Big Bore Kits

840cc kit for a 447 engine with 9.5:1 compression. 750cc kit for a 447 engine with 10:1 compression.

.020" OS, 840cc, 9.5:1 comp for 533 type motors. These would suit those who have Halco (or even 650-XSories) kits which are a few years old and need freshening up.

As above but for 447 motors. These would suit those who have a Halco (or 650-XSories) engine that needs freshening up and who has a 447 crank that's in better shape than their 533.

840cc for 533 motors, 12:1 comp. These would suit sidecar motocross, grasstrack, drag racers, etc.

840cc for 447 motors, 12:1 comp. These would suit sidecar motocross, grasstrack, drag racers, etc.

840cc for 447 motors, 9.5:1 comp. These would suit street bikes with 447 crank.

These are all purpose made, forged pistons that come with pins, clip, and rings. They are not Yamaha TT-500 or Suzuki SP-370 cast pistons that Halco (God rest his soul) used with half the crowns machined away. Normal price is AU$230ea, now AU$180ea.

For those who want to go from 650 or 750 to 840cc, I also have sleeves. Normal price AU$250, now AU$200. I also have copper head and base gaskets, normal price AU$80, now AU$45.

There's an old saying, "All other things being equal, a big motor will always beat a little motor". That's true, and it applies just as much to XS-650s as to anything else. It's possible to take an XS out to over 1,000cc but, before doing anything, you should know that there are three different versions of the 650 motor. The major differences lie in the flywheels, crankpins, rods, and pistons.

Motors with a number prefix of S650 (XS-1 to '73 TX) have connecting-rods marked 256. These are 136mm long (C to C) and take a 22mm gudgeon-pin. All others sold in Australia (TX-A and later) have rods marked 447. They're 130mm long and take a 20mm pin. Not only is the pin 2mm smaller than the earlier type, but it's also 6mm lower down on the piston to compensate for the shorter rod. They each have the same big-end with solid crankpins and oil-slots in the rods. That is, the big-end bearings are lubricated from the outside by orifices in the front wall of the crankcase. These were the only two variants that came to Australia. The flywheels and crankpins are common to both so either rod, and it's piston, will go onto either crank. It should be noted that production of 256 rods ceased several years ago, and stocks have run out. Therefore, piston makers have stopped producing pistons for the 256. If you have a 256, it should be rebuilt into a 447.

The third type, with rods marked 533, came in bikes delivered to the UK and Europe starting from the time that the rest of the world got the 447s. The 533 rods are 140mm long and take a 20mm pin. The pistons in these motors have their pin-hole 4mm higher than the 256s, and 10mm higher than the 447s. The big-ends of these rods are indeed bigger than the other two and the crankpins are hollow, with their own oil-scoops while the rods have none. That is, the big-end bearings are lubed from the inside by oil that has just passed through the mains. The flywheels are also unique to these motors. The crankpin-hole is bigger, and each 'wheel is 1mm thinner through the crankpin hole than the 256/447s to accommodate the 533 rod with it's 2mm wider big-end. At least some of these motors had the prefix of 3L.

Thus, from 1970-73, Yamaha made the 256 only, and from '74 - '81 they made the 447 and the 533. That's two versions of the motor at the same time. In 1982 - '83 they deleted the 533, but continued with the 447 and these bikes were sold in North America alone. There are many times more 447s in the world than the other two combined. Because of the difference in big-ends, the 533 rods (and their pistons) don't interchange with any other. These rods, pistons, crankpins and flywheels must be used as a set. However, all the main-bearings are the same so any crank will go into any crankcase. Thank God for that!

Don't forget also, that the S650's cranks have a 17 tooth sprocket that takes a 102-link timing chain. All others have 18 teeth and a 106-link chain. Needless to say there are also two cams, one with 34 teeth and the other with 36. Obviously, sprockets (removable from the crank and cam) and chains must be used as a set. As an aside, there are also two different chain-tensioners. The S650s have a sprocket-type tensioner which, because of the teeth's pitch, can only be used with the 17/34. Later models have a slipper-type tensioner that can be used with either combo. Either tensioner will fit any cylinder-block.

No-one seems to really know why Yamaha did this, except to confuse us. The point is that when buying pistons, rods, big-end pins and bearings, and timing chains - either stock or otherwise - you must know which motor you have. If the bike looks like it's never been apart, checking the engine number will probably do. If you suspect that it has been apart, it's best to pull the top-end off for a look-see as a previous owner may have put different internals in there. That's OK, but it'd help to know about it before buying parts that should be right, only to find out that they don't fit. It happens occasionally. If in doubt, check yours before spending any money.

