I can only think this was made by aliens, which may explain how the thing managed to run with the ignition switched off. Really the bore is unmarked. All other marks on the piston are linear. There is slight brown discolouration under the crown of the piston. Other than that the only visible fault I have found is that the mating surface of the head was not flat (higher at the outside than the in. Anyway new piston (Honda)+ small end bearing plus gudgeon pin and gaskets arrive tomorrow so hopefully there will be some good things to write about soon. If anyone wants to visit England fore the rebuild and to view the wacky piston feel free.

It looks like to me that the seal spring did the damage. Can we say air leak!

One way to find out is to pull the seals before running it again.

You are talking about crank seals Right? Is there any way you can pressure test the crankcase while the barrel and head are off?

Not really but you could put the cylinder and head back on (no piston)
with no gasket (just use grease) torque normally.


If the spring came off the seal might actually still seal and it might pass the
pressure test, the best way would be to remove the seals and look replacing
the seals at the same time.

You can change the clutch side seal by removing the drive clutch from the
Engine, the other side you will need to remove the side cover flywheel
stuff to gain access, see the service manual.

Hi Hoser,
Just an update, I have just received the parts from Honda so hopefully will have an update after teh weekend

OK, Took me longer to get down to it than I thought but Engine was rebuilt and finished last night.
Used new piston gudgeon pin and small end bearing. Cleaned out all valve cage etc. Compression tested at 150 psi and Engine fired instantly on pull cord as battery was flat. I will put seat back etc over next few days and give it a run at the weekend.
Many thanks for all the advice and support.
By the way if you ever get anyone from the UK asking for suppliers of parts for Pilots I finally found a decent one
The Quad Centre In Buckingham are really helpful.

Did you check the seals to see if it was the sping off a seal that made the markings
on your piston?

Hi Hoser,
Did not check seals but pressure tested the crankcase it held pressure no problem.
I have a feeling having thought about this a lot that the Engine had probably been rebuilt at sometime before using bits a pieces from all over. The piston had those wierd marks but the bore was perfect. the base of the cylinder i.e mating surface had one dent in it where someone had obviously levered it off at some time but the corresponding surface on the crankcase was perfect.
I have learned a lot about this beast over the past 2 months so at least I now have a base level to work from.
Thanks again for all of the advice and pointers
Have attached a pic for your records of Pilots you have helped sort.