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Re: [Fot] TR3 / 4 Piston to Wall clearance

To: Steven Belfer <colordog.1@earthlink.net>
Subject: Re: [Fot] TR3 / 4 Piston to Wall clearance
From: Bill Babcock <Billb@bnj.com>
Date: Fri, 31 Oct 2008 11:42:20 -0700
Unless the pistons are rattling around and have collapsed their  
skirts, or there's blowby you're probably better off leaving them  
alone.  Unless the rings are worn or have lost spring pressure by  
being overheated I'd even re-use those. If you refresh the head  
(replace, regrind, or lap the valves, check and replace any springs  
that have lost too much tension) and the engine passes a leakdown test  
then why mess with the pistons, rings and liners. The rings are  
already seated and functioning. With our old tech engines, I think  
it's often best to just ream out any ridge in the liner and run a  
bottle brush hone down the bore just to disturb the glaze. Check the  
piston skirts to ensure none of them are collapsed. Other than that,  
if it's working it's working. New rings won't function as well as the  
old ones until they properly bed, which could be never.

My old (1950) Vincent Black Shadow has uncounted miles on the pistons  
and rings, mostly with a pretty marginal air cleaner. It got a  
complete teardown recently because the bottom end was noisy. But other  
than refurbing guides, grinding valves and replacing the bottom end  
bearings, pretty much everything went back in for another 58 year  
round. The rings were worn a little, especially near the gap, but they  
were sealing perfectly and had perfect tension. No sign of blowby on  
the piston skirts, no overheating of the rings. I could have replaced  
them for not much money, but chose not to. Not because of the cost  
(trivial, compared to the labor--tearing down a vincent is a MAJOR  
piece of work) but because they were working well. In my experience  
you have more problems getting sliding parts to bed properly than any  
wear issue. And while they are bedding, hot gasses are blowing down  
the piston wall, overheating the rings and annealing them.

Not so with plain bearings and surfaces that get pounded (valve seats  
and valves). If I have an engine apart it gets new bearings unless the  
existing ones are super primo and the valves and springs get looked  
at--always.

On Oct 31, 2008, at 9:58 AM, Steven Belfer wrote:

> I have a well used race motor in the machine shop.  It's got  
> Hepolite Powermax cast pistons and 87mm sleves that have seen many  
> races.
> The liners may need a bit more than a light honing to freshen them  
> up.  What would you accept as the max allowable P to W clearance?
> I don't really want to buy new liners if I don't have to.
>
>
>
> ~Steve
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Bill Babcock
Babcock & Jenkins
Billb@bnj.com
503.936.7660
www.bnj.com

Editor
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Paddlesurfing's Web Journal

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