OS .15 CV-R Rebuild

[RC10th]

This will sort of be a step by step rebuild of an OS engine.

I want to check the bearings as I put it away many years ago without after run oil and it was a bit gunky. It ran ok last time I used it a few months ago but it didn't feel 100% right, still feels a bit gunky so I'll go through it and give it a good checkup.

Hopefully it doesn't need a new bearing but we will see.

[Incredible_Serious]

Following with interest.... hey, maybe I'll learn something.

Alex

[RC10th]

Following with interest.... hey, maybe I'll learn something.

Alex

Hopefully someone at least finds it interesting or that it helps someone have more confidence to tackle an engine of their own.


I'd like to do this all in one hit but I can see time won't allow me to have a full run at it. I'm debating waiting to post it all in one fluid thread or post as I go.

Anyways, engine is out and ready to begin.......

[RC10th]

If anyone has any questions or wants some more info feel free to ask, I wont bite


Here we go......



Firstly remove the 4 pullstart screws. OS engines typically use phillips head screws so I must have swapped these out with allen head screws at some point. I can't stress enough to use a proper well fitting screwdriver as the OS screws heads easily round out.

Once all the pullstart screws are out carefully pull the pullstart away just enough to slip a thin flat blade screwdriver in behind and gently pry it off. OS pullstart bobbins are captured so you don’t really have to worry about this as it won't fly apart on you, however, many pullstart bobbins aren't captured and will likely fling apart if you just try and pull it straight off.

Here you can see the captured bobbin. A plastic clip clips into the housing and retains the spring and bobbin. Nothing worse then having a spring fly apart in your face.

This is the one way bearing that allows the pullstart to pull the engine over to start and freewheel once started. One way bearings typically have writing one one side and not the other, so take note of which way the writing goes or the orientation of the bearing before you remove it. In some engines the bearing will probably pull off with the the pullstart, if that's the case you can just leave it there if you like unless your replacing either a slipping bearing or a pullstart.

Carefully slip the bearing off. Here you can see that there is no writing on the other side of the bearing.

[RC10th]

Now remove the 4 screws from the backing plate, again, make sure the screwdriver tip is a good fit.

Gently and slowly pull the backing plate out making sure not to tear the gasket, if you tear it you'll obviously need a new one. Some engines use an o-ring which is far less fragile.

My poor OS !!!!     It doesn’t look too good inside. I normally just ran it completely out of fuel when I put it away without after run because I pretty much used it weekly. Looking at the starting shaft It definitely needs a good clean !!

Remove the starting shaft.

Carefully remove the gasket without tearing it or scratching the backplate.

If it helps lay everything out in order so you know how it goes back together.

[RC10th]

Time to remove the head. Use a good tight fitting allen driver to remove the engine head screws. If your engine is dirty make sure you clean out the screw heads. If you strip one screw head remove the other 3, it will release the pressure on the stripped screw and it should come out easily. If you strip 2 or more it becomes fun, the easiest way I've found is to drill off the screw heads. There should be plenty of thread left to unscrew the screw stubs once you get the engine head off.

So...... try to avoid stripping them in the first place

Seperate the head from the engine.

Remove the head shim(s) making sure not to lose, crease or scratch either the shim or head surface. If you crease the shim you'll be buying another. Take note in the order you remove the shims so you put them back in the same order. Some engines use a combination of brass and aluminum head shims, if so normally they go brass engine sleeve/aluminium shim/brass shim/engine head.

Notice the glow plug is still in the head It's much easier to remove when the head is still bolted to the engine

Almost all engines have some form of a key/dowel pin to locate the engine sleeve. This is so the sleeve can only go one way ensuring that all the ports line up. If you unfortunately have an engine without a "key" just make sure you line up the exhaust port in the case and you should be ok.

Now we need to remove the recriprocating assembly. The only way to get the piston, rod and crank out is to firstly remove the sleeve. If your engine still has some pinch and the sleeve isn't stuck it should pop up by simply by turning the engine over by hand. If the sleeve is stuck and you can't pull it out slip a plastic zip tie through the exhaust port and turn the engine over by hand to pop the sleeve up. NEVER try and grab the sleeve with channel locks or pliars etc. Once the sleeve pops up you can grab it and pull it out.

