Spare engine queries

[GavinL]

I bought the engine below a few years ago with the intention of stripping and rebuilding as a spare. It turns easily and having taken the head off it appears there is very little wear - no discernible lip on the bores, standard bore dimensions, valve seats ok (photo isn't great), and little wear on the rocker shaft.

A couple of questions:

It's a low compression engine - dished pistons. Can this be rebuilt as a high compression by simply changing the pistons? Is this worth doing?

The pistons and block have 1 or 2 stamped on the pistons and block - what do these mean?

20260308_121818.jpg (2.28 MiB) Viewed 36307 times

20260308_121835.jpg (1.81 MiB) Viewed 36307 times

20260308_122736.jpg (1.97 MiB) Viewed 36307 times

20260308_130101_001.jpg (335.08 KiB) Viewed 36307 times

20260308_130327.jpg (2.29 MiB) Viewed 36307 times

[svenedin]

Yes you're correct it is a low compression engine. Not only from the original pistons fitted but the engine number with "L" for low compression.

The markings on the pistons and block, 1 or 2 relate to when the engine was assembled in the factory. Due to slight variances in machining the bores were measured and stamped with 1 or 2. Then at the work station where the pistons were selected and fitted, a piston was fitted that matched the number marked for each bore.

Low compression pistons can be swapped for high compression pistons assuming the cylinder head is original and has not been modified by being skimmed. To produce the low and high compression variants of the engine at the factory it was cheaper to use different pistons than to have different cylinder heads.

The low compression variant has less power but exactly how much less I am not sure. 5 hp is quoted but not sure of accuracy. These low compression engines were designed to use cheap, low octane fuel which is no longer sold at petrol stations. They are also less efficient and use more fuel than the high compression variant (they do not get so much power out of the same quantity of fuel at each stroke). The only possible "advantage" nowadays is they are easier to turn over on the starting handle!

Stephen