The other ring-related point that should be mentioned is the condition
of the top ring groove in the piston. If it's beat up & has excessive
clearance, the thing will never seal properly, especially at high revs.
I've had to scrap a lot of pistons, where that was the only problem
that prevented reuse.
Glen
(Yes I remember the "old days" when it was common automotive machine
shop practice to cut the groove & fit ring spacers...)
-----Original Message-----
From: Bill Babcock <Billb@bnj.com>
To: Steven Belfer <colordog.1@earthlink.net>
Cc: FOT <fot@autox.team.net>
Sent: Fri, 31 Oct 2008 2:42 pm
Subject: Re: [Fot] TR3 / 4 Piston to Wall clearance
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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