Yamaha TX500 - Piston & Cylinder Inspection
I (Graham) am not sure I have ever seen the XS500 TX500 engine in the bike but of late it has been mounted in an engine stand and Amantis has been hard at work restoring the engine. He has removed all 8 valves, cleaned them up and checked them as well as giving the cylinder head a thorough internal clean before reassembling it. When I first took a keen interest in the engine the cylinder barrels had been removed. On closer examination I found that both piston rings (The compression rings not the oil scraper) on the right hand piston were stuck soundly in their grooves. With some careful use of a thin screwdriver blade (A poor alternative to thin feeler gauge blade) and a lot of patience I was eventually able to ease them off the piston. It was only then that the realisation dawned that a piece of the piston’s skirt was missing. Now you can’t just sew on a piece of material as if it were your wife’s skirt, it means that a new piston is required. Although the piston rings themselves look OK and are nice and flat, it might as well have a new piston kit comprised of piston, piston rings, gudgeon pin and gudgeon pin circlips (NEVER EVER REUSE A GUDGEON PIN CIRCLIP – DISASTER WOULD FOLLOW WITHOUT ANY SHADOW OF A DOUBT). Luckily some piston kits still seem to be available. However before ordering one, we needed to establish the condition and size of the cylinder barrels. To the naked eye the surface of the cylinder barrels looked good, with no obvious scoring or damage but the cylinders could have been rebored or worn in ways that could not be seen. So with the aid of a cylinder bore gauge I measured each barrel carefully. If you ever get the job of measuring a cylinder barrel for wear remember that the places that will wear most are the front and back surfaces of the barrel relative the connected rod that connects the piston to the crank shaft. This is because there is a tendency for the piston to be slung back and forth especially as it changes a direction at a zillion miles an hour. To give you a rough idea a piston travelling up and down just 3 inches, can reach a maximum speed of 4710ft per minute or 53.52 miles per hour. (According to https://www.musclecardiy.com/performance/understanding-piston-speed-in-high-performance-engines/ ) The maths is pretty complex, so I thought it better that they work it out for me. Another astounding statistic is the number of times that the piston changes direction. Now this maths is within my capabilities because it has to change direction twice for every revolution. Therefore at the heady heights of say 7000rpm the piston changes direction 14,000 times per minute. Simple division by 60 shows us that the piston changes direction a staggering 233 times per second hence its propensity to wear the front and back of the barrel!!! Even at a lowly 600rpm the piston has to change direction twice every second.
When I used a cylinder bore gauge to measure our XS500 TX500 is was extremely pleasing to find that not only were the cylinders the standard size that they came from the factory in, but there was no discernible wear on either barrel! Dimensions wise they were perfect.
When Amantis removed the sump (Basically a piece that forms the bottom of the engine) he found a lot of thick black sludge sitting there. This suggested that there was merit in splitting the two halves of the engine apart to inspect and clean its internal components. This couldn’t be accomplished at the last workshop session, because the alternator rotor could not be removed.
However at this week’s workshop session (11/12/2021) a rotor puller was available. Initially the rotor stuck firm to the nose of the crankshaft. Irfon assisted with a quick blast of heat from the induction heating tool. With a few more turns of the rotor puller and a judicious tap with a mallet, off popped the rotor revealing a number of bolts securing the bracket for the counter-balancer chain. Once Amantis had removed the bolts and bracket it was time to split this horizontally split engine. Having double checked that all the nuts and bolts that secured the two halves of the engine had been removed, it was time for (Yes you’ve guessed it) some more judicious taps with the soft mallets. Soon light began to appear between the mating surfaces of the crankcases and finally they parted company. Our decision to separate them was vindicated by the presence of yet more sludge like material within the engine. Upon inspection the internals of the gearbox and the crankshaft main bearings appear to be in fine and serviceable condition. Finally Amantis cleaned all the components. Because this left the components dry and free from any protection against rust, Graham gave them a precautionary soaking in GT85 as it would be a few weeks before the next workshop session following Christmas. Oh and the missing piece of piston was found in one piece sitting in the bottom of the crank cases. Smithy looked on with great interest hopefully learning a few pointers on engine rebuilds at the same time.