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Who to Choose for Engine, Gearbox, O/D overhaul services?

Colonel H

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Dear TSSC members,

I'm new to the club, and new to forums for that matter, so interested to see how this works. I bought a lovely 1973 GT6 MK3 in the summer. Previous owner lavished much love and attention to restoring the body work, but other factors meant he needed to sell before getting stuck into the inner workings. I've had a great couple of months using it, but I think time has come for improvements. Whilst it makes a lovely noise, depressing the accelerator produces more noise, but no discernible performance! Max speed is 80mph at 5000RPM. Interestingly, a rolling road test at CCK Historic in Sussex recorded good oil pressure and good compression, but only 61BHP at back wheels. Another factor is engine temperature gauge is off the scale (hot side) within 3 minutes of driving normally, but a laser heat recorder fired at different parts of the engine indicate its actually running at correct temperature. I have changed the temperature sender unit, but same result. Anyway, I am getting off subject here.

The overdrive has recently failed, and speaking to people 'in-the-know' it sounds as if the Conical clutch is worn out. Symptons are strange rev fluctuations in gears 2-4 (regardless of position of switch, which is now always 'out'), and the car now won't reverse. That was a bit awkward in the Waitrose car park when it first happened!

I have therefore taken the decision to have all three items reconditioned at the same time over the coming few months. Not only that, but I want my engine improved to the best available road-going performance (this one isn't for racing). I have chatted with a few 'suppliers' including CCK in Sussex, Southern Triumph in Bournemouth, Jigsaw Racing in Corby, Slark Race Engineering in Amesbury and the TSSC themselves.

The general consensus is that Gearbox and O/D is a fairly simple overhaul, with no special options. Engine however can have various bells and whistles, all at cost. It seems a 'stage 2' overhaul with the stromburgs to be replaced with SU (from Dolomite, or machined to fit), would create the best outcome at sensible cost (a relative term, but I imagine this will all cost a bit North of £5-6K?). I note talk of Weber Carbs, but they are apparently excessive for these cars. I Also note talk of putting a TR6 engine in the GT6 (and now would be the time, if ever), but no significant advantages over getting my own 2.0 litre engine running well.

I am leaning towards Jigsaw for all this, but all suppliers are knowledgeable, and seem to offer a slightly different angle and opinion on the subject. The purpose of this lengthy post, is to see if anyone else has jumped through these hoops, in particular with Jigsaw, and have any thoughts!

Thank you.

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For the engine work I would go to Jigsaw - but be certain that you know the specification that you want - Mark is very good at persuading you "want" performance upgrades that you may not actually need.


For the gearbbox / overdrive problem, I would take the box to Mick Papworth.

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Thanks Kevin,

Yes, Mark is Persuasive on the phone, but certainly not pushy. I found him to be patient, and helpful....like any good salesman! Presumably with engine renovation and enhancement, there are diminishing returns with every additional feature, with items lower down the list costing extra, but perhaps not adding a significant enhancement. I am a bit vulnerable to exploitation, as I really don't know much about mechanics and engineering, so can't be too specific with my requirements. That's why I seek a trust worthy, reliable supplier who hopefully won't sell me something I dont need! Basically, it needs to be the best available solution for road going, everyday use (despite it only getting used on a Sunny weekend type of basis...but as frequently as often... As I know they need regular use). Hopefully, you get what you pay for at Jigsaw, so I won't rue the decision.

I got a recommendation for Mick Papworth from elsewhere, which is encouraging. It might be a tad tricky dropping the car off at Jigsaw (if that is the chosen place), and asking them to send the gearbox + overdrive elsewhere... Because apparently they do the overhauls themselves. I am presuming there is some benefit from 'one stop shop'....especially if I needed to enact a warranty. There could be some serious sloping shoulders from one company to the next! Apologies for cynicism! Thank you for your advice Kevin.

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For you to be doing 5000 at 80mph, you have the wrong diff!


should be nearer 100mph at 5000rpm in 4th, 100 is at about 4000 in OD 4th.


Can you get a compression test done?  That might give a clue what is up with your engine.


Think before you go the 2.5 route, they are different beasts, the 2,5 is all about torque (that breaks gearboxes and diffs), the 2.0 is much freer reving.





