Vintage Dirt Bike Q & A

Hello, this is my first time posting. I found this site while scouring the internet for help.

I have a 74' MT250 that I started rebuilding 15 years ago and am now finally trying to get it working again. Back then I replaced a bunch of parts, cleaned it up and started to restore it before putting the project on pause due to life getting in the way.

A few years ago I tried to take apart the engine to check it out, replace anything that needed it, and reseal the crankcase. However, I could not for the life of me seem to get the cylinder/jug off of the thing. Eventually I lost interest and didn't touch it for several years.

I am thinking about finally finishing this project so it is not just taking up space. I have removed the engine, put it on an engine stand mounted to a steel fab bench and removed the head. I tried tapping the jug free with a mallet with no success. I made some block off plates to attach to the ports to keep stuff from getting in there and added some handles to them to apply (a lot) more force. Still no luck. The engine would turn over freely so the piston is not seized.

Is there a trick to getting the top end off of this thing? I am afraid to apply heat as I do not want to warp or damage anything. As far as I know the top end has never been removed before.

Thanks for any ideas that you may have!

Its been on there 38 years and is going to take a fair amount of force to get it off. I beat the piss out of mine with a mallet and it became a tiny bit loose. I know this is wrong but it worked for me with no damage at all. I used a flat blade screw driver to very slowly and carefully wiggle the cylnder side to side till it came off. The gasket surface will be fine as long as your carefull. Many a cylinder have broken fins from people doing this but it can be done with no damage to anything just take your time,or just keep going at it with a mallet and also make sure youve removed all the nuts and bolts lol I spent a lot of time trying to remove a head one time only to realize it had one more nut I didnt see, it happens.

I'd be nervous about prying the gasket surface with a screwdriver and no prying on fins.
Figure out a way to apply constant outward tension on the cylinder with bolts and bracing.
This will take some head-scratching and creativity.
Then tap with the mallet and slowly increase the pressure with the bolts.
Heat with a heat gun will help.

My MR175 uses the same long-stud system as the MT250. The problem is the two stud dowels rust into the cylinder and case holes. I got angry once and was able to lift the entire bike off the ground by the cylinder, which didn't budge a micron.

First, I fill up the stud tubes with a penetrant like PB Blaster or similar, then let them sit at least 24 hours. If the penetrant suddenly leaks out really fast after sitting for a while, that's a good sign.

I then use a heat gun around the entire base gasket area. This gets quite hot during operation and is clamped tightly together, so don't worry about warpage.

On the MR cylinder I set up a bolt-type flywheel puller and wood block pushing up under the intake manifold area, with another 2-jaw puller on the cylinder top set up to push down on a wood block on the piston crown at BDC. Then I banged around the cylinder with a rubber mallet, and eventually the cylinder popped up enough to work it free by hand. No sledgehammers, crowbars, or chisels were used.

I doubt your cylinder will be as stuck as mine was, but I never would have gotten mine off undamaged without all the above measures.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks for the ideas. I will give it another go when I get back in town.

I have had penetrating oil soaking in the stud holes for quite some time now. I suspected the locating studs to be rusted and this kinda confirms it. I even tried lifting it off with a hydraulic engine hoist. I had removed the inlet and exhaust port manifolds and bolted block off plates on in place with pieces of steel welded to them to make handles which I could use to lift from. That didn't work and I didn't want to put any more force on it as I didn't want the bolts to break off in the cylinder

It is REALLY stuck on there. Hope this works!

Where exactly did you position the pullers? I don't quite follow how they were positioned.

Thanks!

OK - it sounds like maybe yours is stuck as badly as both of mine were. The idea is to use several pushing/pulling forces instead of one big one, and heat, heat, HEAT! to expand the cylinder aluminum around the dowels and loosen their grip. The base gasket joint really doesn't have much adhesive force compared to the rusted dowels, but it's important to loosen it too so the cylinder comes off straight.

This is my flywheel puller - I use it sitting on a wood block on the case top, with another small wood piece between the bolt head and the underside of the cylinder's intake opening:

http://www.amazon.com/K-N-82-0150-Pulle ... B000FGCYN0

This thing can apply plenty of upward force against the cylinder, with little risk as long as it's pushing up against the main cylinder casting and not a fin. If possible, you can slide it towards the side that has the dowels (on the MR motor this is the left side, I think it's the same on the MT).

This is my 2-jaw puller - I hook the jaws on the cylinder top's two shortest/strongest fins available, pushing down on a wood block or plastic drawer pull on the piston crown at BDC. NOTE: only a LIMITED AMOUNT OF FORCE should be applied to the cylinder with this puller, otherwise you could snap a fin. I've used this successfuly a number of times now, with no problems at all (don't forget the heat !!).

http://www.ultimategarage.com/shop/imag ... 04-400.jpg

Once the two pullers are in place and tightened up, use a heat gun to apply heat evenly all around the cylinder/case joint. I've got an infrared thermometer, and have applied well over 300F to this joint with no issues. Keep applying heat - especially on the dowel side - and occasionally tightenng the pullers (especially the intake-side one) and eventually you'll hear and/or see the cylinder pop loose a bit, possibly with a bunch of rusty penetrant pouring out as well. You can also whack the cylinder with a rubber mallet all round to speed things up, but keep an eye on the pullers as sometimes they move around from the mallet blows.

I'd be very careful how much force you apply to your lifting handles. The cylinder threads are just M6 in aluminum, and can deform easily. It would suck to end up with leaking intake / exhaust manifolds.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks guys! I finally got the cylinder off today thanks to your help.