4F JP piston fitting

[camstevens]

Hi All
Before committing to machining I thought it might be prudent to check people's experience on this;

Drags state;
- 0.0055" clearance (I believe front and rear), and
- "relieve an area the size of a thumb print by .004" just below the bottom ring land on the front and rear faces"

I was wondering if the 0.0055" was conservative clearance, particularly for front pistons, and curious as to the 0.004" relief, front and rear. Is this to account for piston canting over, given the clearance?

Also, the original manual gives a oil control ring clearance of 0.018 - 0.020". This seems a large gap for a piston of this diameter, and may be excessive for more modern materials and oils?

Regards
Campbell

[Roger Gwynn]

The dims given were arrived by experiments carried out by Otto on his cammy who later travelled with me to the annual rally in the Isle of Man via Poland, The Baltics, Finland, Sweden, Denmark, Germany The Netherlands and the UK before returning home to Wurzburg in Southern Germany. The only problem experienced was a broken cush drive sliding member that held us up for half a day.
The clearance is between the piston and the cylinder and is the total, not front and rear as this would be .011", too much even for a cammy. A large piston ring gap has little or no effect on an engine, it is a minimum specified to ensure that the gap does not close up. That does sound a lot though, I doubt if modern rings expand less than old ones, pistons certainly don't.

[camstevens]

The dims given were arrived by experiments carried out by Otto on his cammy who later travelled with me to the annual rally in the Isle of Man via Poland, The Baltics, Finland, Sweden, Denmark, Germany The Netherlands and the UK before returning home to Wurzburg in Southern Germany. The only problem experienced was a broken cush drive sliding member that held us up for half a day.
The clearance is between the piston and the cylinder and is the total, not front and rear as this would be .011", too much even for a cammy. A large piston ring gap has little or no effect on an engine, it is a minimum specified to ensure that the gap does not close up. That does sound a lot though, I doubt if modern rings expand less than old ones, pistons certainly don't.

Hi Roger
When I said "front an rear", I meant front and rear cylinders (1&4, 2&3).

[Roger Gwynn]

Ah! The 4G tends to suffer from the SQ4itus of seizing the rear cylinders, the cammy and MKI used to suffer head gasket failures. You should be OK with the same clearance all the way round.