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If you are not able to remove the engine / gearbox yourself, then having engine and gearbox/overdrive all done by the same place will certainly save costs.


Like all the well known Triumph suppliers, Jigsaw have a reputation to maintain, and they are more than competent to do gearbox / overdrive work as anyone else, and as you say, if there is a waranty problem, you only have one party to negotiate with.

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Hello Again Kevin and Scrpamann (Colin)

Sage advice from both. Thank you. The possibility of an incorrect Diff is looking quite likely. Being new to the car, I know very little of its history, other than I am the 13th Owner (according to V5), but i am trying not to be too superstitious about 13! Again, if I take it to a company such as Jigsaw, or similar, i imagine they can take a look and rectify if required. I could do without the extra expense, but no point in overhauling the other three items, only to have the wrong diff. With regard to the 2.5 Litre engine option, I dont like the idea, and quite glad there isnt an overwhelming argument for having 2.5 fitted. To be honest, I think it is probable that the engine and gearbox are all original for this car (certainly engine number matches the V5...even though the number looks as if it has been stamped on with individual letter and number stamps, and is a bit tricky to read), so I will request that I get my own units overhauled and returned, rather than exchange units.

Colin, the Rolling Road test said compression was 'fine', as was oil pressure, which is why they were a bit bemused by low BHP at back wheels. (61 BHP). I should have asked for a printed report. Perhaps the Diff again? I am not sure how they do their calculations. I agree with your points about mismatching egines with gearboxes. Surely its tempting fate. I am only after a slightly improved version of the original new car, not an entirely different beast. Hopefully that is a reasonable aspiration.

Regards Both,


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Could it be that the degraded overdrive is losing traction when it's 'out' and in direct drive, as well as in 'overdrive' mode?  Both rely on the cone clutch, but I'm no O/d  expert.  


While you still have the car in your garage, it's a simple process to check the diff ratio.   It's been described so many times, but maybe not here on the new Messageboard, so;

Count the number of times the propshaft rotates for one rotation of the half shafts (both at the same time)


Just over four propshaft rotations  = 4.11:1

Just under four rotations = 3.89:1

About three and half turns = 3.63:1

Just over three turns  = 3.27:1

Only the last two were original to GT6 Mk3s


To make these observations, either:

Jack up the rear end and lie underneath(with extreme attention to safety! Axle stands, please) watch the propshaft turn as two helpers turn the back wheels.

OR, remove the gearbox cover and have your willing helpers push the car so that the rear wheels rotate once, while you sit in the cra and count the propshaft revs.


And I can say "Hear, hear!" to the restorers suggested above.




PS  O/d should work only in Third and Fourth gears!  Electric cut-out switch on 'box, disconnects the solenoid in Second and First.  J.

PPS Congratulations on your choice of classic car!  You could only do better by buying a Vitesse!

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My two penny worth,


Think about what you want from the engine,a standard smooth running well set up 2 litre is a great unit.60 mph at 5k revs certainly sounds like too short a diff.


Lack of horses could be anything from worn carbs,split diaphragms(carbs)timing etc.


Nothing wrong with Strombergs if they are set up properly,SU`s wont give you any more power without other mods.


Stage 2.....what does that mean????


If the engine has good oil pressure and compression i see it as pointless rebuilding it and certainly not adding go faster goodies at great expense for very little gain.


If it was me i would take it to a club meet and let someone who owns one look over it,i would be getting the carbs rebuilt,gearbox and diff sorted before i committed £££ to mythical tuning options.


Unless thats what you want of course,your car-your choice,just trying to save you some cash.This isn`t a sleight at Jigsaw or any other supplier,i would just check out what you have is sorted first.



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Where are you located?

If you went to CCK I assume you may be sussex area? If so you should drop in to a monthly meeting. There is a selection of different cars that may be there, but a wealth of knowledge too. (no, not me......I just went ford engine!)


As to mods, steve above is correct. Get the wrong (stage 2 possibly, again there is NO definition of that) and you can end up with a car that is a dog to drive on the road and needs revving hard to get it to go, something people THINK they want, but when it comes to it they are hard to live with. That is from personal experience.....

