Pistons

Piston failure is something which afflicted the earlier Twins. They invariably had high compression ratios and people just did not believe that they needed high octane fuel and the proper spark plug fitment. Today, the problem is less acute because compression ratios have been lowered, fuel has improved (although its octane is lower) and spark plugs, generally, have a wider heat range. It does bear repeating, however, that the correct spark plugs are essential and that ignition timing is spot on.

Pistons are easily removed with the barrel off the crankcase. Make sure rag is placed in the crankcase opening so that nothing falls in. With a pair of thin pliers remove both circlips, one at each end uf the gudgeon pin, from each piston. The gudgeon pin should 'push' out sideways quite easily. If it is stiff, then wrap the piston in rag soaked in very hot water That should heat the alloy of the piston enough to expand it and allow the gudgeon pin to escape.

Once the piston is removed, replace the gudgeon pin in it, so that they are not muddled. (Gudgeon pin circlips should never be re-used. New ones are essential, even if the piston is not being replaced). Pistons can be cleaned of carbon with an old feeler blade. Work carefully. The pistons must not be scratched, nor should there be 'blow by' marks on it. The piston should be measured at the lower end of its skirt, some 90° from the gudgeon pin. Check the specifications for allowable wear.

Check the piston ring thickness in its groove in the piston. Place a feeler blade at several points around the ring. Check the Specifications for the permitted clearance.

Pull off the piston rings very carefully. Start with the top one and peel it off with a spare feeler blade. Rings break as you look at them, if you are not careful. Now check the ring's end gap by placing each one into its respective bore without its piston (and this includes new ones). Use the lowest, used part of the bore and 'square' up the ring in the bore with the bottom of a piston. The Specifications give the permitted gap. File it carefully with a Swiss file if too thin.

Various types of piston are available for each model in question. Remember, however, that the best pistons are usually the ones with which your machine was originally equipped. Remember also that if you change piston type you must consider other changes too. That may mean a camshaft change as well as carburettor and exhaust changes.

At present, for the Twin there are SF, GTL and SFC pistons available. For the 1000 there are

3CL and Jota pistons, but for the 1200, there is only one type (the Mirage does not have high compression pistons).

Piston refitment is quite straightforward if you are careful. First refit the piston rings. Double check the end gap as you refit each ring. Start with the bottom one, which is an oil control ring. It has a spring underneath/inside it which must be there. This ring (and the top ring) can be fitted either way up. Check this. The second or middle ring is a compression ring. This has a sharp nick cut into its lower edge and this must face downwards. The top ring is also a compression ring. Use a little clean oil when fitting the rings.

With the rings fitted, place the piston over its connecting rod small end and refit the gudgeon pin (Check the small end bush wear before reassembly — see the appropriate sub-section). Each piston has an arrow inscribed on its crown. This must face towards the front of the enigine. As a double check, the piston crown's larger valve cutaway (inlet) must face the rear of the engine!

Oil the gudgeon pin and put it in — DON'T force it. Use the same technique of warming the piston with hot rag to help. If you are having trouble then you can remove the piston rings and place the piston in boiling water and then fit it quickly. You'll have to replace the rings afterwards and that is sometimes a little fiddly. Once the gudgeon pin is there, fit two NEW circlips. The circlip gaps should face down towards the crankcase.

Clean the pistons and rings and then oil them generously. Place the ring gaps at 120° to each other on each piston.

Check everything one more time before refitting the barrel. ALWAYS use piston ring compressors.

The pistons on the left are those used in the 'OOC Triple - the top one is the standard 3CL, the c*rer one being the high compression Jota pistor On :~e right are 750 pistons - the top one is a starzsr? SF, the lower one being a high compression SFC

Note the arrow on the pistons facing forward on all models

A theoretical piston and ring cross-section for both models

A theoretical piston and ring cross-section for both models

Use piston ring compressors when lowering the barrel. The cam chain ends are now wired

Note the arrow on the pistons facing forward on all models

Use piston ring compressors when lowering the barrel. The cam chain ends are now wired

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