Installing a top end kit on your Yamaha QT50

This article won’t be a step-by-step how to but will give you the basics on how to install a new and larger top end on your Yamaha qt50 (also infamously known as the big bore kit).


Video on replacing left crank shaft bearing seal

If you don’t know the maintenance history of your qt50, you will want to change the left and right crankshaft seals before doing anything related to your top end. You will need a piston stop and a flywheel puller.

Those seals are 35-40 years old and if they are not sealing correctly, they will allow air to leak past and into the engine. This extra air will make your engine run hot, overheat and ruin your new top end. In addition, bad seals will make jetting a nightmare later. You will never be able to get it jetted right. I’ve linked to everything you need so no excuses – just do it. Trust me, I’ve made this mistake and learned the hard way.


Why bad crank shaft bearing seals can cause problems

Beyond that, you should make sure your qt50 is running great before installing a new top end. A bigger engine will tend to exaggerate any issues you haven’t taken care of prior to install.

Here’s a great article on installing a kit. I’ll add a few things below for a little extra guidance.

1. Get your qt50 off the ground! I tried to install my first kit with my qt50 on the ground. What a pain! I’ve since learned otherwise. Get it up on a workbench so you can stand up and do this right. Heck, you can find a cheap folding table on Craigslist which will do the job. Or an old office desk or get a couple sawhorses and a long piece of plywood. Corollaries to this rule are employ good lighting and try to do the work in your garage or inside somewhere. During portions of a kit install, you are dealing with really small parts. Good lighting is a necessity. Believe me when that circlip flies across the room, you’ll need excellent lighting to find it. If you can do the work indoors, this will allow you to stop and come back to it later if necessary.

2. The broom trick. The broom trick occurs in two stages. First, sweep the floor of your work space really well. Then have the broom handy when your circlip shoots away from you. You will be sweeping the floor again to find it. Echolocation also helps in finding your lost circlip.

3. Needle-nose pliers. You will need a small pair of needle-nose pliers to install the circlips. Better yet, pick up a set of them in different flavors.

4. Check the gap on your rings and make sure it is within specs. If the gap is too small, sand the ends of the rings lightly with sandpaper to increase the gap. A few gentle strokes on each end is all you need. Take each ring and place it in your cylinder and then press it down 1/2″ or so with the piston to center it. Measure your ring gap again with a feeler gauge. If you are really cautious, then measure the ring gap at different places in the cylinder as well.

5. Cylinder ports have sharp edges. You will most likely want to chamfer the edges with a Dremel or give the cylinder a light hone to dull those edges. Basically you don’t want your rings to get hung up by sharp edges in the cylinder.

6. Install the rings on right. Rings are brittle. They will break. Ease one on gently into the first groove. Ease it out and ease it into the second groove little by little. In fact, you may want to practice once or twice with your old rings if you’ve never done this before. Better to break an old ring than a new one. Qt50 rings are interchangeable so it doesn’t matter which goes on top or bottom; however, the ring only goes on one way. The small letters go towards the top or dome of the piston. If you put them on backwards, you will know because the piston and rings will not slide into the cylinder without the aid of a mallet.

7. Circlip installation is hell but get it right. Yamaha qt50 circlips aren’t too bad. They are shaped like a “G”. Grap the straight arm of the “G” with your pliers. Insert circlip at an angle and get the top of it in the groove. Twist and spin the circlip around and pull it entirely into the groove. The straight part of the circlip must be parallel to the piston’s direction of travel (consult pictures in link above for installing a kit). Something I didn’t know – “some circlips have a machined flat side which faces inward”. I don’t think qt50 circlips do. Install one circlip on the piston before you install the piston on the crankshaft arm.

8. Install the piston correctly. The arrow on the piston points down at the exhaust. The ends of the rings rest against the ring stop. Align these before attempting to put the cylinder over the piston. Keep them aligned during install. You can install the piston on the crankshaft arm before fitting the cylinder over the piston. Or you can partially install the piston in the crankshaft and then put the piston/cylinder on the crankshaft arm. An extra set of hands here is helpful.

9. Get an inch/pound torque wrench to properly torque your cylinder head nuts to 84 inch/lbs.

10. Properly break in your kit so that it lasts.

11. A cylinder head temperature gauge is helpful but not foolproof.

12. Go ahead and remove the engine side covers and the footpegs so you have easier access to the top end.