The engine part 2: inspection and repair of the top end

So let’s assume that, with the aid of a decent manual, you now have your engine broken down in large lumps and on a clean surface. Examination and measurement are the two tools that will now be employed regardless of whether the engine is two-stroke or four, has one cylinder or three etc.

If something/anything looks like it’s cracked or fractured you’ll be needing a new one; little if anything that’s a part of the engine or transmission can be welded or screwed back together. The possible exception might be an engine case but at this juncture we’d politely suggest that, as first timer, you’ve probably chosen the wrong bike to restore.

Obvious issues that even the novice can spot are piston with the crowns nibbled away, smears running up the piston’s walls, wear marks in the cylinder bore or piston rings that are either broken or trapped in their grooves. Any of this would suggest an engine in need of specialist input. Cylinder bores are round by design but tend to wear unevenly. The majority of wear is normally at the top of the bore and this is the area the greybeards tend to check first. In reality you probably need to take the piston, their rings and the cylinder(s) to a specialist engineering shop or a good bike dealer and get them to measure the amount of wear.

If you’re lucky it may be that the whole lot is still within limits in which case the bore will just need a light honing to remove any glazing and aid oil retention. If the cylinder/rings/ pistons are worn then it’ll be time for a rebore which again is a specialised job. Oversized pistons and rings will be needed and anyone worthy of the title of engineer will probably want the new rings and piston before they rebore your cylinder. If the bore is too worn or is already on maximum oversize then there are still options.

The most obvious is to find a used cylinder that’s not as badly worn as yours. However if one is not readily available then it’s possible to have a new liner manufactured and installed in the original cylinder casting. This can be done with two- and four-stroke motors and, surprisingly, it’s not as expensive as you might think.

Sitting on top of the cylinder is the cylinder head and regardless of engine type the surface that mates with the top of the cylinder block must be perfectly flush and true. In reality if a cylinder head has been leaking due to warpage its effects are normally glaringly obvious. On an air-cooled motor you have seen a lot of oil leaking down from the head to the engine cases. With a water-cooled engine the coolant may leak into the oil or vice versa. Neither is good news but again your friendly motor engineers should be able to check this out for you. Truth be told warped heads are a rarity but it’s worth checking just in case.

With two-stroke heads there’s not much more to worry about but on four-stroke motors you’ll have cam shafts and valves. With a full rebuild it’ll all need to come apart but to get to this stage you will have already removed the cams. Hopefully you will have noted where each cam cap sits! The cam(s) should be inspected for general wear to their reciprocating surfaces and in particular for pitting. This occurs when the surface hardening of the cam wears away revealing softer metal beneath. It’s possible to get cams resurfaced and reground but often a decent used cam will be cheaper. Most Japanese cams run directly on the alloy of the cylinder head with no other bearing materials used. If oil changes have been neglected or the motor has been starved of oil then these semi-circular areas can become badly scored. Renovation is possible here but it’s not cheap and once again a good used example may be a better and more cost effective option.

It makes sense to remove the valves, remove any build-up of oil or carbon and check for damage such as burning areas, crack or even holes. In almost every case the valves will just need a light clean up and light grinding in. Always replace the valve in the position it came out of and, while you’re about it, check for wear of the valve guides… the manual will tell you how. Finally have a good look at the cam chain adjuster for wear and the chain itself which may just possibly be beyond its service limit.

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