Are you sure you aren't seeing a closer fit at the top due to the ridge of
steel and carbon built up over long service, versus lower in the cylinder
where there is no ridge and you see the fairly large clearance between the top
land of the piston (ie the area between the top ring and the crown) that's
common on most pistons?
To really check clearance you need to ream the ridge out of each cylinder and
remove the pistons. Then measure clearance on the piston below the rings -
that's the portion of the piston that's supposed to fit most closely in the
bore. Check clearance both on the thrust faces and on the wrist-pin faces of
the pistons, and at the top, center, and bottom of the piston travel range.
If you opt to measure diameters directly rather than just measuring clearance
with a feeler gauge, (it's really best to do both) you still need to measure
the bores and pistons at least across the block and in line with the block,
and ditto the pistons - in line with the wrist pins and across the axis of the
wrist pins, in several locations up and down the bores.
In a worn engine, you will probably find the most cylinder near near the top
of the bores - below the area where you reamed out the ridge. Lower in the
bores you will probably note some out-of round, with the bores larger across
the block from the rocking action of the rods and the resulting side-thrust.
The piston skirts will likely be similarly worn. With only minor wear, you
may be able to scuff the cyliinders and knurl the piston skirts on the thrust
faces to decrease the clearance, replace rings if needed, and reassemble.
Good luck !
Karl Vacek
'68 TR250
'67 TR4A-IRS
'64 Amphicar
'16 Ford "T" Touring
'46 Piper J-3 Cub
> Nick wrote:
> Something doesn't look right!
> I just pulled the head on my 74 TR6 to do some valve work and noticed
> that the clearance between the piston and the cylinder wall looks really
> excessive. The motor has less than 10k miles on it. It was rebuilt by a
> reputable shop last year but something really looks wrong. It looks like
> I have almost .020 clearance between the piston and the cylinder wall
> when the piston is at the bottom of the cylinder and much tighter at the
> top. Any advice would be greatly appreciated.
> Nick Gemas
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