Subject: Re: [Fot] A type O.D.

From: "Randall" <[email-masked]>
Date: Wed, 31 Jul 2013 07:55:22 -0700

> We didn't include that warning, because I had never heard of coil bind
> occurring in these units. B I tested the springs with a valve spring
> gauge with a range that didn't allow me to reach coil bind. The
> accumulator springs or so stiff (over 500 lb/in) a couple of hardware
> store washers will increase the pressure about 50 psi. B I shimmed a
> large accumulator OD to about 525 psi with no problems. When the coil
> bind happened, what was the shim thickness and what pressure were you
> at? B Was it a large or small accumulator? B I will add a 
> warning when we
> update the article, but would like the details.

I'm sorry, Larry, I don't recall the thickness of washers.  Maybe around
3/16".  It was a large accumulator, and I had shimmed it from around 300 (no
shims) to around 450 psi (original TR4 workshop manual says that 490-510 "is
required" even though the Bentley gives 380-400)).  At 300, the OD wouldn't
always grab in 2nd gear, so I knew that was too low.

After the damage was discovered, I loaded the main spring (which I believe
was original to the unit, but with a whole lotta miles on it) into a
hydraulic press and measured the length with it compressed solid.  Combined
with all the other measurements, I believe that the spring was reaching coil
bind with the ports uncovered only about 1/4 of the way.  The relief ports
were definitely opening enough to flow some oil during bench testing, but as
I'm sure you are aware, the pump output volume is a direct function of shaft
rpm and they were apparently not able to open far enough to handle the
entire pump output at higher rpm.  I was also using cheap motor oil during
bench testing (saving the MT-90 for the road) so it's possible the oil
change had some impact as well.

Nelson also wrote about finding coil bind, but he found it on the bench with
around 1/4" of shims.  It just hadn't occurred to me that it was possible
for the vent holes to be only partially uncovered when the spring bound.

I'm pretty sure that "root cause" is that my spring was old and sagged from
it's original dimensions.  Unfortunately, I didn't have a new one on hand to
compare against (or I would have just installed the new one).  At the time,
they were backordered at all US vendors (and the one I ordered from the UK
didn't come in for over a month).  (Moss actually changed the part number
and doubled the price, so I'm guessing that a new manufacturer had to be
found.)

-- Randall 
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