Case Gasket Replacement

Bummer. I guess a picture is a link.

I've put together a little writeup on how I replaced my case gasket, but the pictures really kind of make it.

Do I need to have a certain number of posts before I can include them?

Bikeguy Joe said:

Yep...around 5 or so.

Click to expand...

Cool. Thanks.

I've just replaced the case gasket in my engine and I took some notes and pictures for later reference, so I thought I would share them here.

I cleaned the engine up some before I hauled it inside, but you can still see in this picture where the case was leaking. There's a fairly large area where no gasket material is evident. Since it was leaking behind the drive shaft, I figured that it must also be leaking between the crank and drive shaft.



I started the teardown on the right side of the engine with the clutch.



After removing the cover, I removed the retaining screw and flower nut, which freed the outer clutch plate and counter-spring. I made a tool for rotating the flower nut before realizing that it could be manipulated by hand if the clutch is held in the disengaged position.



I jammed the sprocket with a scrap of chain so I could remove the nut and lock-washer that hold the clutch assembly in place.

Then I jammed the sprocket the other way so I could use the gear puller to remove the clutch assembly. The seal on the bearing was in tough shape, but seemed to be holding up. With some persuasion, I was able to remove the woodruff key as well. I could see that a little bit of gunk had leaked through this bearing, but not enough to cause any trouble for the clutch.



I removed the screw that holds the gear to the crankshaft by jamming the magneto with a punch. The slot in the screw was rather poorly machined, which made this trickier than it might otherwise have been.



I used the gear puller again, tapped the key out, and inspected the oil seal. It seemed to be in good shape.



Turning my attention to the left side, I jammed the sprocket again and removed the nut and lock-washer, then applied the gear puller one last time.



It turns out that I had been enjoying the benefits of an automatic chain lubrication system!

Removal of the magneto frame was straightforward, just pull four screws and out it comes. First, though, I took the opportunity to jam the magnet and remove the nut from the crankshaft.



The magnet itself puzzled me a bit. Every time I made some progress lifting it, it would snap back into place. After some thought, I tried a faucet puller that I had purchased for a different project. It turned out to be just the right thing. I think that I've seen a similar tool at AutoZone, if you're looking for one.



With the accessories removed, I turned the engine back upright and started working my way down. It looks like it was leaking some around the spark plug. The more I saw, the more impressed I was with how well the poor thing had been running.



First things first, I pulled the plug. It didn't look quite as white to my eye as it does in this picture, but it did look like it had been running a bit lean.

I unscrewed the head nuts and came up with three studs and one nut. Close enough. I pulled the head off and had a look inside. Thankfully, there were no surprises.





I pulled the jug off. The profiles of the transfers in the jug and case are different. The shape of the gasket is different than either.



I blocked up the piston and put a rag around the case. I carefully removed the retaining rings and the wrist pin.



Now I set about removing the jug gasket which was very well adhered to the case. I used a plastic scraper, a soft wire brush, and a syringe of alcohol. I find syringes to be very handy around the shop for dispensing small amounts of liquid without spilling. I buy mine from Techni-Tool. They have blunted needles in various sizes as well as little plastic cones that can be cut to size. If I wasn't going to split the case anyway, I'd have plugged it at this point. A lot of the gasket ended up in there. You can see my offset studs on the front mount in this picture. I made them to clear the down-tube on my bike. They could stand to be cleaned up some.

It was long work, but in the end I prevailed over the jug gasket.



Not knowing how snug the crank and drive shafts would be in their bearings, I decided to take the screws out of the case and try pulling it apart. The drive shaft turned out to be pretty snug.

So, I blocked up the case and started tapping it out toward the left side of the engine. I didn't want to try it the other way because of the very small section that the flower nut attaches to.



I ran out of room in the little cup-shaped piece that I had put under the drive shaft and had to move over to the vise. The jaws installed are a pair that I made out of aluminum. If they were harder, I probably would have put something over them to protect the case. I used a length of brass round bar to protect the shaft and surrounding areas from my sometimes erratic hammering.



Eventually, out came the drive shaft and one bearing. Yuck. I decided to come back to the drive shaft later.



The crankshaft was only a sliding fit with it's bearings. I guess that was for the best, because I'm still not sure how I would have gotten it out of the bearings -- they are pressed in from the inside. I suppose I would have had to dream up some way to hold only one half of the case and tap or press the shaft out from that side.

You can see where the gasket failed between the crank and drive shaft. It's definitely at the thinnest point between the two. In fact, it looks like the gasket wasn't an exact fit and that it might not have fully covered the mating surfaces at this point. I didn't find any gasket residue on either half of the case, so maybe it squeezed right out when they tightened the case screws at the factory.

I removed the oil seals carefully and tapped the bearings out with my trusty brass rod. It turns out that the crankshaft bearings are open on both sides while the drive shaft bearings are sealed on both sides. Both seem to be standard 6202 type bearings. The crankshaft bearings had some aluminum bits in them, so I cleaned them. I like to put one bearing at a time in a plastic bottle that has a good sealing lid with about half an inch of alcohol (or other solvent that's compatible with the plastic) and give it a nice brisk shake. I get pretty good results with this method and it really saves on solvent.

The case gasket came off easier than the jug gasket, but it was still long work.



I was still curious about those transfer ports so I lined up the jug with half of the case to see how much of an opportunity it was. Looks like a pretty big one, if there's enough thickness in the case to match them up.



Well, there was no use avoiding it any longer. It was time to make a gasket. I put half a sheet of artificial vellum over the left side of the case (the right side of mine has a protruding lip) and locked it in place with the case screws. I started by rubbing a finger along the edges, but ran out of dirt and had to fall back on a pencil.