Pull the sleeve out.

[RC10th]

The only way to remove the piston and rod is at top dead center. Carefully rotate the crank to TDC.

Reach in with a finger and slide the rod out towards you, it may help to grab the top of the piston and bottom of the rod with your thumb and index finger so you can wiggle it up and off the crank pin.

Make sure you take note the orientation of the piston and the rod as you take it out, it needs to go back in exactly the same way. Most pistons have a crank relief cut into them which face the front of the engine. If your rod has an oiling hole in the side it faces the front of the engine. These OS engines have an oiling hole dead center on the bottom of the rod so pay attention to piston orientation.

The crank should slide out without too much trouble.

[RC10th]

Loosen the nut or screw on the pinch bolt and give it a slight tap to free it up. If the carb protests removal give it a gentle twist and it should pull right out. You could take the carb off at any time, this is just when I decided to do it. Set the carb aside for now, we will rebuild it later.

Remove the pinch bolt.

This is the rear bearing. I though it would need replacing but it actually feels pretty good. I wasn't going to remove it but for the sake of this thread I'll remove the bearings anyways.

Heating the case up in an oven is probably the best method but I have been doing it this way for a long time without any problems.

Be careful not to burn yourself !

Using a heat gun heat up the engine case, make sure you heat up the case equally on both sides. I normally alternate between 5 seconds each side untill it's hot enough to remove the bearings. Don't go overboard on the heat, just enough so the bearings fall out.

Give the engine case a sharp tap on a block of wood and the bearing should fall out. If it doesn't you should heat up the case a bit more. Flip the case and tap it on the edge of the block so the front bearing falls out. Once the bearings cool check them for wear, if they are crunchy or have excessive play they should be replaced.

Bearings removed and a bare engine case.

[RC10th]

Congratulations, you have now successfully stripped down your engine !

Next up, clean and rebuild.

[RC10th]

This engine doesn't require much clean up so there's not much to clean. I normally use motor spray or brake cleaner to clean all the parts. I sometimes use green scotchbrite to clean rust off parts. These probably didn't need the scotchbrite but I used it anyways, just be careful not to go too heavy on bearing surfaces or the crank pin.

Parts all clean.

Remember the oil hole I told you about in the rod earlier? Make sure this hole is clear.

Install the bearings in the case the same way that you removed them taking note that you put them back in the same way. The front bearing is pretty straight forward.

The rear bearing is open (or more open) on one side which gets installed toward the rear of the engine, this is so it gets oil while the engine is running.

[RC10th]

I highly recommend some quality after run oil to assemble an engine. I've had and used this Prather Products after run oil for years.

Put a couple of drops down the snout of the engine and let them run into the front bearing, also put a few drops on the rear bearing.

Slip the crank in and make sure it spins nice and smooth. Put a drop of after run oil on the crank pin.

Put a drop of oil on either side of the rod to lube the wrist pin and check for smooth movement. Also put a few drops on the piston itself and make sure it's coated on the top and sides.

Slide the piston and rod into the case making sure it's facing the correct way. Make sure the crank pin is at the top and slip the rod onto the crank pin.

Oil the inside of the sleeve.

Getting the liner in can sometimes be tricky but whatever you do don't force it. Keep the piston at the top of its stroke and gently twist the liner back and forth if need be while you wiggle the piston into the liner.

[RC10th]

Here is the flat in the back plate I was talking about earlier, it needs to be installed at the top. I have seen people shatter pistons because they put in on wrong.

Line up the notch in the starting shaft with the crank pin and slip in the backplate.

Tighten the screws evenly in a criss cross pattern and turn the engine over by hand a few times to make sure all is well.

Make sure the head shim and head surface are squeaky clean and slip the shim on.

Install the head and make sure you tighten the bolts evenly and in a criss cross pattern.

Slide the one way bearing on the starting shaft

Install the pullstart

[RC10th]

At this stage we can either put the carb back on as is or rebuild it. I don't think this carb needs to be rebuilt but if anyone is still interested after all that I'll rebuild it.

[RC10resto]

Nicely done.
Thanks for taking the time, great pics and instruction

[Incredible_Serious]

Nicely done.
Thanks for taking the time, great pics and instruction

What he said.....