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

From: Larry Young <[email-masked]>
Date: Fri, 02 Aug 2013 15:42:17 -0500

I normally shim them with plan hardware store washers that are about 
1/16".  These old springs always seem to be a little week, but I have 
never had to use more than two to get an OD back to spec.  I checked 
several manuals (but not the TR4 workshop manual) when we wrote the 
articles. There were often discrepancies between different sources. I 
think the accepted spec for a large accumulator is about 360 psi and 450 
for a small accumulator.  The change to the small accumulator was made 
to give a more gentle engagement for the IRS cars.  I can't remember 
exactly what I did to get my large accumulator to 525.  Since the inner 
spring doesn't do much, I may have used a small accumulator spring as 
the inner spring or used a plug to shim the inner spring more than the 
outer spring.  Sam and I should revise the article, if it only takes 1/4 
inch to reach coil bind one must be very careful.
Larry

On 7/31/2013 9:55 AM, Randall wrote:
> 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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