I cut out the template and transferred it to a piece of gasket material, which I also cut out. I pinned it to the right side of the engine and trimmed it up. You can see that even on this new gasket the spot between the crank case and drive shaft area is still very thin. I should note that the material I used was 3/32" thick. The only decent-sized parts of the original that I got off were saturated with alcohol, but they appeared to be about half as thick. Thankfully, nothing appears to be too precise on these engines and the case doesn't seem to mind being about 50 mils wider than it was.

After looking at the finish on the mating surfaces of the case halves, I decided to use gasket sealant on the entire surface. It seems to have worked out well enough, but if I had it to do over, I would try thinning it down with alcohol before applying it. The material that I used was quite thick and I ended up using more than I might have liked. The sealant was very tacky and really made it easy to get the gasket lined up how I wanted it and keep it there.





I realized that the bearings should be installed before the crank (!), so I tossed them in before applying sealant to the final two surfaces and re-joining the case. I installed all of the case screws and went around perhaps six times to get them all snug. A small amount of sealant seeped out. I should have used less, but I don't think it will prove to be a major problem.





The grimy drive shaft was still gazing mournfully at me from the workbench, so I put it in the vise and removed the adjusting nut with a pair of channel-lock pliers. That's definitely not the right tool, but it had the advantage of being present in my toolbox. If you're ambitious, you could make or buy the correct spanner. It turned out that the tension on the spring was relieved before I ran out of threads, but I still stood to the side and donned my safety glasses.

A pin holds the inner shaft and the ferrule (I guess… I'm not sure what to call the part opposite the adjusting nut) together. It was a sliding fit, held in only by grease and gunk once the spring was removed.



I carefully removed the seals from the drive shaft bearings with a small flat-bladed screwdriver and cleaned them out before re-greasing and re-assembling them.



Here are all of the drive shaft parts cleaned and laid out, just before I re-assembled them.



I drove in one bearing, then drove in the drive shaft and the other bearing. Then I went back and re-seated the first bearing. I used a 9/16" deep well socket. It would have been much better to use a soft tool to drive the bearings from the outside edge. A good tool could probably be made on the lathe from Delrin or a similar material. Of course, an arbor press would also be better than a hammer, but that's still on my wish list.



I lubricated the crankshaft bearings with 2-stroke oil before installing the oil seals. I was able to get the inner lip over the edge of the shaft by rocking it in a circular motion. Another socket served to seat the seals, but I used only steady hand pressure here rather than the hammer. It seems to be important to press the seals in very straight. I think that the outside edge would tear easily if they were put in crooked.

I made the jug gasket in the same way as the case gasket. I re-installed the piston before applying the gasket and sealant. I didn't want it to get too dry before I installed the jug and head and tightened it down. I re-used the head gasket, which caused some issues later.

Time to re-install the accessories. I started with the magnet and magneto frame, since I wanted to use it to jam the crankshaft for installing the small gear on the right side.



I remembered the crummy slot in the screw that held the small gear in place, so I put it on the milling machine and cleaned it up. It ended up going on much easier than it came off.





I put the clutch assembly in place and noticed that it… wobbles… quite a bit through each rotation. This explains the "whoosh whoosh" noise that I've been hearing. Does anyone know if this is normal? It looks like maybe the backing plate isn't machined square. I'll leave that for another day.

I realized at this point that it was the sprocket and not the magneto that needed to be jammed to install the nut on the clutch assembly, so I went back to the left side and installed the sprocket. Then I returned to the right side and finished assembling the clutch.



Since I was starting to imagine that this might not be the last time I take this engine off the bike, I decided to take the opportunity to cap the white wire and connectorize the black and blue wires.



Throw the spark plug and the clutch cable stay back on, and it's an engine again.



I put it back on the bike and fired it up. It ran about as well as before, which was a bit of a disappointment. Then, I heard a crack as if it had backfired. Curious, I watched while revving the engine a little to see if it would do it again. Sure enough, it did and I happened to see a little puff of smoke waft away from the spark plug! Examination revealed that it was leaking around the head gasket as well as the spark plug. I pulled the head off and applied sealant to the head gasket. Probably a better move would have been to replace it and perhaps use the aluminum paint trick. I also removed the spark plug, cleaned the mating surfaces, and re-installed it using more torque than I really felt comfortable applying to this little aluminum head. But, I did feel it really seat this time. Hopefully the threads don't come out with the plug the next time I pull it! I wonder if it's practical to helicoil the spark plug hole.

Anyway, it now starts easier than ever before and is running much better and doesn't seem to be leaking. The sealant is holding up so far on the head gasket, but I'll keep an eye on it.

It smells a bit like a pine air freshener when it warms up, which I guess is sort of a feature.

corgi1 said:

Good spread,and interesting imfo.,,,,I am wondering If there is enough clearance ,at the thin part between the crank case and drive shaft passage ,too allow for a wider gasket cut creating a slight mushroom effect on either side,thus helping the gasket not to slide out under pressure,,

Click to expand...

Thanks.

There's quite a bit of clearance on the drive shaft side, but the crank side has a lip. I'm hoping that the sealant will help it stay put.

The lip does seem to help with the alignment, more with the crank shaft than the drive shaft. It's certainly better than that picture Jim showed where the case halves were practically in different zip codes. Of course, it's no substitute for properly installed dowel pins if you really want to do it right.

My motor is from boygofast.

Yeah, it struck me a little funny that his had the recess but no lip. On my engine, the lip and recess are as cast, not machined, so it's not exactly precision alignment. But, it does get it in the neighborhood, which is better than it could be.

I didn't take any measurements to see how good the alignment was while I had it apart. Maybe if I tear it down again I'll check it out.