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MAKING THE DECISIONS - WHAT TO REPLACE?
by Pete Snidal, self-appointed wrenching guru (C) 2000
Planning on taking your motor apart? Got it apart already?
Once that happens, you soon discover the major trick to monkeywrenching:
deciding what to replace and what to keep. Your standards will vary, of
course, depending on your criteria for longevity and intended use - building
a racing engine to last the Daytona 500, and for which you'll be spending
hundreds of thousands of dollars just to enter, will demand higher thresholds
than throwing together a grocery-getter which you hope will last you another
6 months until the kid's braces are paid off. But here are the general
- 1) Valve Guides: You put a valve in the guide and see
if you can get any "wiggle" - feel any side clearance. This with
a dry valve and guide, once you've cleaned it all up. If you feel clearance,
you can then use a micrometer to check and see if the valve is worn. (But
it's usually the guide.)
- 2) Valves: I have a thing with a suction cup on a stick
and grinding compound. The procedure of "lapping" the valve and
seat has value only in checking to see if Ican get a proper sealing surface
(a "ring" about 2mm wide on both their surfaces. If Ican, I know
Idon't need to have them refaced on a valve grinding machine, and having
a valve refacing machine recut the seats. If the valve or seat is too far
gone, the valve will sit too far down into the seat, and either or both
will have to be replaced. The machine shop with the refacing gear will
help you make this decision. (nb: once the refacing is done, "lapping"
with grinding compound is totally unecessary - there is an old wive's tale
still in vogue in some circles which says this will improve things, but
it won't - conditions are totally different when the engine is hot and
running fast than they were when you lapped the seats cold, so there won't
be any improvement.)
- 3) Rings: You take them off the piston, and put them
in the cylinder squarely, at top, center, and bottom of stroke. Using a
feeler gauge, measure the "end gap" - the distance between the
ring ends. If it's over the specs in the manual, you replace the rings.
If new rings show too much gap, you know your cylinder is too far oversize.
(Variations in end gap in different parts of the cylinder will also give
you an idea of cylinder wear - remember that gap differences will be 3
times diameter diffs. Well, 22/7)
- 4) Piston clearance: You put the piston in the cylinder
(both dry and clean) and using a wire feeler gauge, check for skirt clearance.
(at right angles to the piston pin) If it's more than the manual says for
piston clearance, you get a new piston and try again. If it's still too
much, you have the cylinder rebored to the size of the new oversize piston
you exchanged for the first new one you just bought. Never having had a
good set of wire gauges, I've always used a narrow blade gauge.
- 5) Big end: Check for side clearance of the top of the
rod at different places - top dead center, middle, and bdc. See manual
for decision-making here. If the side play is getting up there, check for
end play - if you can feel any up-and-down clearance at all, it's time
for disassembling the crankcase and having a new big end pressed into your
crankshaft. You have this done at a knowledgable motorcycle repair shop.
- Other Stuff: Cam bushings, cam followers, oil pump drive,
all these and more can be decided upon by the same basic methods. Check
when dry and clean for sloppy fit axially, and check the manual to see
if end play is a consideration - although you can often figure it out for
yourself (would it make a difference if this thing moved end to end? Is
there anything here that is designed to prevent end play, but is excessively
These are the basic considerations to use in deciding
what to replace when you have your motor apart. This is only a guideline;
you should of course have a copy of the service manual before you attempt
to do any kind of rebuild on any kind of engine, and read this carefully
before embarking on your maiden voyage into wrenchland. Happy monkeywrenching!