72 MAR

[yellow_cad]

I just acquired a 72 MAR that seems in pretty good shape. It does smoke a bit and drip some oil so I understand that I need to do the crank seals and gaskets. What special tools do I need to do this? Also any tips would be greatly appriciated. When it is all tight, what is best to run in the gear box? I have some Redline MTL. It was missing the decompressor but I found one NOS. Does the decompressor go in the hole at an angle to the ground? Thanks for any help on this. Jim in Folsom, CA

[brian h]

I just acquired a 72 MAR that seems in pretty good shape. It does smoke a bit and drip some oil so I understand that I need to do the crank seals and gaskets. What special tools do I need to do this? Also any tips would be greatly appriciated. When it is all tight, what is best to run in the gear box? I have some Redline MTL. It was missing the decompressor but I found one NOS. Does the decompressor go in the hole at an angle to the ground? Thanks for any help on this. Jim in Folsom, CA

[brian h]

I have rebuilt two Mk I and one Mk II Ossa MARs, the essential tools you will need is a flywheel puller, a tool to hold the flywheel,a tool to hold the clutch hub and a standard two legged puller for the front shock absorber.

You need to be carefull with the gearbox shaft shims and keep them together when you strip the engine, otherwise the engine is held together with screws and a good screwdriver wil work fine, the engine is easy to assemble and there is a good book to help you. Google Ossa World and you will find the Clymer Publication: OSSA service Repair Handbook - it will tell you just about everything you want to know and web sites for parts.

I use the later 80 weight Castrol Gear oil which works fine. The decompressor fits the 14mm tapped hole on the RH side of the cylinder head - the spark plug is always central, the decompressor works well as Ossa brakes are not good when wet.

Hope tis helps

[yellow_cad]

I have rebuilt two Mk I and one Mk II Ossa MARs, the essential tools you will need is a flywheel puller, a tool to hold the flywheel,a tool to hold the clutch hub and a standard two legged puller for the front shock absorber.

You need to be carefull with the gearbox shaft shims and keep them together when you strip the engine, otherwise the engine is held together with screws and a good screwdriver wil work fine, the engine is easy to assemble and there is a good book to help you. Google Ossa World and you will find the Clymer Publication: OSSA service Repair Handbook - it will tell you just about everything you want to know and web sites for parts.

I use the later 80 weight Castrol Gear oil which works fine. The decompressor fits the 14mm tapped hole on the RH side of the cylinder head - the spark plug is always central, the decompressor works well as Ossa brakes are not good when wet.

Hope tis helps

Thanks for the tips Brian. Just a few questions: What is a good source for the special tools? Does the clutch need to be removed if I am just replacing the crank seals? I have the Clymer book, Vintage Dirt Bikes (OSSA is one of three bikes covered). Is this the book you are referring to? I went to OSSA World but can't find any books. When you talk about the decompressor being used in place of the brakes, what exactly are you referring to? How would this work? Jim

[woody]

It is best to split the engine to replace the crank seals. You can do it without but it is difficult to get the seals in without damaging or distorting them and another consideration is that you have no way of knowing whether the tensioning spring has popped out of the back of the seal during attempts at getting the seal in. Besides, if you have a leak it could be the centre gasket or a stipped or damaged sump plug, both which need a stip down. Other possible leaks - oil seal behind the gearbox sprocket, worn sleeve that sits in this oil seal, clutch pushrod seal in the end of the mainshaft, gear shaft O rings.

Removing the clutch and cush drive is straightforward but the problems start with the sleeve that the cush drive sits on. This is a tapered fit on the crankshaft and it can come off easily or it can be a bitch if it's stuck on really hard. An ordinary puller is no use as there is nothing to attach the legs to, you can't get behind the collar of the sleeve as it is right up against the crankcase. There is a proper puller available which you can get from Keith Lynas. See his parts page.

http://www.ossaplanet.com/partspage.html

You can make something that will do the job in order to use a 2 or 3 legged puller but much better to buy the proper part. However, be prepared for a mighty struggle if that taper has bitten hard.

Also, on the MK1 MAR (up to 1973) the ignition flywheel nut has a normal right hand thread, later bikes, MK2 onwards (1974 on) had a left hand thread (or it could be the other way around, long time since I did one) Make sure you know which engine you have as if you tighten it further thinking you're undoing it, you can shear the end of the crank off (yes I did, many years ago... when I didn't know there were left handed threads) Although you say your bike is a 1972, check the engine number with Keith or the frame number chart on the site below (engine/frame numbers always matched from the factory so your engine number can be used on a frame/chassis chart) Just hope your number is in the run as some of the numbers aren't. If you google Ossa Chassis number you will get links to other charts as well, so check a few if you don't find yours at first. Keith will be your best bet though.

http://hem.passagen.se/ossa/ossaref.html

The gearbox sprocket nut also has a left hand thread.

As mentioned before in the previous post be very careful when splitting the cases as there are shims on all the shafts, both ends (usually) including the kickstart shaft and selector drum. Also the crankshaft. You're supposed to re-shim the engine everytime it is reassembled but you will probably be ok putting them back where they came from (don't mention that to Keith...) Shimming is an absolute bitch of a job. There is also a thick shim/washer that sits under the outside cog on the gearbox countershaft on the clutch side. Watch for it if that cog falls off the shaft. The gearbox needs it. Whilst it's apart renew the O rings for the gear change and kickstart shafts. The O ring on the right side crankcase for the gear change shaft can only be accessed with the engine apart. You don't want to miss that one and have it leak when reassembled. The other gear change O ring and kickstart are in the clutch cover.

The manual you have is as good as any, unless Keith has others.

A decompressor valve releases compression from the cylinder when opened and somehow this increases engine braking. If the rear wheel is on a grippy surface, if you apply the decompressor with the bike rolling along in gear it will slow the bike down reasonably effectively, making a hell of a noise of escaping air at the same time. In dry conditions it can be used as an effective brake on the rear wheel going down hill, as the wheel stuggles to turn against the increase engine braking. If the ground is slippery or muddy though, the rear wheel has nothing to turn against and the increased engine braking will just lock it up and stall the motor.

Edited September 1, 2011 by Woody

[yellow_cad]

Thanks Woody. My engine number looks like it starts with an M but all of the numbers on the charts start with B. Aside from that my number range is correct for a 72 MAR. I'm new to 2 strokes; how much smoke out of the exhaust is OK? Since it will run and I can warm it up, will a compression test tell me anything?

[woody]

That's ok for the numbers, engines are prefixed M and frames B, but the important thing is the number. They matched from the factory so you can use your engine number against the chassis chart to date it. I don't think the MK1 MAR has the number stamped on the frame anyway, it used to be a stick on plate.

Run it on 50:1 modern 2-stroke. I use Castrol TTS, it doesn't gum up rings or exhausts. It shouldn't smooke much at all on that. However, it could be smoking for a number of reasons - too much oil, gunged up middle silencer, burning gearbox oil leaking through the clutch side crank seal.

If it has only been ridden on light throttle over the years the exhaust could have a bit of gunge in it. Sometimes a long run on an open throttle will be enough to burn out the residue and stop the smoking so it may be worth taking it out somewhere to try that.

If the bike responds and runs ok but just smokes, it is unlikely you will need to worry about a compression test. Similarly if you can feel a lot of compression on the kickstart. If it doesn't run clean, also check the carb for over fuelling, stuck float etc.