Pat:
The seller on eBay didn't really lie; The Volvo OD is the same
unit used by Triumph. Some of the fittings are different to allow using
it with the Volvo speedo, tranny, etc, but the OD itself is the correct
OD. A "real" Triumph OD will have a serial number starting in 25, which
indicates a 25% overdrive. Volvo used a 27% or a 28% ratio, so the
serial will start with 27 or 28. You will never notice the difference in
the drive ratio when using the OD. It is only about 75 RPM slower at
3000 RPM.
Volvo didn't use the same mounts, so the rear mount is not
drilled or tapped. This is very easy to do. Any hardware store will have
the tap, and it only costs a few dollars. The only other issue is that
the speedo cable for the Volvo is different, so the cable adapter is
different (you can buy the correct one from Moss). All of this is pretty
easy stuff to correct.
What puts some people off is that the speedometer is incorrect
after the installation, because the Volvo speedometer is different. Even
correcting this is pretty minor, and can be done in a few minutes.
The drive gear for the Volvo OD is 21 teeth. The TR6 uses a 20
tooth, and the Spitfire 18 teeth. I checked my speedo after the
conversion to a Volvo OD using highway mile markers and my watch, and it
is almost exactly 15% low (10 miles an hour at true 60MPH). Since the
Volvo speedo gear is 21 teeth, I need 15% fewer teeth to get my speedo
close, so the Spitfire OD gear is the one to use. The reason that the
TR6 speedo gear will not work is that the drive gear inside the OD is
also different from the TR6 OD. So simply using the TR6 20 tooth speedo
gear will not get you what you want. Since the Spitfire also used the
J-Type OD, the gear is a drop in. By the way, the Spitfire J-Type OD is
not identical to the TR6 OD either, and differs in ways very similar to
the Volvo OD.
Swapping the gear requires jacking up the car, disconnecting the
OD cable at the tranny, removing 1 small bolt. You then withdraw the
gear, insert the new one and reinsert the bolt and reconnect the cable.
Simple. About 20 minutes, even for the mechanically challenged.
The spitfire and TR6 drive gears are available from Quantum
mechanics, for $40.
Cheers,
Vance
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net] On
Behalf Of Patrick Bitton
Sent: Friday, January 07, 2005 6:35 PM
To: 'Robert Liam Gannon'; 6pack@autox.team.net
Subject: RE: Volvo Overdrive Unit
Good point Robert. I bought one from someguy on Ebay(person in New
Zealand). He advertised it as a TR-6 overdrive. Well, the flange had to
have new holes drilled. Then I had a local Triumph mechanic install the
overdrive on the transmission. That was fine. I then installed the
tranny with OD on my engine. Once my restoration project was
finished...I took the car out for a drive and noticed that the
speedometer was giving wrong readings. With a little bit of research...
it turns out it was from a Volvo and not a TR-6.
Therefore, even if the person advertises it as a TR-6 OD, make sure that
he guarantees that it is. Have it checked out and you will not have to
learn the hard way as I did.
Now, all I can do is keep up with the flow of traffic and I won't get a
ticket. It will be too much of a pain to replace the gear. Maybe in a
few years, when I have nothing to do.
Cheer,
Patrick
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