Key Points During the Upper Part OH of the Engine | Assembly Ideas without Damaging the Cylinder Base Gasket.

Whether it is a 2-stroke or 4-stroke engine, the maintenance of removing the cylinders from the engine is highly difficult and requires knowledge and experience. Great care must be taken not to damage the piston rings during assembly, and at the same time, in the case of a 4-cylinder engine, care must be taken not to damage the cylinder base gasket. One idea is to set the gasket in the crankcase with the cylinder side attached.

Every mechanic has a different way of inserting pistons in a 4-cylinder engine.

Installing the piston pin clip is the most stressful part of the installation process. If it slips out of the clip groove and falls into the crankcase, it would be impossible to see it, so a paper rag must be pressed into the hole in the crankcase. I recommend using a cloth rag because there are more gaps than you think even if you push the paper rag in tightly.

Some manufacturers and tool stores sell piston bases as products, but any material can be used as long as the pistons are level. Some mechanics find it easier to insert the 2 cylinders first with pistons 1, 4, 2, and 3 at the top dead center.

When replacing piston rings due to low compression pressure or increased blow-by gas, it is common to remove the cylinder head and cylinder from the crankcase of the motorcycle engine. The 3-story structure of the cylinder head, cylinder, and crankcase should be a common sight for riders who have been interested in tinkering with engines since they started riding 2-stroke mopeds and Honda horizontal 4-strokes. On the other hand, most automobile engines have the crankcase and cylinder as a single unit, and the cylinder head is placed on top of it.

The biggest difference that you will face when assembling both types of engines is that most motorcycles "put the cylinder over the piston", while automobile engines often "insert the piston into the cylinder". In automobile engines, where the cylinder and crankcase are one piece, the pistons and connecting rods are inserted into the cylinder one by one as a set, whether from the bottom of the crankcase or the top of the cylinder. At this time, the connecting rod is not yet assembled to the crankshaft, so it is not very difficult to insert the piston into the cylinder.

In contrast, motorcycle engines are generally assembled so that the cylinder is covered by connecting rods and pistons that have been assembled on the crankshaft beforehand. The piston then swivels around the piston pin, which increases the difficulty of inserting it straight into the cylinder without tilting it.

In a single-cylinder engine, the cylinder can be tilted according to the tilt of the piston, taking into consideration the interference with the stud bolt, but this is not the case with a 4-cylinder engine. However, this is not the case with a 4-cylinder engine. There are some special cases such as the 270° crossplane crank of the Yamaha YZF-R1, but usually the number 1 and 4 pistons are set 180° out of phase with the number 2 and 3 pistons, and multiple pistons must be placed in the cylinder at the same time.

If the piston enters the cylinder in a tilted position, there is a risk that the piston rings will be damaged at the end of the cylinder sleeve, not to mention that the 2 will interfere and get stuck. Piston ring compressors are available to prevent this, but if the cylinder is covered with the piston with the compressor installed while it is pivoting, there is still a risk of damaging the rings.

In the case of the Kawasaki Z1/Z2, there used to be a special manufacturer's tool, the piston ring compressor, which could hold 4 pistons at the same time. This was to align the 4 pistons at the same height, shrink the piston rings, and then cover the cylinder.

The difficulty of inserting the pistons in 4 cylinders at the same time has led to a pattern of assembling one or 4 pistons and 2 or 3 pistons as a set, and it is indeed a manufacturer's idea to prepare a special tool to prevent the 4 pistons from tilting. However, since the bore and cylinder pitch varies from model to model, there seems to be no general-purpose tool that uses this method.

Temporarily fix the piston base to the cylinder side to avoid damaging the base gasket.

The piston base is subjected to the weight of the piston, and also to the possibility of being pried out of position when the cylinder is inserted. One way to eliminate such concerns is to temporarily fix the cylinder base gasket to the cylinder side. If a liquid gasket is used, it may stick and be difficult to remove when the cylinder is removed, but with rubber bands, there is no need to worry about this.

Even with the piston base, the angle of the individual pistons may not be perfectly aligned. In such a case, the cylinder is inserted halfway and adjustment is made. At this time, there is no gasket on the crankcase side, so you can concentrate only on inserting the piston into the cylinder.

Insert the piston rings, also using the tapered skirt at the lower end of the cylinder sleeve. This method often works well for the top and second cylinder, but if the oil ring is a 3-part type, be careful not to bite the thin side rails. If the side rails are clearly stuck against the sleeve, the cylinder will not fit, but if the side rails are partially protruding, a little harder push may be enough to bend the rings and get them in. Of course, if you start the engine in this condition, the cylinder will be scratched, so it is important to insert it carefully. Once the cylinder is in place, turn the crankshaft to make sure the piston strokes smoothly.

Remove the piston base and cut the rubber bands before the cylinder is completely lowered. The rubber bands can be easily cut off and collected by pressing a screwdriver handle against the top edge of the cylinder.

Use an old piston pin or something similar as a collar to press the cylinder against the crankcase to prevent it from lifting when the crankshaft is turned. If it does not turn smoothly and you feel something dragging, do not hesitate to remove the cylinder and check for ring damage.

In order to insert all 4 pistons into the cylinder at the same time, it is very important to support the pistons so that they do not shake their heads. To do this, a piston base set on the crankcase is a must.

It doesn't matter if it's a special one or an off-the-shelf one, but if the piston base can hold the piston so that it can be inserted straight into the cylinder, the assembly process will be much easier. Of course, it is essential to compress the piston rings, but since the lower end of the stock cylinder sleeve is often skirted so that it is wider, if you compress the rings a little when they are close to the rings and insert them into the hem of the skirt, the cylinder will try to go down by its own weight along the stud bolt. In some cases, the cylinder will go in smoothly.

This is not recommended as it is a task that should normally require the use of a piston ring compressor, but some experienced mechanics will dexterously assemble it by compressing the sleeve skirt every time it touches the 4 high piston rings and using the taper of the skirt part.

When using a piston base, it is common practice to set the cylinder base gasket, which fits between the crankcase and cylinder, on the crankcase beforehand. However, this can cause the piston base to push against the gasket and damage it. This is why the technique of temporarily fixing the gasket to the cylinder side was developed.

Using a liquid gasket as an adhesive is one of the most common methods, but what I'm going to introduce here is a method using rubber bands. It's not so much an exaggeration as it is a simple matter of hanging a number of rubber bands on the base gasket placed on the underside of the cylinder to prevent it from falling off. This is the only way to prevent the piston base from damaging the gasket and preventing it from peeling off and hanging down even if the piston ring insertion is a bit tricky. It also prevents the gasket from sticking out of the engine, which often happens when gaskets are used together, and helps improve the quality of the restoration work.

Insert the piston with the gasket suspended by the rubber band, and cut the rubber band when the piston base is removed, and the gasket will fit perfectly into place. Another advantage of using rubber bands is that if you cut the gasket in one place, you can recover it without any debris falling into the crankcase.

Procedures such as not using a piston ring compressor and suspending the gasket with rubber bands are not described in the service manual. However, it is a sufficient means to achieve the objective. If you combine a piston base with a quadruple piston compressor, you should be able to do the job even more reliably and with even less risk. Knowing that such techniques exist, which can be called field wisdom, there may come a time when they can be applied to a variety of tasks.