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My 1948 Prefect has a 100E engine. To retain the motor mounts, it has the timing cover from the original engine with no timing marks, of course. How can I find exact TDC on a sidevalve engine so I can add a timing mark?

Not easy as the pistons are not visible through the plug holes.

The 10hp engines' timing was determined by a pin screwed into the timing cover. This was unscrewed and turned around then pushed into the hole and pressed inwards and the engine slowly turned by hand. When the pin moved inwards as it engaged an indentation in the cam sprocket, that was the basic timing position. The 10hp cam sprocket is one of only two parts used in the 100E.

So does my 100E actually have the dimple in the sprocket? I always thought it did not. Having the front cover from my original engine, I still have the pin. If it is that easy then I'm almost home free.

Yes, it should be there. Part Number E93A-6256_A.

Thank you many times. For some reason I feel like I should have known that all along. Anyway, now I do and can quit worrying about it. No mark needed. YEA!

No problem; it's what sites like this are good at - sharing knowledge.