Author Topic: MX360 Build of the Parts Bike (Read 4279 times)

Makotosun · « **on:** January 26, 2017, 06:08:26 pm »

I assume I am not alone in that I sometimes acquire a bike for its parts, then after staring at it in the garage for awhile, decide it really needs to be rebuilt, not scrapped

And so it begins.

Some time ago, (about a decade) I purchased a 1973 US model MX360, mostly because I wanted the wheels and front suspension for one of my DT400 projects. It was cheap, didn't run, didn't even turn over. Kind of rusty and decrepit. In other words, Perfect.

Kind of cute in its own way.

Well, other projects came and went, my focus being the streetable models, as here finding open dirt to ride can be a challenge, but a street titled old smoker can go anywhere a car can go . . .

My son, now 14, would walk to the back of the storage where it sat, and climb into the saddle, way before he could really even reach the footpegs. Most recently, we decided to take a closer look. An hour with a few wrenches and we had the lump of motor out and headed home to the garage to find out if the damage that had it locked was repairable. What we found was this:

Surprise of surprises, the piston was not locked or rusted to the cylinder. The problem was about the time the piston top melted, the main bearings self destructed and threw the crank out of alignment so it wouldn't turn. Most surprising, the cylinder was still on standard bore (probably a replacement considering the rest of the bike condition) and the carnage in the cases was limited to a stretched balancer chain and bearings! So the decision to bore it, fix the internals, and build the bike was made.

I will break these up into several posts for ease of posting and of reading.

Makotosun · « **Reply #1 on:** January 26, 2017, 06:24:59 pm »

The goal of the build was inspired by probably my favorite dealer brochure pic:

There is no way I am going to do a full on restoration, though it would be possible with what is still good on the bike. The only super high dollar item that is missing is the chain guide that hangs down from the rear swinger. They are out there, but more than I want to spend. The goal is a really nice play bike for trail riding. Eventually it may get a new ignition with outer flywheel to make the beast a bit more tractable. For now, the goal is make it run, then have some fun.

The tank shows normal racing wear, but it has a nice patina, and no rust inside. For now it will be kept as is. The side panels will be painted with new number plate backgrounds, and the fenders which are very rough will be replaced. Seat is a recover, but is fine. I do have a nice proper cover with Yamaha on the back, so it may get that if I fall in love with it.

The motor at this point has been gone through - fresh crank and rod, most all new bearings, new seals, NOS balancer chain, and a few other bits. Transmission was like new. The autolube was unhooked with the tank missing, but it will be restored to use.

The top end was bored to second over to clean things up. It was on a standard piston, but was really out of round. Cleaned up very nice and an OEM piston was sourced and installed. The only really non-stock part I am going with is some Boyesen Power Reeds that I had laying around.

I like the look of the polished fin edges, so got out my tools and spent some quality polishing time.

The goal is to make a fairly close low dollar cosmetic, but very nice mechanical bike.

Just a little tip - if you start one of these projects and have problems getting motivated to work on it, find a 14 year old who thinks he wants to ride it, and you can borrow all the motivation you will ever need!

We started assembly of this (Not including the motor as shown above) about a week ago, and progress has been brisk.

First we got the motor into the frame, then were able to check the old button ignition for spark - Woo Hoo - we have a fire in the hole!

Some new Fly Vintage bend bars up front and some fresh paint and bolts make the cockpit presentable.

For now we will use stock rear shocks. I had a set that had nicer springs, so bolted them up. Yes I know the reservoirs should be reversed $:-\$

Makotosun · « **Reply #2 on:** January 26, 2017, 06:32:43 pm »

We decided that until we get the motor going, and decide whether to hang onto the beast, we just spooned on some old but serviceable knobbies from the back of the shop. Metzlers with some aggressive treads.

You may have noticed the kick stand was bent in the first picture, like most of them are. I decided, as I have done on several of my twin shock DT's, to replace the stock with a 77-79 monoshock stand which only needs the mount tab hole to be enlarged a bit to be a direct fit.

The stand works better, holds the bike at a more correct lean angle, and the big pad keeps it upright in soft stuff.

Wheel bearings were all a bit on the sketchy side so were replaced with a set of All Balls Racing double seal versions.

Perfect fit and only about $25 for both ends of the bike. Brakes got a full disassembly, regrease the pivot shaft and fresh factory shoes. I like to be able to stop, and not have failing pads make that decision for me.

Where we left off last night was pretty satisfying.