Improving the Triumph TR6 drivetrain
This is clearly one of the more complex tasks of building the
ultimate Club-Sport TR6, and
one that clearly avoided by the general vendors. A lot of very expensive hardware is required,
machine work, and skilled technical labor. In this area, I have tapped the expertise of Dave Wingett,
Bob Lang, Mike Munson, JK Jackson, Kas Kastner, and tidbits from the Group 44 garage.
The TR6 drivetrain was one of the fundamental weak points of
the Group 44 and Kastner-Brophy
race teams. However, effort here not only rewards the driver with a vastly safer and more
robust car, but one that now handles far better than any other modification that can be made
to the car. The reason? The combination of LSD and correctly-matched swaybars is an
My own recipe
Stock transmission and J-type O/D, ceramic-coated housing
Gear-shift lever shortened 2.5"
Tilton hydraulic release bearing
Racetorations12lb steel flywheel
Close-ratio gears (from a Triumph Dolomite Sprint)
4.1:1 crown and pinion gear
Corvair rear hubs
Racetorations finned differential cover
Personally, I consider the TR6 gearbox to be among the finest in the world. It has a short-throw,
a very direct, mechanical feel, and does not demonstrate any of the rubberiness that modern
gerboxes demonstrate. The ratios are very well-chosen for the torque of the motor, although
they can be a little pedestrian for the high-performance motor. Alas, the gearbox can be
expensive to rebuild, especially if one has over-drive. Alternatives exist in the form of a Toyota
5-speed, but I find the Japanese gearboxes very remote feeling.
When rebuilding the Triumph gearbox, attempt to source as many
NOS parts as possible.
E.g., today's synchro rings are junk, and you will probably go through them in 10,000-20,000 miles.
The J-Type over-drive has a ratio of 0.797. The following table shows the gear ratios
for stock pre-1973 and post-1974 gearboxes. I have also included the special close-
ratio gearset that Kastner had made, the Quaife A and B dog boxes, the readily
available Triumph Dolomite box, and the Jerrico box. Note that the Quaife and Jerrico
boxes allow for clutchless gearchanges, and are brilliant. But at around $3,000 per set,
make the simpler Dolomite set at $600 much more attractive.
The Dolomite gearset uses a 23-spline
1" input haft, rather than the stock 10-spline
1" input shaft. This necessitates the use of
another clutch. The TR7/Dolomite clutch works
fine. However the TR7 clutch seems
to have the same thow-out bearing problems as
the TR6. See "Clutch" below.
I run Redline MT90 transmission fluid.
This does not contain any sulphides, which
are known to corrode brass synchro
The Kastner box has a very high first because it is was designed for rolling starts, and would be
wrong for overdrive..
I am not keen on lightweight aluminum flywheels (about 12 lbs with ring gear) because of the potential
to walk on the bolts, even if pinned and correctly torqued. Lightening the stock cast-iron flywheel
is also not an option. The stock flywheel and ring-gear on late motors weigh nearly 30lbs, and
the most that can be removed from this flywheel brings it down to 24lbs. For anyone proceeding
on this route I would strongly suggest that you have the flywheel magnafluxed, yearly!. Cast-iron is
brittle and can develop small cracks. Machining the flywheel and spinning at high rpms will only
exacerbate this problem, with the potential that the flywheel can explode, right near your ankles. Thus,
steel and aluminum are the only avenues.
Stock (1974) flywheel is on the
with ring gear, around 30lbs
Lightened stock flywheel in the middle.
Weight with ring gear, around 24lbs.
Note that most of the machining is towards
the outside, as can be seen from the "larger"
dome in the center of the flywheel.
Racetorations steel flywheel on the right.
Weight with ring gear, about 12lbs. Note
the extra bolt holes for the crankshaft,
which is important for high-revving
motors. The flywheel is a piece of art,
having been nickel plated. Too nice
to hide inside your motor!
I have drilled the crankshaft for 8 bolts. I use only ARP bolts.
