The hard work really has been done with the inspection and prepping the parts.
Make sure that the rag is still in place in the crank case. We don’t want anything falling into the bottom end.
First the base gasket needs to be lubricated, I do this so that it helps to seal the surfaces and it has an added benefit of stopping the gasket sticking, if you need to take the top end apart again the base gasket will be reusable. So coat the gasket in petroleum jelly or your other preferred lubricant. and install it on to the crankcase – careful as often base gaskets need to go on a specific way
Carefully Install 1 circlip into the piston – make sure it does not spring out! (wear safety goggles!)
If you didn’t need to drill your piston, dip your little end bearing into 2 stroke oil and install it into the conrod
Dip your wristpin into 2 stroke oil and start to push it into the side of the piston that doesn’t have a circlip into it. This is normally possible with your fingers.
Place the piston over the little end of the conrod with the arrow on top pointing to where the exhaust will exit (normally the front of the engine), line the wrist pin up with the little end bearing and push it the rest of the way into the piston.
Ensuring the rag is still stuffed into the bottom end, very carefully install the 2nd circlip. It’s very important that both circlips are seated fully into the circlip groove. and that they do not spring out as you are installing them (wear safety goggles!).
Install the piston rings onto the piston with Wossner piston kits the rings can be installed either way up & both rings are the same. Line the ring ends with the locating dowel.
Remove the rag from the crankcase
Carefully rotate your engine so that the piston is near the bottom
Wipe a layer of 2 stroke oil all the way around your cylinder coating the bore.
We are going to lower the cylinder onto the piston
I use my fingers for the rings but I know others use ring compression tools – I just find fingers easier
Compress the top ring ensuring the ends still line up with the dowel
And slide the cylinder over the top ring, if you are using a 2 ring piston then do the same for the 2nd ring.
Now making sure the cylinder is the correct way round, push the cylinder all the way down onto the base studs – sometimes you need to rotate the engine a bit to raise the piston and give yourself a bit of play.
Check the cylinder nut torque settings in the resources guide on phoenixmotos.com, setup your torque wrench and torque adaptor (the torque wrench needs to be at 90 degrees to the spanner for the correct torque to be applied) and use the setup to tighten the cylinder nuts to the correct torque setting. Tighten each nut slowly and use a star pattern.
Now the head gasket, there are a few different types. I apply them all dry except the o ring types with which I use petroleum jelly as a lubricant. And make extra certain that the o rings are seated correctly in the grooves in the cylinder. for all others just double check that they are the correct way around.
Install the head, find the head nut/bolt torque settings in the resources guide on phoenixmotos.com, setup your torque wrench up and start tightening the nuts. Tighten slowly and in a star pattern so the head is torqued down evenly.
Check that your kill switch works by inserting your spark plug into your HT lead, resting it on your cylinder head & turning your engine over. when you turn your engine over your spark plug should spark & when you press your kill switch & do the same it should not spark. It’s so important that your kill switch does work in the event of a problem.
Install the spark plug, check the compression (I usually just do this with my hand after a top end rebuild, you can normally tell the difference straight away but a compression tester is better). Reconnect the HT lead / spark plug wire.
Reconnect the power valve if there is one, and install the necessary covers – check phoenixmotos.com for bike specific instructions.
Reconnect the coolant hoses and tighten the jubilee clips (do not over tighten) if your bike is water cooled.
Fill your radiators with coolant being mindful to watch for any potential leaking coolant from hoses or your head gasket – if it does you will need to check the sealing of the hoses or torque of the head nuts etc. Hopefully you won’t need to strip back down.
Re install your carburetor, slide the air intake side into the air boot first, push it back (use the heat gun/hair dryer if you need to soften the boot) and then slide the output side into the inlet manifold.
Twist the carb so that it is in the correct orientation and insert the slide into the top of the carb, you should feel the slide hit the bottom – which is what you need. If the slide does not drop into the bottom all of the way then you may need to take the carb back off as the needle jet may not be lined up with the main jet. It’s critical that the slide drops all of the way down or will be like starting your bike with your throttle wide open and with a brand new engine build too, this is not what you want!
Make sure your carb is angled correctly & tighten your jubilee clips
Connect your fuel source up to your carb
Install your exhaust – use a copper gasket if you have one / O-rings. You do want your exhaust to seal at the manifold and where the silencer attaches to the expansion pipe. If the connection is not great and can’t be resolved then some high temperature gasket sealant isn’t a bad idea!
It’s a great idea to add an hr meter to your bike unless you already have one. This way you can easily keep track on the amount of hrs of run time since your maintenance jobs. Keep a track of how
I just want to mention Squish clearance as well. In most 2 stroke dirt bike cylinder heads you have a combustion dome and a squish band. The squish band is the outer part of the diagram and sits close to the piston crown. The distance between the piston and the squish band is critical. The steps I have taken you through in this guide relate to building your 2 stroke engine back to stock specification so I haven’t covered squish band measurements or recommendations in here but if you were doing any modifications then this is an area to be particularly careful of. There are squish clearances in the resources section on phoenismotos.com & this topic is covered in our 2 stroke tuning guide also available on phoenixmotos.com
Moment of truth and the run in period
You are now ready to start the engine.
Have a general check around the bike make sure everything looks ok – no coolant leaks etc..
Turn the fuel on and turn your choke on or use your tickler if you’re on a really old carb!
Make sure fuel isn’t pouring out of the overflow pipes – re check your carb if this happens
Make sure the throttle snaps shut after you open it and listen carefully for the slide hitting the bottom of the carb this is really important.
I like to lean the bike over until fuel pours out of the overflow tubes as it ensures the carb float bowl is full
Now you are ready – slowly kick the bike over a few times without the intention of it starting
Now start the bike.
There are lots of theories for run in. Personally I like to follow this routine:-
Let the bike start to warm until you can hear the idle change signifying that you turn off your choke.
Alter the idle screw so the engine is not going to stall
Let it warm for a couple of minutes
Give the throttle a few blips up to approx a third revs.
Let the engine idle and keep blipping the throttle every so often until the engine is hot then switch off
Let the engine cool to room temperature – at least 1 hr
Re torque the cylinder and head nuts, check the coolant level
Restart the engine and follow the procedure as above but this time you can take the revs up higher to 2/3
Again once hot switch off
Let the engine cool to room temperature – at least 1 hr
Re torque the cylinder and head nuts, check the coolant level
The next run, let the engine warm up until hot and then ride the bike. Start riding at low revs and then mid range. I’d then advise a taking the engine up the flat out for some short bursts.
Bring the bike back and turn it off.
Let the engine cool to room temperature – at least 1 hr
Re torque the cylinder and head nuts, check the coolant
The bike is now ready to ride. I’ve raced bikes at this stage before and had longevity out of the top end.
Assuming you aren’t racing the bike this same day. The next time I rode the bike I’d want to take it to a track, warm it up to hot before practise and then in practise I would not scream the motor too much but not be afraid of letting it rev hard. Then I’d be happy to race the bike.
Whether you race or not I’d always be in the habit of letting the bike warm up to get hot before actually riding the bike especially if planning to rev it hard (which is fine) this way you will only put your engine under stress once all of the parts are thoroughly warmed up to working temperature.
Well done! You did it! Top end rebuild complete and successful! Don’t forget to check phoenixmotos.com for resources and extra guides, maybe you want to try a bit of tuning or a full rebuild next!