A few extra BHP at 5000rpm usually means a whole lot of torque is lost at 2000rpm.


You probably need to drive a well sorted standard GT6 to see what that should be like. You may be pleasantly surprised. 


If I was building a GT6 for touring/road use, I would look at a very standard specification, keeping the strombergs (they are as good as SU's, if you change to sprint larger SU's again you will be gaining top end power at the expense of "normal driving") and using the MK2 GT6 cam and matching head, which you may have already.


But really, find a std GT6 before spending large amounts of cash on something you may not like.



Now, if you want a car for trackdays etc, certainly go the whole hog,.....

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Just my opinion but it sounds like you have only driven your car in a " faulty" state. The diff sounds like it may me the answer but the advice to talk to owners of standard cars and see how they run is wise. A well sorted standard car with set up Strombergs is a joyous thing. Obviously it's your money but 5 to 6k is nuts. New cylinder head, carb rebuild and set up, gearbox rebuild and possibly the correct diff and you will be a happy man and thousands better off!. From that point you could modify at your own pace but probably wouldn't want to .


I use Dave Saunders at Triumphspares in Spetchley near Worcester and what he doesn't know hasn't been discovered yet. I usually have to twist his arm for me to spend any money as he won't do unnecessary work.


Please try a ride in a standard car as there are plenty in the club and extremely knowledgable owners and I think with your willingness to spend your car will be sorted.

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Dear John, Steve, Jon, Clive, Mike,

A raft of interesting thoughts there. Many thanks.

John, I haven't been around the bazaars enough to hear about the diff ratios, and the self assessment method, so that is a great piece of info. I'm not well equipped with tools, but do have axel stands, so once I have worked out where it is safe to jack up the car, and position axel stands, I will give that method a go. Sadly work takes me abroad tomorrow for a couple of weeks, so there will be a delay before I report my findings. With regard to the Vitesse, it is indeed a fine looking car, and on hot days in the GT6, with it's fairly cramped cabin, a slightly larger, open top Triumph does hold appeal! One day maybe!

Steve, in my heart of hearts, I know you are correct, and that perhaps I should do the carbs, gearbox, O/D, and seemingly most importantly check I have the correct diff, before I commit to £££ on engine overhaul. The more reckless side of me suggests it is worth getting it all done in a 'oner', the main appeal being I then consider it a new car, with me as first owner. Body work and paint is now 'as new'. I don't intend selling, so perhaps it is an investment in a long future. But you are right, that sentimentality comes at a cost, where perfectly good cheaper options exist. I gather a vote in favour of doing it all together, is that I will save quite a few mechanic hours removing and replacing gearboxes and engines if it all gets done simultaneously, rather than over time. Possibly a weak argument.

BTW, I gather engine overhauls have two generally accepted levels. Stage 1 and Stage 2. The latter being a more extensive (and expensive) overhaul including things like improved gas flow (but I am not sure what that involves, or it's true merit).

Jon... Good call, and I've done it. GPS and speedo match almost perfectly at all different speed ranges.

Clive, I live near Salisbury (Amesbury to be precise). I enjoyed my visit to CCK in Sussex, not least because It was a fabulous drive on the A272 there & back. They are great hosts, and clearly know their stuff over a range of Classic Cars, but not specialists in Triumph, over and above other Marques. Your thoughts are similar to Steve (above). Points noted.

Mike, reading your advice compounds what has been said above, and as a result of all your comments, I have now decided to hang-fire on a full engine overhaul because there is actually no categoric evidence it is required. The measures you have collectively mentioned above will cost much less, and by the sound of things, may well produce the outcome I need. Thanks for recommendation of Dave Saunders. I will touch base. Course of Action is therefore as follows.


1. Ideally try and find a normal, well set up, GT6 MK 3 close by. Compare.

2. For my own interest... Ascertain if I have correct diff. Act accordingly.

3. Gearbox and overdrive to overhaul (gearbox needs doubleDclutch to go down)

4. Carb rebuild, and any sensible improvements that can be done to the engine without a rebuild (cylinder head perhaps).

5. Re Assess.... But by the sound of things...hopefully no further action imminent.


Gentlemen, thank you for your time, effort and advice.



