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TR6 TR6 Overdrive

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jjbunn

jjbunn

Jedi Knight
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I'm not having much luck finding an Idiot's Guide to TR6 Overdrives. I know that at some point I want to add back an overdrive to my '71, and I understand that the period correct Overdrive would be an "A" type. But these are very hard to come by, and expensive. On the other hand, the "J" type is more readily available, and Volvo units can be used. For example, there is one on Ebay at the moment:

Ebay "J" Overdrive

Apart from the unit itself, there are other bits and pieces, like the wiring harness (which I have), column switch, relay (which I also have), solenoid (does this usually come as part of the O/D unit itself?), speedo drive gear, and so on.

Most importantly, how easy is an O/D to fit? How do I find out if my existing transmission will take an O/D unit, and what the dimensions and relevant part numbers are, will my transmission tunnel be big enough, et cetera? How could I decide if the Ebay O/D linked above be suitable (assuming it is as advertised)?

In short, is there a resource that covers this topic, that someone can point me to? I'm in no hurry to add an O/D but I'd like to be informed so that if one turns up for sale at an acceptable price I can make a decision.
 
You should look up Quantum Mechanics on the 'net. They have various kits for adding an OD to a non-OD box. I don't recall the details but the output shaft has to be replaced and there is an adapter housing that fits between the OD and the main case. The Volvo unit needs some parts for the speedo drive to work properly in the TR. All the TR6 tunnels are the same so the OD will fit. If you put a J into an early car you'll need an adapter for the trans mount. I don't know if all the Volvo units are the same so I can't tell you which will work.

Bryan
 
Julian,
Best reference I know of is at
https://www.quantumechanics.com/categories.php?op=newindex&catid=11
(scroll down to the "required changes" section)

Any TR6 gearbox will take the J-type OD, if you can find the right mainshaft to fit your gearset. I believe these folks
https://www.overdrive-repairs.co.uk/
are capable of making a special shaft to work with earlier gearsets, but it's probably easier to do as John suggests and source a gearset to match the commonly available mainshafts. As I recall, the TRF TR6 parts catalog has a lot of information about which gears can be used with which shafts.

On a scale of 1-4 wrenches, I would definitely rate this a 4-wrench operation. There are special tools required (which can be made), and you will basically be doing a complete gearbox teardown and reassembly. IMO you should also check and adjust the hydraulic pressure, since some Volvos used less than the TR6, which takes yet another special tool.
 
Overdrives are very nice to have for longer distance driving. That said, I would never pay the ridiculous money they are demanding today. You can buy a lot of gas for $1600.00.
 
DougF has a very good point. I had a BGT and put an overdrive in it. That car was geared such that I used the OD around town and it made it much more relaxing to drive. In driving my TR6 around this summer, I found that with its taller gearing and 6 cylinder engine, I didn't miss the OD. If I were going to go a long way on a highway, the OD might be nice but it isn't nearly as important as it was in the other car. You may want to drive your TR6 a bit before deciding to spend the money.

Bryan
 
As others have said, Quantum mechanics has a really good site to go to for OD info.
Randall has stated it quite correctly, that it is a 4 wrench job adapting a volvo OD to a TR. Unless you are really handy it is a pretty big job. Im sure quantum would be happy to do it for you ($)
Keep looking around, it is amazing what comes up on E bay these days........................oh ya, you can sell that relay, its not used on the "J" type...
 
heliguy said:
As others have said, Quantum mechanics has a really good site to go to for OD info.
Randall has stated it quite correctly, that it is a 4 wrench job adapting a volvo OD to a TR. Unless you are really handy it is a pretty big job. Im sure quantum would be happy to do it for you ($)
Keep looking around, it is amazing what comes up on E bay these days........................oh ya, you can sell that relay, its not used on the "J" type...
Click to expand...

Sounds like good advice ... so what exactly should I be looking for? A complete O/D transmission, rather than just the O/D ?
 
Complete Overdrive and Transmission.
Otherwise you have to change the main shaft in the transmission to a longer one, (see Quantum Site) for the over drive.
I bought a rebuilt transmission and overdrive from John at Quantum. He really stands behind his stuff and having an overdrive is nice. I like being able to use it on the freeway and have the revs drop by about 700 rpm, it so much quieter.
As my mechanic friend says, that engine will run all week at 3300 rpm, which I think roughly will put you 70 MPH or so.
I am sure these numbers are not exactly right, but you get the idea. I am sure one of the gear heads out there have the exact numbers and will provide them.
I would suggest finding and talking to local people to perhaps find a knowledgeable local rebuilder that may have a trans/OD.
BritSport here in Seattle does and they are good too, as I found out later, after dealing with John, which has been great but across the country, it's a long distance to ship and such.
 
By the way, I have a Quantum O/D in my car and I love it. I've been to John's shop ten times and love talking to him.

I just offered you an alternative, but he is terrific.
 
jjbunn said:
Sounds like good advice ... so what exactly should I be looking for? A complete O/D transmission, rather than just the O/D ?
Click to expand...
Yeah ... the problem with that is that you don't know what's inside unless you tear it down anyway. I bought one on eBay for my Stag (which is similar to the TR6 box but different) that proved to have been assembled by a gorilla.

Overall, it's probably simpler and cheaper to install one of Herman's 5-speed conversions. Everyone I know that has one is very happy with it. And if you pick the right box to go with it, the gear ratios are almost identical to a TR6 + J-type.
 
