Replacing Sunbeam Cylinder Liners

I have had quite a few Sunbeam twins through my workshop, which for various reasons have required new cylinder liners. The liners are normally a light interference fit into the engine block, so pressing the old ones out and pushing the new ones in are not daunting tasks, even for the amateur mechanic, provided that you spend a bit of time thinking it through. The use of a close fitting dolly and a hydraulic press is essential, however.

An interference fit means that heat must be used to expand the engine block before the old liners are pressed out or the new ones pressed in. Attempting to do it cold will result in damage to the inside of the cylinder block causing the new liners to be a sloppy fit, which may later cause them to fail. The same principle applies to the replacement of valve guides and bearing bushes.

It is generally recognised that +0.040” is the largest permissible over-size for Sunbeam liners. Consequently, the largest piston that is supplied by Stewart Engineering is +0.040” over-size and I have seen a number of S7/S8 engines that have been bored out to that extent. Some of those have been cracked and I believe it may be because they had grown too thin and too weak to withstand the continual flexing that they have to endure. The thinner they are, the more they will flex. They are made from cast iron because of its excellent wear characteristics but continual flexing is not what cast iron does best.

Steel dolly to press out liners

Both liners were cracked

A few months ago, a customer brought a bike to me because it had been burning a lot of oil and he could not find an obvious cause. He expected to find excessive wear in the bores or in the valve guides but neither was evident. Subsequent investigation revealed that vertical cracks extended from top to bottom of the thrust face of both liners. The cracks were not easy to see when the liners were in place but they were obvious once they had been pressed out. For that reason, I no longer recommend to my customers that Sunbeam liners should be bored beyond +0.030”.

Being worn beyond the point where they can be re-bored is not the only reason for fitting new liners. I have seen several liners that have a horizontal crack just under the lip at the top. This can happen for two reasons: either the liner is a loose fit in the engine block and has been flexing in the bores or the front of the engine block has become distorted because of strain on the front engine mount. This can happen if the brace plate, which should be fitted under the front engine mount, has been omitted. The brace plate is not an optional component but it is common to find it missing.

Fitting new cylinder liners is a straightforward job but there are a couple of things that need to be borne on mind. An obvious one is that the two liners are different in one respect. The notch for the locating pin is in a different place. New liners are supplied by Stewart's without the notch so one has to be machined into the rim. The position of the notch is important since it determines the alignment of the recesses that are designed to prevent collision between the valves and the liner. The notch must also align perfectly with the locating pin and clear marking of the liner can facilitate that. However, it is a bad idea to mark the liner with a scriber. The material that the liners are made from is brittle and a scribed line is the perfect seed from which a crack may later develop. A felt tipped pen is a safer option.

My first job after graduation was with Hawker-Siddeley Aviation, sadly long gone. One of their favourite training films was a slow-motion view of a Harrier engaged in an engine test on the ground. Although nominally stationary, the whole aircraft was in motion. The airframe twisted, the wings flexed and the tail waggled from side to side. The purpose of the film was to dispel the notion that there is such a thing as a completely rigid structure. Every object will flex when subjected to force. If a strong object is subjected to a weak force then it will flex only a little but it will still flex.

Cylinder liners are no exception to this. If you place a bore gauge into a cylinder liner and squeeze the liner between forefinger and thumb it will become oval. Even gentle pressure can change the shape by more than a thou. An error of one thou can be sufficient to cause a seizure in a new bore.

When one component is pressed into another with an interference fit, both components will flex. Generally, the outer component will become larger and the inner component will become smaller. That is why valve guides and bronze bushes, that are held in place by interference, must be reamed after they are fitted, to achieve the correct clearance. The same is true of cylinder liners. They always shrink when they are fitted. Furthermore, because the liners are not a perfectly cylindrical tube (they have a lip at the top) and because the material of the engine block is of varying thickness and strength, the shrinkage will vary over the length of the liner with varying degrees of ovality. One new liner that I recently fitted varied in diameter by up to 0.003” in places. That is enough to cause serious problems, not least because the bore is unlikely to be very straight.

Valve recesses are not symmetrical

Dial bore gauge

Conveniently, we can buy new cylinder liners from Stewart Engineering, who supplies them pre-finished to the “standard” bore size. This is very unusual. Most suppliers of cylinder liners supply them several thou under-sized, so that they can be bored to fit your new standard pistons.

In fact, cylinders should always be bored to fit the pistons that you intend to use. The reason is that pistons of the same nominal size, supplied from different manufacturers, can vary significantly in actual diameter. Even pistons from the same source can vary. On two occasions, I have received “matched” pairs of new pistons that were more than a thou different in actual size. Nothing can be taken for granted when buying new pistons. The notion that there is a reliable “standard” bore size is flawed.

I haven't yet mentioned the subject of clearance. Everybody knows that insufficient clearance can cause tightening or even seizure in a newly rebuilt engine but excessive clearance can also be problematic. It isn't just a case of an annoying rattle when the engine is cold. It can result in rapid wear of the bores and even cracking of a piston skirt. Most manufacturers specify the recommended clearance for their pistons. It is usually in the region of 3 to 4 thou, depending on the composition of the alloy and the structure of the piston. It is always a good idea to follow the maker's recommendation if the bike is intended for “normal” riding.

So, to get back to the liners, what can you do with a newly fitted cylinder liner that is nearly but not quite the correct diameter for your new pistons? Owing to the way that cylinder boring machines normally self-centre, it is impossible to bore a liner that has a very small but varying error in its diameter. In many cases, it may be possible to rectify the variations in the bore diameter with a hand-held hone. But that is a skilled, manually intensive and time-consuming job. If the only liners that are available have been pre-finished to the “standard” bore then, unfortunately, the most practical and reliable solution is to use first over-size pistons and bore the liners to suit.