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I always take my diffs/boxes etc. to Mike papworth,he will do any other work you want as well if you would rather have it done all in one shop.


If the carbs are off its no biggy to have the head off to check the valves-convert to run on unleaded (if it hasn`t already)mines run on 99 Octane without additives for years and is fine.


He`ll even pick it up if you cant get it there.


No i`m not on commission,just satisfied that he`s honest about whats wrong and a good bloke aswell.



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John your list of Diffs and reference to the ones fitted to GT6 are not quite correct.


GT6s had either 3.89:1 or 3.27:1 diffs fitted. 


3.89:1 fitted to most overdrive cars


3.27  fitted to non overdrive cars, plus overdrive cars in certain markets (eg Switzerland) and some of the Mk2s.



The 3.63:1 diff was fitted to Spitfire 1500 whether overdrive or non-overdrive.


The 3.63 is in my opinion a good upgrade to an overdrive GT6 as it's slightly higher without losing much acceleration on a standard engine. late spitfire diffs are also the strongest.


The 3.27 diff is the weakest variant, although it's a good diff for uprated engines or 2.5 litre conversions it is the easiest to break with additional power so is not without some risks.

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Thank you for that update Cookie, it is only as a result of this forum that I have become aware that I probably have the wrong diff. I will be fascinated to see. By your comments, I should in theory have a 3.89.... And if I don't, I should source one, or perhaps consider a 3.63.

When I say 'consider' ..... what exactly am I considering?! I assume the bigger the ratio, the faster the car might go, but with less torque. The smaller the ratio, a slightly slower speed, but more torque and grunt? Perhaps handy for overtaking, which is something I don't risk at present. Prior to this forum, I would probably have thought it can't make that much difference, but seeing that most have identified my diff as a probable culprit for low speed (80mph) at 5000 RPM, I need to pay attention to the subject.

Are diffs like gearboxes, in that they need occasional overhaul, or are they fairly maint free (just incase I need to source a replacement for my car)?



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Diff ratio won`t give you anymore torque or grunt,the shorter the diff, i.e Triumphs shortest 4:11-1 will make the engine rev higher to reach the equivalent speed of a setup with say a 3:89 or 3:63.


It may accelerate faster but it will run out of revs quicker with a short diff.


I am intrigued to know what ratios you have,i was playing with the Mintylamb calculator,see:



Even with a Standard overdrive box and a 4:11 diff 80mph is only 3900 revs.I wonder if something is slipping?


Unless you have 10 inch wheels i`m baffled.


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you have checked the speedo but you need to also check the calibration of the rev counter.


should have 3.55:1 on the dial  but accuracy is dependant on its condition,  the inards are same as the speedo its subject to many problems with cables , magnets and drive disks.



a well set up std GT6 will cruise at 80 quiet  happily with a good level of reserve for a blast up to 3 figures 


if you count  the turn of prop , make sure both wheels turn together or you introduce a error 


the easiest way is put in to gear ,overdrive Off ,   push car one one accurate turn of the road wheels,  ( mark the engine driven fan  or crank pulley ,get someone to count the turns of the engine , then use Johns figures to reckon up what you have 


that way no jacking, lifting, or removing .


plugs out may make the push easier.




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Mike Papworth has done two diffs for me, one in a TR6 and one in my current Stag. I drove a 150 miles each way to make sure it went to someone who knew what they were doing. I would have no hesitation in using him again. He also sorted 2 minor jobs that he noticed whilst the car was with him but only after I gave the go ahead. Photos of what has been done and takes the time to explain how and why.


My Stag drives soooo quietly now it is a pleasure to take get behind the wheel and drive, apart from it being a Triumph of course ;)

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Thanks to the Cookie Monster for corection.

And as  I've just found my calculator, 8000rpm at 50mph is 6.25mph per 1000rpm.  

That implies enormous slip somewhere in the drive train, and only the gerabox and O/d clutches are possible culprits.

Try looking at the O/d filter for "black hairy bits"!

See Canley's Tach page and your WSM: http://www.canleyclassics.com/y.asp?xhtml=xhtml/infodatabase/overdrivesdtype.html&xsl=infodatabase.xsl



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