Questions to ask on a rebuilt engine are. 1: Is there any warranty?  2: What did the rebuild consist of? 3: Is the seller a trader or a private individual 'fronting' for a trader?

The most popular type of rebuild is an un-touched bottom end, with old bearings and oil pump still in it, fitted with new barrels, pistons and heads. This will last anywhere between one month and a year before the bearings give up and it dies.

The next is any old engine, painted silver.

There are some engines which have had new bearings, rings, exhaust valves and seals and gaskets which, if the price is right, will be good value and last for a long time if cared for. Cared for means: oil changes at correct intervals, correct fitting, correct timing, regular tappet adjustment and sensible carb choices.

Checks.... 1: End float. This is the determiner of wear. Grab the front bottom pulley and pull it towards you, then push it away and listen/feel for the clunk. If you can barely feel movement (should be .15mm) end float is good. If it gives off a healthy CLUNK! it's bad. Such an engine should be very cheap.

2: Cracked heads. shine a torch down the holes with the plugs in them. Uncracked heads will be a dirty grey around the plugs, but cracked heads will have black staining where the gases have been escaping. It'll still run, but it may not last. Again, this should be a very cheap engine.

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