LT77 gearbox - C and D Series - dimensions different?

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DaveRover3500s

Member
Hi members. My P6B 3500s had an LT77 conversion back in the 80s (I by chance net the guy who owned the car and did the conversion and still in contact). A 17a D series box was used (23/2600cc SD1 box). The box started to get noisy then lost 5th and bad 2nd and 4th. I toyed with idea of getting it overhauled but by chance a said to be known good condition box came my way, a 14a C series box out of a TR7 with low miles. I took it and decided to just get boxes swapped out with new clutch fitted at same time (turned out old one near perfect!) This was done for me as I just don’t have facilities. New rear output shaft seal was also fitted. The box performed perfectly well and with new selector rubbers has very solid gear change - not sloppy like my old one was. It is smooth and no bearing noises. All good… except! I found the gear lever seems to sit naturally more to drivers side- it does not come out of opening in centre. As per pics. What this means is 5th was difficult to select as it was fouling badly the plastic centre console- I had to unscrew console and leave it loose. The D series box I took out did not have this issue. Looking at the mountings the box simply used the mount that the D series used and there is no right left adjustment- it had to mount where it mounted could not push box over toward passenger side - a 10mm movement would done it. So I decided to nip a small part of the lower shock absorber bulbous part of gearstick that I worked out was fouling the plastic centre console in 5th -as can be seen by pic. This has just about cured issue - not perfectly but enough. My question is why the difference- is there any gearstick left right positional adjustment that I or chap that fitted it is missing? I read the cases of D series is different that C series and mechanic said boxes were slightly different though mounted same points ok.

It’s bugging me to be honest and I am now considering getting the old D series box and remote overhauled and refitting it (cost = ouch) then sell on C series as a known good box ready to fit with It’s overhauled remote. I know D series box is stronger so long term a good thing but I really don’t give car any stick it has easy life! The C series box definitely seems to sit in the tunnel more to the right drivers side.
 

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Try looking at the engine mounting rubbers, see if they are still intact and not broken away from the metal plate. If they are still intact, loosen the nuts on the chassis side and lift and let the engine settle again, or rotate the engine to centre the gearlever.
 
This problem sounds like its just related to the set up of the spring wires and related bolts on the remote. There is a very good section in the SD1 manual on the process to get these settings right. Most should be doable from underneath. Its also possible that the lever retainer cup thing has broken and is allowing the lever to move more than it should. Check out this piece on the 'tensioning cup'.
It also talks about exactly your problem. Much valuable info there.
I an early TR7 box in my P6B, which has occasional 1st gear selelction issues, but a rebuilt 25a...C box on standby. Still dont know what 25A means...
I have an SD1 lever cover, but it wont screw onto the lever , which is 5/16 unc, IIRC.
 
25A prefix gearboxes were for CKD SD1 cars, with the various A and B etc. suffixes. Whether it also applied to other CKD models, I don't know. Vitesse were 28A prefix
 
colnerov said:
Try looking at the engine mounting rubbers, see if they are still intact and not broken away from the metal plate. If they are still intact, loosen the nuts on the chassis side and lift and let the engine settle again, or rotate the engine to centre the gearlever.
Click to expand...
Hi Colnerov.
I put new mounts on when I rebuilt the V8 in late 2022- they are in perfect condition - I understand what you mean re loosen chassis side nuts and lift engine and rotate engine a tiny tad- that may be way forward thank you.
 
jp928 said:
This problem sounds like its just related to the set up of the spring wires and related bolts on the remote. There is a very good section in the SD1 manual on the process to get these settings right. Most should be doable from underneath. Its also possible that the lever retainer cup thing has broken and is allowing the lever to move more than it should. Check out this piece on the 'tensioning cup'.
It also talks about exactly your problem. Much valuable info there.
I an early TR7 box in my P6B, which has occasional 1st gear selelction issues, but a rebuilt 25a...C box on standby. Still dont know what 25A means...
I have an SD1 lever cover, but it wont screw onto the lever , which is 5/16 unc, IIRC.
Click to expand...
Hi JP928. Great thanks that’s lot info and I have been through it now and in the past - I can sort of understand what all info is telling me but not fully if am honest as I have never touched remote set up before. The reverse and all gears are perfect with no problems between them a very good gear change much better than my sloppy old remote- I put all new rubbers in remote. I don’t want to mess with remote set up as is in case I mess it up! I showed a really knowledgeable Rover Club member who had fitted many LT77 boxes and knew loads about them- he said was really tight nice gear change and suggested I could alter the gearbox mounts a bit- to set box more to passenger side. May be I could I need to have a good look myself when on a ramp I guess. The photo is of a friends C series 17a box - he said his gear lever and remote also seem to naturally be slightly right of centre - which I can tell from the pic. I believe the remote set up info says that gear lever should sit between 3rd n 4th - so this appears correct. He is going to use SD1 mount far as I know so I guess he can endure the gearbox is perfectly central so stick comes up centre on dust cover… incidentally I think my set up is using the original P6 dust cover…. Not sure. I really appreciate feedback given me food for thought. I may be worrying for nothing - the set up works perfectly gears are great no notching etc and with the small cut out (looks brutal but I am confident not effected stick) the 5th gear is ok - a little tight but ok once used to it. I just want to get it as it was on the D series box so May try small engine mount adjustment that may be all it needs.. many thanks and happy new year! Dave
 

