
Robert McCoy

Glenn, I tried to edit my erroneous post but couldn't do it. I misread what backplate and flywheel you were using.

Check the archives regarding the tunnel/cover mods. I think some extended the old covers by cutting it and using sheet metal. I don't know if the 4 synchro models had the tunnel covers.

Bob
Robert McCoy

"I read somewhere that I could use the original solenoid with the late model starter by wiring two terminals together on the starter. True? Which terminals?"

You can either leave the solenoid in-situ with the two thick cables on the same stud (either stud) and extend the white/red down to the new starter, or reroute the battery cable to the new starter and extend the brown and white/red wires to the new starter if you want to remove the original solenoid.

As FRM points out you really should have a relay between the ignition switch and the new starter as the solenoid on those take significantly more current than the original solenoid, which will probably burn-out your ignition switch eventually.

However the original solenoid is already a type of relay, so you might just as well leave it and its connections as they already are, connect the cable that went to the original starter motor to what would normally be the battery cable stud on the solenoid of the new starter, and link that stud to the spade on the new solenoid, which I think is what you are referring to.

The ignition switch will energise the original solenoid as before, and that will extend 12v down the original motor cable to the stud on the new solenoid, and also to the operate spade for the solenoid. The new solenoid will operate and extend 12v to the new motor as it normally would, so cranking the engine. The only difference to the factory installed pre-engaged starter is that the stud on the new solenoid that would normally have the battery cable and the browns connected to it, and hence have 12v all the time, will only have 12v when cranking, but that's all you need.
PaulH Solihull

Thanks for the clarification, Paul. That's what I thought I remembered from my brief installation of a gear reduction starter last winter. I did not keep that starter because it didn't fit right and it sometimes failed to engage the flywheel ring gear. I agree with Fletcher that it is silly to retain two solenoids, but at this point I just want to bring the rental car back to the agency as soon as possible and clean up the details on the MG later. I am thrilled that I can remove and install the later stock starter without removing the distributor and oil filter holder.

Regarding the engine restraint parts, my car does not have them in the area near the rear engine mounts. Possibly they were removed by a previous owner, or maybe it was not stock on GTs? The car does have a bracket fitted to the top of each front engine mount that seems designed to keep the engine from jogging forward in a collision or panic stop. I actually had to make a panic stop several weeks ago and suffered no ill effects, so I'm not highly motivated to add the rear restaint. -G.
Glenn G

Silly or not, I've been using my original fender mounted solenoid, as a starter relay, for over 25 years with absolutely no problems. It retains the ability to crank over the engine, without having to wire in a remote starter switch, whenever you need to crank over the engine for service work. By simply jumping the two starter terminals together, whenever the ignition switch is turned to start, both the starter solenoid and armature start to function at the same time. When I set up my car, I had access to a '68 GT parts car, which is where I sourced the backing plate, flywheel and starter motor. I put $1 into the starter motor to get it working; a brush spring had broken. That starter motor hasn't been apart since I installed, it along with the 4 synchro/OD transmission. My original ignition switch, which is now 45 years old, is still as tight as the day I bought the car 40 years ago. The system will function as good as or better than the factory design. For the last decade I've been running a supercharger and the starting system doesn't mind it a bit. RAY
rjm RAY

This thread was discussed between 23/03/2012 and 06/04/2012

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