We constantly hear from customers who have holed a piston or have bought a machine with
a blown top end
and seem to believe that they can simply install a new piston or valve and go
along their merry way with no problems.
It just isn't so! Anytime a motor blows a piston, you
should completely dismantle the motor and clean everything
to white glove specifications.
On a British Twin, this is especially true. You must remove the plug from the sump
trap in the crankshaft. DO NOT attempt to squirt solvent through the crank shaft to bypass
removing the sludge
trap, doing so will ensure your motor will lock up in about 10 minutes.
Remove, clean, replace the sludge trap,
and replace the plug. The aluminum from that piston
gets everywhere. It is also not uncommon to need an oil
pump when a piston blows.
The piston particles tend to screw up the scavege side of the oil pump, then it smokes
badly, because the engine cases are full of oil. while this doesn't happen every time,
it happens often enough that I
would strongly recommend tearing the engine down and
cleaning it as described, replacing oil pump and rod
bearings while it is apart, otherwise
you may screw up a crankshaft the next time it seizes (which will be very
soon if the sludge
trap is clogged with piston debris)
If you are going to buy pistons, why not do it right? If you are simply replacing the old pistons
with the same size
you may have the same problem. It only costs $30.00 per hole to have the
cylinder bored and fitted to the new
oversize pistons.You CANNOT tell anything by simply
looking at the cylinder. It has to be measured with a
micrometer. Excessive clearance leads
to shortened ring life, broken rings, broken piston etc.....
Another common mistake, is believing that you can perform an acceptable valve job at home.
unless you have a machine shop at home. DO NOT attempt to install valve guides.
You will damage the head
by driving out the old guide. Heavy aluminum deposits on the outsideof a guide show that it has been removed
the wrong way. That means the next guide will fit
that much looser. If you attempt to drive in too large of a
guide, you may crack the head.
The correct way to remove a valve guide is to glass bead the head (to remove the traces
of carbon on the
guide, as carbon build-up will remove aluminum on the way out)
Heat the head to 275-375 degree, then
remove the guide. Then the guide has to be miked
to determine the size of the replacement. It is almost
impossible to install a valve guide at
home and have the valve seat properly. While the bike will run,if
you simply beat a new guide
, it probably will not run right(popping past an unseated valve, losing 1
cylinder because of valve
sticking in loose guides etc)
The reason you can't do it at home is because the seat must be RE-CUT ANY TIME A GUIDE IS REPLACED. THEY DON'T GO BACK IN AT PRECISELY THE SAME ANGLE THAT THEY COME OUT. You must also consider the possibility that the valve stuck open when the piston blew
(from the aluminum particles deposited on the valve stem) and hit what was left of the piston. You
normally can't tell this with the naked eye, but it is very apparent as soon as the valve is chucked up
in the valve machine. Even a slightly bent valve will cause major problems and should be discarded.
Although, I know it is not what you want to hear, You should completely dismantle and clean the engine.
DO NOT BEAT THE CRANKSHAFT OUT OF THE CASES WITH A HAMMER, THAT WILL
DAMAGE IT, YOU WILL LOSE OIL PRESSURE AND YOUR BIKE WILL DIE THAT
Machine work is no place to try and cut corners. Send the head out to us for a valve job.
Send the cylinders out to be bored and fitted to new oversize pistons and replace the oil pump.
If you try to take shortcuts, it will
probably cost you MUCH more in the long run, especially
if the crankshaft or cases are damaged the next
time it blows up.
If you wish, we can perform any or all the operations I have described above. You can send
your parts by
UPS, or send the whole motor if you prefer. If you don't want to use our services,
try and find a QUALIFIED
MOTORCYCLE shop in your area. (be forewarned: many shops
are not able to do decent valve work
and even less automotive shops can)
We do this type of work everyday. we are highly qualified on all phases of machine work,
or any other type of repairs.
If you would like more information about any of our services call or e-mail us.