Randall's statement is corroborated by this which appears in the Moss UK
catalog (available as a pdf download):
"22/1753 is the only safe unit to use on the TR4 IRS, as none of the earlier
units feature the "cushioned" engagement. This modificaiton was introduced in
1965 to smooth out engagement, it also lessened the likelihood of the
engagement to rip the differential mountings off the chassis."
Clearly listed for both TR4A IRS and TR6 from CD01 to CD15651, I have never
seen the 22/1753 for sale or up on ebay. My 4A has an older banger, the 1712
model (or 22/61712), not likely original to my car. So I'm looking for the
'correct' one. The earlier ones are fairly common.
Curiously, Moss UK lists a separate part number for TR5 - TR250, but they don't
list the Laycock model number, and the part number is different from that
listed for the 4A-6 (1753) which you would expect would work for the 5/250.
Also curiously, the part number stocked for the 4A in that catalog seems to be
linked to the later TR6 model(22/1985) not the the 1753 which raises the
question if they are obtainable at all.
So does anyone out there have the 22/1753 in 4A - 5/250 - 6 ??
Or a 22/1985 ??
cheers
January
66 TR4A CTC74217LO
Date: Fri, 9 Apr 2004 13:40:43 -0700
From: "Randall Young" <ryoung@navcomtech.com>
Subject: RE: TR4a OD Trans in a TR250??
> My understanding is, it is the size of piston and spring in the
> accumulator was changed.
That's right. There's also a sleeve that goes in the original piston bore,
to carry the smaller piston.
> I think this affects both the pressure and volume of oil
> available to bang the od engaged.
The change is primarily to the volume of pressurized oil available to move
the operating pistons. The actual operating pressure is higher (at least
for early TR6 vs TR3, but I assume the change was for the 6-pot engine); but
the smaller accumulator piston has the effect of letting the pressure fall
much farther when the OD is first engaged, to give a softer engagement. The
pressure quickly builds back up though, to lock the OD clutch against the
increased torque of the bigger engine. The net result is a lot less 'bang'
when shifting into OD.
Randall
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