The International Cessna^® 170 Association

1951 170A, 1900 hours, overhauled the C145 400 hours ago
#5 cylinder compression now 60/80 (others all 75 or better)
leaking pask the exhaust valve
same valve repaired in 1998 and 2005, by grinding
find this unacceptable, perhaps scary
ordered a Millenium assembly
see no other way to avoid worry
the larger Milleniums have had problems, but not this one, I think
anybody find fault with replacing the whole thing?

Thanks, again, for advice.
Jim Heidere

Repairing the valve by "grinding" may be an abbreviation of the work performed or it may be the summation.... but there are other possibilities that you might also consider.
The labor involved is not that different whichever way you go, so the temptation to replace an entire assy is great. And the entire replacement will also address valve-spring replacement issues that might have been the root cause of the repetitive failure. (Valve spring tensions are more important than many folks realize, yet you'd be surprised how many cylinder rebuilds will ignorantly re-install the same old springs without any testing of them whatsoever. A weak set of springs can lead to valve leakage leading to seat/face erosion and eventual failure.)
There are other reasons that can cause a particular valve to repetitively fail... A bad lifter or an incorrect, excessively long pushrod. They are rare but they can happen, so if you replace the entire assy with a new one, at least visit those issues.

Why do you find it unacceptable to grind a valve seat? Isn't the term "grind" in this case just being used loosely to describe machining the seat?

What can cause a valve/seat interface to become compromised? Buildup on the apron of the valve? Other material galling the seat? Misalignment causing a lateral drag as the valve seats? I'm just looking for causes other than the lazy or weak springs George mentions.

I read in my logs that not too long before I took possession, the #5 exhaust valve had been "ground" to remedy a low compression, which returned to 76. This is on a new set of Millenium jugs that were installed in '99 with the overhaul.

if the clearances between the valve stem and the guide are too loose then the valve will wobble and not seal consistently on the seat, causing burning/leaking. very common on Lycomings and the bigger Continentals.

George is talking about the pushrod being too long, should be checked each time it's apart for the proper lash - if the rod is too long it holds the valve slightly open - again, burning/leaking. the lifter is there to keep constant tension on the pushrod pushing on the rocker which rides on the valve tip - if it keeps bleeding down then slop which equals hammering on the tip.

nothing wrong with grinding a valve and seat (do them both at once) but if it keeps happening then you need to find the underlying cause - something in the geometry isn't right. replacing the cyl assy at least starts the seat/guide/valve combo from scratch, but always check the valve seating even on new before installation.