XJ750X Motor Rebuild

Can someone school me on building an XJ750X motor. I've put together 2 strokes but this would be the first 4 cyl, 4 stroke. I've got the bottom 2 crankcases split/stripped and ready for paint. What do I need for proper bearings etc..... BTW I got the crank and 2 cases as parts as mine through a rod through the cases hense why I don't have matched bearings etc. Also I plan on replacing all wear items. So no reused bearings or seals... I've got the service manual but it's not overly detailed.

 

You must love that engine, grind crankshaft, new shells, new wrist pins, new rings, with hone or new pistons with rebore, the gasket set alone will make your eyes water.

 

The head gasket alone for my non YICS 650 is $90! I can get an entire Fel-Pro gasket set for a small block American V-8 for that.

Rebuilding the entire motor is going to cost well into 4 figures.

 

That's why I finally decided to get an ebay engine for my air cooled 700.

 

Yep I'm aware of the cost. Looking for the how to.

 

A manual might be a good start

 

Just noticed, an X model?! Wow, you jumped right into the deep end of the pool!

 

Plastigauge for measuring the shells. You'll need to join the crank cases together with with the crank and shells installed and a piece of Plastigauge between the shell and the crank. Everything will then need torquing down to the correct spec. Take the cases apart, remove the crank and retrieve the Plastigauge, compare the pieces to the chart in the pack, they will tell you what clearance you have between the shell and the crank, from there you can work out which size shell will give the correct clearance.

I don't know if you can order shells in different sizes. Someone like Chacal or NW Vintage Cycle, or Yambits in the UK parts may be able to help you there.

Depending on how far into this you want to go, you could in theory order shells to ensure you have the same clearances across each set of shells. You'd then be on your way to having a blueprinted motor.

Of course you'll also need to do the same for each con-rod big end. You'll need new wrist pins for each small end. No point using mismatched pins and rods. New pistons will need new rings and the bore for each will need to be honed.

In the 650 engines, most of the bearings are of the fairly big ball variety. I can't remember ever seeing a needle type. I'd imagine the 750 and 750X are fairly similar, so shouldn't pose a problem. As for seals, there's one at each end of the crank, and I think one behind the alternator. With the X being water cooled there could well be more. The manual will have more information.

Top end rebuilds are fairly simple, the X has a 20 valve head. Install new valve guides whilst you're in there, and lap the valves in. I don't know what type of mechanism it has for opening and closing the valves. the air-cooled bikes are shim over bucket. Working out the shims is a breeze with the head off the bike as you can turn the cams independently of each other. If you do decide to sort out the shims whilst the head is off the rest of the engine, do leave the cam chain on. You'll have to take the cams out or split the chain to get it back on and there's no point splitting the chain if you have the entire engine in bits. In fact, you may as well install a new cam chain whilst it's all apart along with new guides if you really want to replace all wear items.

The vast majority of workshop manual don't go into huge amounts of detail as they assume the reader is a trained and qualified technician. For a hand held guided approach you want either a Haynes manual or a Clymer.

That should get you started. If you need more than that I'd look up blueprinting, even if you don't get quite so anal about the clearances the basic process is the same.

Take your time, be patient, there's a fair bit of repetition involved. A light tap from a soft faced mallet can make joining cases easier, they don't need huge amounts of force. If they aren't going together then something else has gone wrong. During the final build, use liberal quantities of assembly lube and check that moving parts do actually move. If they suddenly stop being able to move you need to need to back track and find out why.

Personally I'd buy a whole engine, however from an academic point of view I get why you would want to as there's a lot to learn and a lot satisfaction from an engine that runs well. For the effort I don't know what gains you'll get with blueprinting these engines, they don't rev that high and they're quite low stressed compared to more modern engines, so if you do go ahead with the build, just get it in spec and save yourself the hassle of messing around with plastigauge.

 

Thanks man. I think I've got the crank issue partly sorted. Looks like mine originally came with a yellow, Blue, 2 Brown and a green shell for the main crank bearings. Crank looks good so I hope a quick clean and a polish keeps that in shape as I can't see any scoring or wear on the crank surface. I'd reuse the original bearings as they don't look worn but they have been kicking around loose for a couple years and a couple have marks in them from jamming other parts against them etc. Rather just go new and be safe. Doesn't look like Yamaha sells them anymore but maybe Chacal can get some. I'm assuming if I get the same color code and then plastigauge them we'd be on track as long as the clearance is in the tolerance level. Just trying to focus on the bottom end will worry about the top later.

 

Best fit a new starter clutch & guides.