As another note, wrapping the bell-housing with a kevlar jacket
makes an effective shatter shield
in case one does lose the flywheel. Try to make the jacket 0.5-1" thick.
TBD. Maybe I will be able to use a hydraulic throw-out bearing. Alternatives include the
TR7 Borg & Beck clutch, Centerforce II pressure-plate and cover from a Pinto 2300 (which
uses the same 23-spline 1" input shaft as the TR7/Dolomite close-ratio gear set I am using).
The tranny does not need much work, unless one is shooting for a 160rwhp motor.
The rear diff certainly benefits from a Salisbury LSD (even with stock motors) and
makes the car more fun to drive. The Salisbury design is superior to the Quaife and
Fantom designs, both of which allow more slip to the outside wheel than the Salisbury.
My car has O/D, so the diff will also
receive a 4.1:1 crown-wheel and pinion.
Without O/D, this combination would
mean high rpms while on the freeway and touring.
Be careful not to source Taiwanese
manufactured crown and ring gears, as they are of
poor quality (source: a metallurgist friend).
Rebuilding the diff with the 4.1:1 gears and
Salisbury LSD is not cheap; count on $1600.
Always use new ring-gear bolts, and safety
wire them. The safety wire is not to prevent
the bolts from becoming loose, but to prevent
them from falling out.
The clutch-type limited-slip units have been known to run very
hot. The factory
effors ran finned diff covers. I painted mine black to enhance heat exchange. I use
Redline 75/90 gear oil.
A note on the Racetorations finned diff cover. Racetorations
claim that it holds more
oil to help the diff run cooler. I don't know what glue they sniff, but this is not the case
on the unit they sold me. Their unit is a poor-quality casting of the factory finned
unit offered for the TR4A, and will not fit the TR6 diff without modifying the location
of one of the bolt mounts (by TIG-welding the hole and redrilling).
Corvair rear hubs
The rear hub is a serious weak point on the TR6, and *will* fail sooner than you
think. My approach is to replace the hub with Corvair hubs, though I have heard
that Datsun 240/260Z hubs will also work. If you stick with the stock hubs, I would
suggest magnafluxing them at the end of every season. Be very attentive to the diff
carriers, and weld gussets to these well-known failure points.
The TR6 hubs (below, right) are patently not safe for competition or hard driving.
There have been a number of TR6/4As involved in accidents when the hub breaks free
in a corner, with disastrous consequences.
The design of bolting the hub to the outside could only have come about after a
long design session in the pub. Kastner and Group 44 replaced the original hub
for Corvair and Mercedes Benz hubs (below, left), though I have heard that Datsun 240Z
hubs will work just as well, but I do not know of anyone who has gone this route. Expect
this little modification to cost around $1400.
The bearing carrier on the left is aluminum,
and basically a bolt-in except that some material
must be relieved from the trailing arm to allow
the beefier u-joint carriers to pass through. A simple
trial-and-error job with a die grinder. The mounting
flange is thicker than the stock unit, necessitating
longer studs in the trailing arm. When replacing
the studs, helicoil or keensert with 5/16"-18 studs.
The stock studs are UNF, but it is better to use
coarse thread into the casting for strength. Fine
threads are usually used for high-vibration situations.
The outer axle yoke is modified TR6. a shop will
"cut" the cap area to accommodate an "inside curclip"
(as opposed to the stock outside circlip). The difficulty
here is a) not damaging the axle splines during all
this manhandling, and b) finding a good core that is not cracked.
The Corvair outer hub (from the IRS models)
uses a 5-bolt on 4.5" bolt pattern. If you want to
use Triumph wheels, then the hub has to be drilled for the 4-bolt on 4.5" pattern.
Finally, the center of the hub is larger than stock, so the
center hole of the brake drum needs to
be opened up and so too also with your road wheels. If you use Al-fin or 240Z alloy finned drums,
you will know that the alloy drum consumes all of the stock hub landing; so too with the Corvair
hubs. This has issues with rendering the wheels to be no-longer hub-centric, but that will be
covered somewhere else.