>
>> Its too late for me, but what is your opinion on Bronze guides and the
>> whole matter of danger to the valve train using unleaded?
>
>Bronze guides and precautions against unleaded fuel are two different
>issues, so lety's look at them sperately.
>
>Rapid valve guide wear has always been a problem in the XPAG/XPEG series
>of engines. Lack of lubricant isn't the problem. There are no valve stem
>seals in the modern sense of the word, so more than enough oil works its
>way down into the guides. Lack of proper retention of lubrican, however,
>is a problem. The real problem is incorrect geometry between the rocker
>arm and the valve stem. This puts a lot more sideways thrust on the valve
>stems than there should be, which accelerates wear on the guide bores and
>on the valve stems.
>
>Ideally when the valve is at half lift, the rocker arm and the valve stem
>should form a 90 degree angle. That is seldom the case in any production
>line engine, but even with all new parts the geometry on an XPAG/XPEG is
>almost always way out of whack. This has to be corrected by either
>machining the bottoms of the rocker shaft stands or by inserting shims
>under them, depending on which direction the geometry is off. On an old
>head that has slightly sunken valve seats due to many rebuilds, the latter
>is more common.
>
>O
>
>Getting the geometry right does improve valve guide life, but it's still
>far from ideal. The friction surface between rocker tip and valve stem
>top is pretty small, and the rocker tips aren't very hard (especially if
>they have been reground to compensate for wear). Once the rocker tip
>wears, it imposes more sideways thrust on the valve stem even if the
>geometry is right. What we really need is thicker valve stems or a thrust
>cap to install on top of the stem. Even better, how about roller-tipped
>rockers!
>
>
>
>Phosphor bronze guides help to reduce wear because they retain lubricant
>better, and because the material has a lower coeficient of friction. They
>do expand more when they heat up, so the cold clearance between guide bore
>and valve stem has to be slightly greater (as you discovered), otherwise
>when it gets good and hot the guide will clamp onto the valve stem. When
>this happens, the moving valve tries to take the guide along with it.
>
>Rapid valve seat deterioration due to use of unleade4d fuel is a different
>issue. Even with leaded fuel, XPAG/XPEG valves and seats go bad pretty
>quickly. The 30 degree seat angle is partly to blame. It doesn't provide
>as good a "wedging" seal as a 45 degree seat, so sealing is not as good as
>it could be and there is more chance of hot exhaust gas leaking past the
>seats. This causes the seats to erode more quickly. Recutting the seats
>to 45 degrees helps considerably.
Chip, thanks for the reply. I guess I will answer my own questions in a
number of years(if I Get that long) when I next remove my TF head. I did
have the seats recut 45 degrees,So at least that something. Do you think"
relead " will help when I take longer trips?
Thanks again
Mike
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