TR3driver said:
jjbunn said:
Sounds like good advice ... so what exactly should I be looking for? A complete O/D transmission, rather than just the O/D ?
Click to expand...
Yeah ... the problem with that is that you don't know what's inside unless you tear it down anyway. I bought one on eBay for my Stag (which is similar to the TR6 box but different) that proved to have been assembled by a gorilla.

Overall, it's probably simpler and cheaper to install one of Herman's 5-speed conversions. Everyone I know that has one is very happy with it. And if you pick the right box to go with it, the gear ratios are almost identical to a TR6 + J-type.
Click to expand...

I really prefer the concept of the overdrive, rather than a 5-speed. The purpose for me is not so much to improve drivability, but to have this somewhat arcane bit of electrically operated speed control.

Looks like I should maybe shelve the idea for a while.
 
I added a volvo j-type overdrive to my '71 TR6 tranny without difficulty. Tearing down the transmission is the most difficult task. Rebuilding was a snap so long as you pay attention to details. I did not rebuild the overdrive unit preferring to purchase an already rebuilt unit. I still had to disassemble the overdrive to switch out the speedo drive gearing to match with the TR6. The whole process was very satisfying and I have a much better understanding of my mechanical systems.
 
71tr said:
I added a volvo j-type overdrive to my '71 TR6 tranny without difficulty. Tearing down the transmission is the most difficult task. Rebuilding was a snap so long as you pay attention to details. I did not rebuild the overdrive unit preferring to purchase an already rebuilt unit. I still had to disassemble the overdrive to switch out the speedo drive gearing to match with the TR6. The whole process was very satisfying and I have a much better understanding of my mechanical systems.
Click to expand...

That's very encouraging! I thought that just one shaft needed to be changed in the transmission: does that involve tearing down the whole thing? I suppose it does ...

What parts did you need to obtain to do the complete job, do you remember?
 
jjbunn said:
That's very encouraging! I thought that just one shaft needed to be changed in the transmission: does that involve tearing down the whole thing?
Click to expand...
Pretty much. There are only 3 shafts, and to get out the one that has to be changed (the mainshaft), you have to remove the other two first. Then when the mainshaft comes out, it still has roughly 1/2 the gears still attached to it, that have to be transferred to the new shaft. Since there may be some slight variations in shafts, you should also check the clearances on the new shaft.

TRF has a kit of the standard items to replace (although I would add a few copies of the circlip that holds the gears on the mainshaft, in case you make a mistake and have to install it again). On sale til Jan 1 for $300 :
https://www.zeni.net/trf/TR6-250GC/75.php
 
Other than the Quantum rebuild kit being cheaper, does anyone have an opinion on the merits of the Quantum vs. the TRF one?

In the new year I hope to start rebuilding an early TR6 OD box that I recently obtained.
 
If I were to do this over I would not purchase the complete tranny rebuild kit from TRF or anyone else. When you make this conversion the overdrive unit replaces the tail piece of your non-overdrive tranny meaning you don't need the third large mainshaft bearing that comes in these kits.

There are some good write-ups on rebuilding the tranny and adding overdrive. Most of them suggest you tear down the tranny and evaluate the parts before buying replacements, I tend to agree.

While there are a number of parts to be acquired for this conversion everything revolves around a new mainshaft. This means you have to take the tranny apart, pull the gears and put everthing back together on the new mainshaft. This sounds daunting but it's not and while you have everything apart just go ahead and replace any worn parts. Generally you replace things like synchro rings, thrust washers, mainshaft bearings, some smaller shaft bearings, circlips and possibly some shim washers to get the proper clearances.
 
I'll add my experience to the prior comments re John E & Quantumechanics: I installed one of John's rebuilt A-Types a year ago and am delighted with the unit.

Living where I do it is a long fast ride to almost anywhere that's out-of-town. Being able to cruise at 75-80 mph without feeling like I'm flogging the engine is quite nice.

I too prefer the distinct nature of the original type OD vs the Toyota conversion though several local club members have that and seemed quite happy with the result.

I would not hesitate to do business with Quantumechanics again.
 
Adding an OD gearbox to a TR6 may not be a project for the uninitiated, but it's a common mod: many, many folks have done it successfully. I also might add that the "driveability" of the Laycock de Normanville (I just love that title!) OD unit is, without doubt, far superior to any 5-speed gearbox.

With the J-type, you can enjoy final drive reduction rates of up to 28% or .72 top gear OD ratio (I had one in my GT6). That's a significant reduction in cruising RPM. For example, the fifth gear ratio in most of the common conversion boxes is around .82 or .80, at best.

With the J-type, it's a real thrill to engage OD in third AND fourth gear, effectively rendering a 6-speed gearbox. Plus, many passengers have never seen an OD gearbox in action: it never fails to wow passengers when you flip that OD switch!

To me, the correct OD gearbox completes and compliments the overall LBC driving experience. There's nothing like it, and what's more, it's part of the heritage of the marque.

I strongly suggest that before going to a 5 speed conversion you take a spirited ride in someone else's J-type equipped TR-6. It's well worth the effort.

After the test ride, I'll bet you choose the OD over the 5 speed! /bcforum/images/%%GRAEMLIN_URL%%/driving.gif
 
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