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Re: [Fot] A-Type Overdrive Slipping

To: "Fot" <fot@autox.team.net>
Subject: Re: [Fot] A-Type Overdrive Slipping
From: "Randall" <tr3driver@ca.rr.com>
Date: Sun, 2 May 2010 18:26:42 -0700
> Later, 
> Laycock re-developed 
> the overdrive with larger pistons (hence more pressure) and 
> these larger 
> pistons did not have piston rings - they came with o-rings.

I believe the piston size got larger during the TR2 run (when OD in 2nd &
3rd was added); but the O-ring seals did not come in until 1964 or so.
Anyway, my original TR3A OD had the larger pistons with cast iron rings on
them.

> I have 325 psi oil pressure

Too low, IMO.  The TR4 workshop manual (which is for the same OD as used by
TR3-3B) says 380-400 is required.  At 420 psi (with Redline MT-90), I can
shift into 2nd OD at full throttle and no sign of slipping.  Note that the
factory specified pressures as high as 500 psi for some applications (early
Stag), so I don't think there is any concern over the unit being able to
handle the pressure.

The point about setting the operating valve is a good one; but incorrect
setting will normally just cause the unit to shift slowly or not work at
all, rather than slip in overdrive.  Best practice, IMO, is to ignore the
"setting lever" and measure the ball movement directly.  Opinions vary on
how much the ball should move (See Nelson's article for a study of behavior
at various lifts), but I use .030 - .040" based on an old article by Del
Border (which was actually for a big Healey, and said 1/32" - 1/16").  The
TR4 OD I have in my TR3 gives almost exactly 1/32" lift with the setting
lever lined up, and does have some wear in the linkage, so I feel this gives
credence to the numbers that Del quoted.

Randall
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