Mike,
It's OK to replace only the parts that are broken, though that leaves
unanswered the question of which others are equally weakened.
You can probably find a single piston and a rod. Abingdon Spares and
the members of this list are places to start looking.
But first... if you check into the engine section of the workshop
manual you will find that there were a number of slightly different sized
pistons used originally. Depending on the size the boring of the bores
measured, the original pistons might have been over-size when the engine
was assembled. It seems to me that the way to approach this problem is
to measure the bore, then work backwards from what you measure to get to
what you want for a piston.
The rod may be OK. A machine shop should be willing to magnaflux it
for you and check it for straightness.
Once those challenges are met, you can get into balancing. I don't
know how precisely these components were balanced in 1954, though
certainly not as well balanced as we can do today. Removal of TD
pistons is fairly easy, in that they drop right out the bottom of the
engine, so you could have all of them checked and balanced before
reassembly, and that gives you opportunity to replace the rod bearings
while the engine is apart. You will need a narrower-than-usual ring
compressor to fit in the space between bottom of the bore and the
crankshaft, but the job can be done in an engine that has not been bored.
Main bearings should be OK, assuming that the filter did its job,
unless there was undue shrapnel in the sump, something you would want to
check very carefully by pouring the oil through a white cloth and using a
magnet. Changing mains in an XPAG engine requires its removal from the
car. All this other work can be done with engine in place.
XPAG valves break all too often. I would replace all of them, not even
bothering to magnaflux--just get new valves. You would want to check the
guides since the head is apart anyway. Some advocate softer springs
since we don't drive these cars as sportingly as when they were new.
Bob
> Last fall, or early winter, I took my TF on one last drive - it
> turned out to be a disaster. As I was driving along I heard a "bang!"
followed
> by a very loud metallic "clack, clack, clack,clack." I thought I had
> thrown a rod. I had the car towed home and put in storage over the
winter.
>
> After a few other projects this spring, I made enough room in the
> garage to begin tearing down the engine. These photos show what I
found: The
> head of a valve had broken off in #4 cylinder and must have shot
around in
> there. It banged up the head just a little bit, and smashed the top
of the
> piston, as you see. Amazingly, it didn't score or scratch the cylinder
even a little
> bit.
>
> So I'm looking for advice. Can I just replace the single piston and
> rod? Can I still get original weight pistons? Where should I look?
(I'm
> located near Abingdon Spares.) I will plan to replace all the rod
bearings
> at the same time.
>
> Or should I replace all four pistons? Money IS an object here, but
> I don't want to do all this work and fail to do an adequate job.
>
> Obviously, I'll plan to have the head rebuilt, P&P'd.
>
> This is in my 1954 TF 1250, car has 33k original miles.
>
> Thanks for any advice,
> -Mike Eldred
> Wilmington, VT
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