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One other pic! This is of side tunnel drivers side where the remote adjustment bolts are - as you can see there is so little room tunnel was cut out around them - shame not a tidier job done re round hole large rubber P6 grommet. I have now made up stainless steel plate and self tapped it in with sealant - an inspection plate. It’s moulded so plate does not touch bolts that protrude - otherwise would transfer vibration from box. It can be seen how close the box is to the side of the tunnel - I defo have to get under car on ramp and see how the box is sitting am convinced it’s not central to tunnel! Dave
 

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dont forget that the original box was not centered - look at the cast iron lump in the mount, its asymmetric. The diff input flange is also not centered, hence the uj offset in the prop shaft. My LT77 does not touch the tunnel sides anywhere, and I can get at those adjusters.
This my box ready to go in.
mCYL9kj.jpeg

See if your spring adjuster bolts look similar in height.
Here is the iron lump, obviously offset. The bolt holes go into the mounts on the tunnel.
0EFHAX1.jpeg

Here is my lever ,,
1EpPXrC.jpeg
 
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Something else to consider. The rear gearbox mount has a big effect on the angle the engine sits at - my 4 speed was difficult to get into reverse becasue the rear mount was very soft, and the lever hit the sides of the tunnel hole - once fixed it was ok. I looked at the slope of the throttle shaft from the rear support to white coupling (almost horizontal), and played with the gearbox rear mount until it got very close. Also the gearbox output drive flange should be nearly parallel to the diff flange, vertically, within 3 degrees IIRC. If your box is at the wrong angle it could be causing selection problems.
SCYbFXa.jpeg
 
Hi again JP928. Thanks for your replies and the pics - again very helpful. I see you have gone for the P6 mounting at rear of the box. Mine has the SD1 mounting with the rubbers at angle further forward - as per Chris Gregory 1989 conversion guide. So its clear I need to investigate why my gearbox is not is the exact place it should be in that puts the gearstick in the centre of the opening - as it was when the D series box was installed - This requires me getting her up in the air and see what can be done - I will check the spring adjuster bolts by taking my dust cover off the top. Thanks again i have learned lots as usual and this will be handy as a local P6 member is doing a LT77 conversion on his 3500S soon. The article Chris did is excellent.
 
When I fitted my 5 speed gearbox on .I ended, I had to use a scissor jack to jack the tunnel side out about 1/4". This enabled me to get the gearstick almost central in the hole inside the car. Quattro then explained how to set the stick to be between 3&4. It took 5 mins.

Remove the round rubber dustcover to have a better view.
 
Hi both - thanks for info re the tunnel jacking out a bit and the manual for remote gearstick adjustment. I think with info i have its clear the box is not exactly central to where it should be - rather than a gearstick issue as it already sits perfectly between 3rd n 4th as per manual info, selects gears well including reverse and 5th. It just seems to be 1cm ish more over to drivers side in tunnel that means gearstick does not pop up centre of dust cover or hole. But the D series one originally fitted did - that's the oddity that i can now resolve - yes will take cover off have better look from above. I am so much more armed now with better insight - how quickly this forum has benefited me! Thanks all - I will of course feedback once i resolve issue - with pics! But will be a while garage has little heating and its ruddy cold out there this week! I will get interior refitted and seats in and get HJ mobile again soon as poss! Dave
 
colnerov said:
Try looking at the engine mounting rubbers, see if they are still intact and not broken away from the metal plate. If they are still intact, loosen the nuts on the chassis side and lift and let the engine settle again, or rotate the engine to centre the gearlever.
Click to expand...
Hi Colnerov! Well you were spot on with your analysis! I had a look at the air filter box after I refitted it (new filters) and noticed it was not sitting level- the engine was tilted down on drivers RHS. As I put new mounts and locknuts on when I rebuilt the engine I knew they were ok. So I loosened off both mounts and used wood and bottle Jack to lift engine on drivers side- it lifted of course. I then manhandled the engine and basically got it level - I could see the drivers side mount was sitting 1cm higher than it was. I checked the gearstick and bingo it had centred and gear selection was spot on. I tightened up engine mount nuts as tight as possible removed bottle Jack and engine now is sitting visibly level. Hopefully this is a full fix however I want to get underneath and check cotton reel gearbox mounts it may be one side needs packing a bit. There appeared no movement on the mounts but I guess must have been when gearbox was replaced. Thanks for help once again - really pleased to get this sorted! Dave
 
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