Some past engine modifications.

This is just a sample of some modifications that I have done to improve oil sealing by the application of "O" rings to eliminate various gaskets and the usual application of sealants.  In addition to ensuring good sealing the use of "O" rings simplifies assembly and disassembly.

O-ring around base of the barrel, replaces original paper gasket.

The mouth of the crankcase was counter-bored to accommodate the O-ring shown above.

  

The original pushrod tunnel relied on a precise gap to squeeze a flat polymer gasket by a given amount.  In general the gaskets could not be reused and in use they would harden and often leak.  This shows the barrel to head joint but the barrel to crankcase was sealed similarly. I have never liked this design, my improvements shown below.

Machining a groove in the crankcase around the base of the push-rod tunnel for an O-ring.  I hated the original sealing system which was prone to incorrect assembly.

O-ring fitted to the new groove.

A simple O-ring groove as I used at the bottom of the barrel was not feasible and so I adopted an insert sleeve sealed by O-rings in both the head and barrel.  This necessitated boring a circular recesses in both, shown here for the head.

Showing the recess bored in the barrel and the sleeve inserted into the head.  This system makes assembly fool-proof and results in a leak proof seal.  As a modification to existing components it requires precise setting up before machining the recesses to ensure accurate alignment.

Not exactly an obvious oil sealing modification, but this breather helps produce a slight negative pressure in the crankcase and so would rather let outside air in rather than let inside oil out.  One way valves in breather lines are not uncommon but in most cases they are too small.  I prefer to use reed valves from a 2 stroke engine which are designed for high flow.