Anyway, back to big-bore kits. The first is to simply install oversized stock pistons. Hardly a big-bore kit but if the top end is worn, bringing it back to new would be a performance improvement. Call your local Yamaha dealer and price pistons, rings, and gaskets. As you read on, subtract this figure from the price of the big-bore kits to arrive at the true cost of a steroid conversion. While we're talking about the true cost of a high-performance part, don't forget to compare carb diaphragms to slide carbs. If your diaphragms are holed, that brings the price of a Mikuni round-slide down to a very reasonable level. If you really want to scare yourself, add up the cost of points, a condenser or two, and an advance mechanism and compare that to a Boyer electronic unit. OK, now that we have a starting point - a stock rebuild - let's continue on from there.

First is a 750cc kit featuring Wiseco forged pistons. They come in 80 and 81mm for the 447 rods only and have a compression ratio of 10:1, which is about as high as you'd want to go with a street motor on unleaded fuel. These are available as a stock item and we always try to have at least 2 on the shelf at all times. These are very common in the bikes raced on the dirt tracks of the USA. Buy the 80s now and the 81s when reboring time comes around. 750cc kits require larger sleeves to be installed into the cylinder block and this is the only machining required. The crankcase accepts them with no modification and the sleeves have plenty of meat for a rebore. Fitting a 750cc kit requires the removal of the top-end only. Unless the bottom-end is to come apart for other reasons, it need not be touched. Same goes for the head. Although it has to come off, if it's in good condition it can be left alone. Generally speaking, unless the bike is for racing or very high performance street use, the valves, cam, and carbs are quite adequate for 750cc.

If 750cc isn't big enough, the next step up is 840cc but there is additional work needed to fit an 840 kit. The crankcase needs to be slightly opened up which means it must be split. For this operation, the studs must be removed but that is something that should be done to all motors anyway. It allows the top end to be removed while the motor is still in the frame. The upper case is then sent for machining. The pistons are made by Ross, have a diameter of 85mm, come in 9.5:1 for the street and 12:1 for racing, fit the 533 and 447 motors, have moly top rings and ductile iron 2nd rings, and valve reliefs that are 2mm oversize allowing for bigger valves without further flycutting. Unlike the Wiscoes, these pistons were custom designed by 650-XSories and feature a crown that’s a perfect fit into the 650’s head. These pistons could survive in a nitromethane engine.

The other method of raising the capacity is to increase the stroke. This will necessitate a plate under the cylinder-block, thereby increasing the motor's overall height. Fitting the motor into the frame becomes more difficult, you'll need to make a new top-mount, and your carbs will no longer fit into the side-panels very well. Also, stroking (or even de-stroking) must only be done in increments equal to the pitch of the timing-chain. Stroking involves machining the crankpin-hole in the flywheels further out from centre and is quite expensive. The plus to this is that it gives more cubes, the minus is that it alters the rod/stroke ratio for the worse, increases rod-angularity and decreases piston-dwell. All of these conditions are undesirable, but acceptable (up to a point) in the search for more cubes. Depending on the bore, stroking can raise the capacity to around 1,020cc. This is the absolute limit. It's major surgery and not something to be taken lightly. If you want more than 750cc, go for an 840. Some engines lend themselves to stroking but not boring but the 650 lends itself to boring rather than stroking. Only if you need more than 840cc would you ever consider stroking.

Do you need to rebuild the crank and replace the rods? That depends on their condition. Remember though, that we're talking about a bike that's at least 20 years old. Unless the crank has been rebuilt in the last few years, or the bike has incredibly low mileage for it's age, it may well need to be done. Many owners baulk at the cost of having the crank rebuilt and though it is expensive, once done, it's done. Unless you severely abuse the bike, it should last as many years as it has already. It's a job you'll only do once. Unless you're sure that it's OK, have it inspected before doing anything else.

How much of this do you really need? To sum up, a 750cc kit alone will give a marked improvement in performance. Next are pipes and air filters. Beyond that, you may want to consider a cam to suit, or an 840cc kit with a cam. Beyond that, you can go to stainless valves, slide carbs, lightened rockers, or stroking for really big cubes. In all cases however, we recommend a Boyer ignition. There's not much point trying to improve the performance of a motor if it's incapable of firing correctly, which it is. You won't believe how bad the stock ignition system is until you install a Boyer. They're not cheap, but worth every cent. As mentioned earlier, if you were going to rebuild the top end anyway, don't forget to subtract the cost of stock parts from the big-bore prices.

650-XSories can also supply R & D valve-spring kits, Barnett clutch kits, and stainless valves. 650-XSories requires a 50% deposit with all special orders, with the balance payable COD. Parts in stock will simply be sent COD. Prices can vary according to the exchange rate of the A$. That's life.