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Collection, Preservation and Display of Old Lawn Mowers

Compression Pressure

Ransomes Twenty Four

In comparison to my Suffolk 43DL which is a solid turn over the above was relatively easy. I took the precaution of doing some compression checks please see attached photos. The higher pressure was after i squirted some oil in the cylinder before the reading.  I used a a drill to rotate the engine.

What is an acceptable pressure for these engines?  Engine data (acceptable wear limits etc) appears scarce.






Thanks for looking


wristpin Tue, 21/09/2021

I’ve never bothered too much about single pot small engine compression figures and as far as I know neither Suffolk nor Villiers ever issued figures. There’s a lot more to be learnt from a leakdown test particularly on engines with automatic compression release  - most other than SIF and Villiers. 

DJD Tue, 21/09/2021

To get a truer reading you need to open throttle fully, also try same with a hot/cold  engine. If no pressure difference on a low figure to start with, it's often a poorly seating valve, generally the exhaust one.

nospark Fri, 01/10/2021

Rule of thumb is compression ratio X 14.7 (atmospheric pressure in PSI)

In practice gauge reading will be slightly less, depending on volumetric efficiency of the engine, it gives an idea.

DJD Sat, 02/10/2021

Low compression side valve engines such as these will be quite low anyway, also try testing dry against another go with oil down the bore.