Is this to be a racing engine? If it's just for general driving it's
probably fine, but every ring increases friction and robs horsepower.
Piston measurements are almost impossible to get right with a
micrometer. The more modern the piston, the more true that is. Pistons
are tapered and frequently cam shaped. The clearance needs to be greater
near the exhaust valve since that is the hottest part of the piston.
Used pistons get even harder to measure since heating/cooling cycles
distort the pistons without really making them unusable.
If this is for a race engine, toss them and get something better. If
it's for a street engine, visually inspect to ensure there are no seize
marks, make sure the rings are all free, decoke and check the skirt
clearance with a feeler guage at the bottom of the cylinder (away from
the ring wear area).
> -----Original Message-----
> From: Bob Kramer [SMTP:rgk@flash.net]
> Sent: Wednesday, March 11, 1998 10:09 AM
> To: fot@autox.team.net
> Subject: Fw: Re-use of Pistons
>
> I received the following question and I'd like to pass it on to others
> more qualified to answer. FWIW, I have heard that the 4 ring pistons
> are not a good as the 3 ring variety, something about them being for
> low compression tractor use not the TR engine. I let my machinist
> check
> out my pistons so I've never gotten into this subject. I'll reserve my
> own thoughts (until I'm enlightened by the collective wisdom) in order
> to preserve the illusion that I know what I'm doing!
>
> Bob Kramer, Austin TX
> Hill Country Triumph Club
> TR6x3, TR250 x3, TR3A vintage race
> rgk@flash.net
>
> > William F Daehler@DELCO on 03-10-98 03:52:46 PM
>
> > I'm looking for a little help again. I'm trying to decide whether
> or not
> > to reuse my old pistons or not. The are 86 mm, have been cleaned
> up,
> and
> > have new Deves rings on them. The sleeves have been cleaned up too.
>
> I had
> > the rebushed the wrist pins too. The pistons are the old 4 ring
> style by
> > the way, the forth one near the bottom of the skirt.
> >
> > I have measured my pistons and sleeves per the workshop manual. It
> takes
> > a while to get all that calibrated equipment in one place, but I
> did,
> by
> > borrowing the right stuff from work.
> >
> > I'm in spec, but as defined by the work shop manual. The sleeves
> are
> quite
> > cylindrical, and all points are in spec. I'm nervous about the
> pistons
> > though. You are suppossed to take the measurements A-A and B-B
> across the
> > thrust face.
> > A-A is on the skirt, taken just below the compression ring. Fine.
> B-B is
> > taken at the bottom of the skirt. Fine again.
> >
> > If I measure at the top of the piston, or perpendicular to the
> thrust
> face
> > (gudgeon pin circlip to gugeon pin circlip) I show 0.025" less.
> But this
> > is a measurement in an area that's undefined and not controlled.
> >
> > I don't know what dimensions new pistons are supposed to be. Am I
> > observing a wear condition? Are pistons suppossed to be tapered and
> oval
> > shaped?
> >
> > What do I do? What do you know? What have you succeeded with? What
> have
> > you failed with? What do you recommend? Have you ever gotten into
> any of
> > these measurements? Do you want more information?
> >
> > Will Daehler
> > 63 TR4
> > 414 768-2170
>
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