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MG MGB Technical - Converting to 3 syn OD - gearbox mounts?

Hi all,

I am currently fitting a 3 sync OD box to my 67 BGT that previously had a non OD box. I have the box attached to the engine and have swung it all back into place. However, although the engine is sitting on the motor mounts that gearbox X member appear to be around 1 inch too far forward. Has anyone else had this problem? Did the non OD and OD boxes use the same positions on the chasis for the X member mounts?

Cheers
Iain
67 BGT
I D Cameron

Ian;;
I have never done that exchange but I have seen different positions and bolt holes for the rear cross member.I converted an MGB to a V-6 amd used the rear most mounting holes on the frame (tub).
Sandy
Sandy Sanders

Hi Sandy,

What year of car? I do know that later RB cars had different holes, but I don't know about the early cars.

Iain
I D Cameron

Iain,

All cars had the two sets of holes for OD and non OD boxes - the OD box uses the front set of holes which are indeed about an inch different.
Chris at Octarine Services

Oh dear, my car only has 4 holes, two on each rail. Did the cars originally have 3 or 4 holes on each rail? No sign of any others. I believe that there is a captive plate that the bolts screw into. Is this correct? If so I may try drilling new holes and sliding the plate along. Of cirse, it may not slide if the passage is blocked. I think this could cause me a problem!

Iain
I D Cameron

Ian
Years ago I did the same conversion to my 65 roadster.

Put the new box and O/D withe engine and then PANIC the holes were in the wrong palce for the cross member.

Consulted an expert like Chris - he said look again the correct holes and captive threads WILL BE THERE but probably covered with underseal and more underseal etc. Proded gently with a screwdriver and found the correct holes. RELIEF!

So please look again for the holes and threads will there no need for any drilling.
George
251 GRW
G R Wilder

Ian
Years ago I did the same conversion to my 65 roadster.

Put the new box and O/D with the engine and then PANIC the holes were in the wrong place for the cross member.

Consulted an expert like Chris - he said look again the correct holes and captive threads WILL BE THERE but probably covered with underseal and more underseal etc. Proded gently with a screwdriver and found the correct holes. RELIEF!

So please look again for the holes and threads will there no need for any drilling.
George
251 GRW
G R Wilder

Well, I am not so lucky as you George! There has been extensive restoration of the chasis rails and I can gaurantee there are no missing holes! I have scratched to shiney metal and I have a problem!! I can see 3 possible solutions, advice welcome!!

1. Give up and put the non OD box back in.
2. Drill holes in the correct position, but without the captive plates how can you attach the bolts? (The current threads are not moveable, possibly welded into position).
3. Tack weld each side of the cross member front and back (so four tacks in all) and accept that if the box needs to come out I'll need to grind the tack welds off.

Right now I am favouring option 3 as I have no intention of pulling the box again, although the engine will be in and out a few times in coming months.

Any opinions on a solution?

Cheers
Iain
I D Cameron

ian
there is a diff cross member that has two sets of holes for the gearbox mounts which will then put the crossmember in the correct place
ste
Ste Brown

Afraid not Ste! I spent a while this evening playing around with different mounting pieces from later cars to try and get the X member lined up with the holes. But, and it's a big but, I have realised that the X member cannot physically go where it was with the non OD box as it will now foul the OD (which is of course far bulkier than the non OD box at the back). So I definetely can't use the current holes in the chasis, even if I could get them lined up.

I had a chat with a friend about option 3, the weld, and have been convinced that this would struggle to hold unless it was a solid weld, which would make it almost impossible to remove the box in the future. If I do this I will live to regret it, so I think it is out!

Therefore I need some way of fixing the X member in the new position. Suggestions so far include cutting the floor above the chasis to drop a new captive plate in and make some sort of U plate that is bolted through the chasis and has threaded holes already in place. I can't see either option being easy and as the car has to be on the road by the 16th of June I will begin putting the other box back in tomorrow.

Any bright ideas for a longer term solution are most welcome!!

Cheers
Iain
67 BGT
I D Cameron

iain
cant you just drillnew hole then make a bigger hole in the floor under the seat so you can get a socket downthe hole to tighten up then put carpet back or a grommet
as you can see in pic without the flors in it gives you a better view under there
drilling holes is neat an quick to do just for the sake of removing the seats
ste

Ste Brown

Ian
I am really sorry to read that your GT shell does not have the correct fittings for the O/D gearbox cross member - it would of been made with the fittings so some one has altered it.

I do think that the suggestion from Ste is the best solution as it is very near to the original design.

I do think that before drilling holes it would be worthwhile
a) finding a car locally that you can measure to find the true position of the holes (you could use mine but I live in North EssexSuffolk borders area).
b) ask British Motor Heritage two questions
1) Can they give you the position of the holes
11) the correct size of bolt and thread for the application

I am sure the position is quite important and that it may not be quite where the cross member lays when offered up to the shell because of many reasons so it is best to find out what the original design specifies.

Good luck

George Wilder

G R Wilder

Iain

Sorry to hear about this setback. Installed the OD in my 67 GT last fall and I love it.

Once you get the crossmember perfectly lined up and can mark the frame, would it be possible to drill a hole all the way through the frame and put a bolt down through from the top? If it were spot welded on top, it could act like a stud and allow you to use a nut on the bottom to pull the crossmember up.

