[TR] GT6 / Spit rear end options Re: Triumphs Digest, Vol 7, Issue 293

[aribert neumann]

Dave:

Grass Roots Motorsports magazine built up a rotary powered Spit a number
of years ago.  I became a subscriber when I learned that the project was in
the works.  They (highly) modified a Miata or Rx7 center section so that
there were no changes to the Triumph frame.  At that time,  I had destroyed
2 diffs, in 4 yrs, with my 2.5L Spit bodied GT6.  Using their articles as
guidance, I built up my own,  While GRM also went w/ a custom rear
suspension w/ coil overs and custom upper control link, I kept the GT6
suspension but upgraded to a later Spit swingspring set up.   I have a
3.66:1 ratio diff (lowest available that I am aware of) with the GT6
overdrive.  I am pleased with the overall ratio.  My diff consists of:
1.8L Miata diff housing (or 2nd gen RX7 - very similar in appearance), 1st
gen RX7 diff carrier, ring & pinion from a mid '80s RWD 626 and axle stubs
from a Miata (I don't remember the exact years for this, only a couple of
years where the inner CV joints are bolted to axld stubs instead of having
spines that stick into the diff).  For drive axles, I am using Miata inncer
CV joints and VW Transporter outer CV joints (should have used smaller
dia joints from a Rabbit GTI - I needed joints that mounted to a flange,
most CV joints include a splined stub).    To make all this work with the
Spit / GT6 frame, the aluminum Miata/RX7 diff housing has the mounting legs
cut off, aluminum blocks are welded to the housing to match the Spit/GT6
diff frame mounting & the transvers leaf mounting (and then machined)  and
the steelTriumph front mounting plate on the diff carrier has to be welded
onto the cast iron Mazda diff carrier.

This is not a trivial endever.  I have a mill and lathe and several
welders.  I borrowed a rear frame from a Spit, I outsourced the aluminum
welding - I don't have a TIG (yet) - and my then employer did not have a
fab shop that I could slip into during lunch to take care of my business.
I also outsourced machining the splines on the drive axles to go from Mazda
inner to VW outer.   My then employer sent me down to the Atlanta area to
the Lockheed Martin windtunnel - not far away, in Peachtree, GA, is a Mazda
salvage yard - I was able to pick up all needed Mazda parts in one short
visit and had them shipped back home.  This was a winter project for me
when the car is off of the road anyway.  Currently I have about 20K miles
on the custom rear end.   THe biggest difference I noticed was the greatly
reduced rolling resistance with CV joints vs the original Roto-Flex
couplings - the first few drives were downright spooky when driving at low
speeds and then coasting (declutched).   Downside is that the Japanese diff
leaks oil - more than my trans or engine!  I intend to pull it this winter
to look closer at where it may be coming from - I think there may be a pin
hole somewhere in one of the welds on the housing.  But then I have
intended to do so for atleast the past 3 winters and did not get around to
it.

You might contact GRM to see if they still have back issues of the Ro-Spit
(their name for the project car) build, specifically issues with articles
about the diff and the rear suspension.



> -----Original Message-----
> From: Triumphs [mailto:triumphs-bounces at autox.team.net] On Behalf Of David
> Porter
> Sent: Tuesday, October 14, 2014 5:23 AM
> To: Triumphs
> Subject: [TR] spit/Gt differentials
>
> I'm a bit displeased with the 2 rear end ratios available for these
> units. I've seen a number of interesting conversions on other marques,
> like Miata IRS under a Sprite. Anyone know of "similar" conversions for
> the Spitfire or GT6 that they have pictures or contact info for?
> Dave
> --