That should give the strength you need without really affecting the frame rail.
BEC Cunha

Iain,
Just wondering - could it be your cross-member. I've looked at some in my inventory and they are definitely not all the same. Wish I knew what particular 'B they came off of. Some have two holes for mounting the transmission buffers (others have only one). Looking from the side, it is quite apparent that the vertical buffer supports also vary in their offset from center.

Is your cross-member original to your 'B? Assume you don't have it wrong way around (i.e., 180 degrees out).
Steve Buchina

Steve, it is not the crossmember. As you can see in Ste's picture there should be four holes each side. I have only two as the chasis has been restored in the psat. Presumably the PO didn't think it was worth drilling holes in the chasis that they did not need.

Ste, many thanks for that picture. It is a great help to visulaise what is in there! It does make me wonder if the original plates have been used. If they have the captive threads may be there, just hidden by the crossmember. I may drill a hole above and see if I can see in there and find if the hole is there. If it is then I can simply drill out the chasis at the relevent place to open up the access to the thread. Alternatively I do like the idea of a hole above that I can simply put a large thick washer and a bolt in from above. Covering with a gromit under the carpets would allow it to be easily removed in the future. Again worth thinking about. I may take the seat out on one side and explore this evening!

Ste, is that picture an old one from your archive or is your car currently in that state? If it is, would it be possible to measure the distance from the front to the second holes? It would help in determining where to drill exploratory holes!

Cheers for all the suggestions.

Iain
I D Cameron

iain
i will measure a shell today to get the measurenents as my car is finished
ste
Ste Brown

iain
from centre of the forward hole to centre of next hole was 31mm and 70mm to the next one
ste
Ste Brown

Thanks Ste.

Right, I'm winning!! I have drilled two 24mm holes in the floor pan above where I want the X member to be mounted. When I got it there I found a heck of a mess! At some point the thened with new metal, but they have not completely removed the old metal and so it is still in there! Anyway, after a lot of drilling I was able to free the plate that the X member bolts to so that it can be moved back and forth. Sounds trivial, but we are talking a good hours work!! I can now move the threaded plate above the new holes I have drilled. I just need to open those holes up a bit and then that should be one side sorted. Then I just need to do the other side, but that should be easier as I now know what I am doing! This is assuming the same trick will work! For reference you don't need to remove the seats as the X member is actually a fair bit forward of the seats, so that is a bonus! I think the aim tomorrow night will be to get the side I have drilled bolted up. Mark exactly where I will need the other sides holes to be and then get it all drilled out. All being well I will then be in a postion to mount the gearbox (propoerly as well!) and I can get on with connecting up the engine!

Will report back when/if successful! Cheers for the help. Useful as usual!

Iain
67 BGT
I D Cameron

Hi:

Just curious, did you replace the rubber mounts? There is a diiference between the GT and roadster gearbox mounts. I bought the wrong mounts for my roadster, and when I needed them. they didn;t fit.

Rich Boris

Rich Boris

I've seen this thread a bit late in the day, but I was going to suggest drilling pilot holes where the holes should be to see if the thick plate was there and, if so, tapping a thread in it.

The option would have been riv-nuts.

At least I've learned something in advance of my own OD conversion.

My '67 roadster currently has a 4-sync non-OD and I was planning to revert to 3-sync with OD, but I'm now favouring 4-sync OD as it will hopefully be more straightforward, cheaper and more durable.

Anyone interested in my 3-sync OD 'box?
Dave O'Neill 2

Dave,

I suspect that the 3 sync OD box and the 4 sync box will use the same holes, so no problem for you on that front. I did initially drill the plate with a view to tapping it, but then I managed to free the plate so that I could simply move the plate and use the original threaded holes.

I am fitting a supercharger soon. As a result I have modified my OD so that it will only function in 4th gear, in the same way as the V8 box was modified in the factory cars. However, the D type OD is known to be weaker than the LH type. If you race and the 4 sync currently fits without knocking on the tunnel then you may be better to stick with that. 3 syn OD boxes are quite valuable!

Iain
67 BGT
I D Cameron

Hmm, I seem to have had my user ID knocked out, so I'm having to post this as a guest.

After a lot of fiddling the cross member is finally all bolted up and in place!! I now have 4 extra holes in the floor of the BGT, but I guess I can use these for waxoil!!

Is it my car again, or is it very difficult to get at the front propshaft bolts to tighten them up? It sits right above the fixed cross member at the front and I can't get in very well. I have had a go, back I think I'll need to buy a small stubby 1/2 spanner to get better access.

Now the X member and the prop are sorted I have started to reattach everything else. Ran out of time this evening and it will be Monday before I have another go, but hopefully it will be running then. I really hope this OD works!!!!!!

Iain
67 BGT
I D Cameron

It works!!!

All fitted and drove to work today. After a year without OD it is great to have the extra flexability again! The box itself is really tight with no noise at all from any of the gears. You can even get the car into first quietly! A huge improvement in both basic box and of course having OD. Highly recommended!

Iain 67 BGT with OD
I D Cameron

This thread was discussed between 04/06/2007 and 12/06/2007

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