MB5 Engine Swapped Build

I picked up this rough but complete MB5 a few months ago and its been sitting on the backburner until I concocted a plan for it. I recently started messing around with it so I figured I'd start a build thread to push me along and get some creative input along the way.

Lets kick things off with pics of the bike as I got it.


Re: MB5 Engine Swapped Build

The power plant is being donated by a 95 Yamaha Blaster that my dad got for me as a kid. I spent a lot of hours ripping it up on that thing and broke it more times than I can count. I was planning to rebuild it again at some point but realized that quads aren't really that cool and occupy way too much garage real estate.

I got as far as rebuilding the engine with all new top, bottom, and trans internals before I sold off the rest of the quad.

So now I have this super fresh aircooled engine waiting to drop into something fun and a beat up tiny motorcycle that needs a bunch of work.


Re: MB5 Engine Swapped Build

Here's the last day that motor ran. Right before I pegged a tree and bent a buch of important stuff.


Re: MB5 Engine Swapped Build

good luck!!!! i just finished my mb5 build with the stock engine :)) i was considering an engine swap as well but decided to just go stock. good luck with those frame welds!

Re: MB5 Engine Swapped Build

Mb5 will be much funner with a swap... doing a similar thing with one of my ARs right now! Good luck!


Re: MB5 Engine Swapped Build

That AR is sweet! Is that a kx85 engine?

Re: MB5 Engine Swapped Build

I think the blaster engine will compliment this bike really well. The porting is pretty

simple and it has a wide, linear powerband which should make for an easy riding bike. Power density is relatively low but it should have no problem cruising 70 strapped to a 180lb street bike. It has a 12v 55w output lighting coil which will make wiring super easy.

They also didn't change for the 20 years they were produced so parts are a dime a dozen and they're practically bullet proof

The main mounting point on the blaster engine utilizes the swingarm bolt. The spacer between the bushings on the MB5 swingarm seemed to be pretty close to the same diameter to the blaster swingarm bolt. I figured this would be the easiest way to establish a starting point. I cut the spacer in half and sanded down the OD until it slid easily into the mounting boss. Then it was more or less located in the frame and I just had to get the other mounts figured out.


Re: MB5 Engine Swapped Build

Here it is mocked up before I started on the frame modification. I figured it would definitely need a down tube with a cradle under the engine for support. I wanted to keep it easy to drop the engine for maintenance so a bolt-on subframe section would be the best route.

20200519_184534 (1).jpg

Re: MB5 Engine Swapped Build

I chopped off the original upper engine mount and welded it on further forward to give me a place to start the engine cradle.


Re: MB5 Engine Swapped Build

Here's how the engine cradle turned out after I got it welded up. I like the lines, its very low profile and flows pretty well. The swing arm mount is carrying most of the stress so I think this should be strong enough.


Re: MB5 Engine Swapped Build

Plenty of room for the exhaust to drop out under the bike.


Re: MB5 Engine Swapped Build

Damn, this is looking good so far!

Re: MB5 Engine Swapped Build

oh hell yeah, I was really expecting to click this and see some tweaker asking how to put a rebel 250 motor on his mb5 that was "given" to him.

A blaster motor is a really smart choice for a swap on this bike, it'll be super fun. Engine cradle looks great so far.

Re: MB5 Engine Swapped Build

That cradle is the tits. This mb5 is gonna rip. Nice work. Iam subscribed.

Re: MB5 Engine Swapped Build

Ha, I might do some tweaker shit every now and again but I try not to show that off.

The engine is working out much better than I hoped given it started in a quad. Everything lines up really nice with only minor trimming of the frame around the kickstart boss.

I still have the blaster frame, it is pretty heavily modified so I decided it was expendable. That's been making life really easy on this build because I've been cutting mounting tabs off and reusing them it instead of making my own.

The only "problem" so far is that the engine needs to be offset about 20mm toward the clutch side in order to get a straight chain line. Hardly a noticable amount while sitting on it but I bet I'll feel it riding.


Re: MB5 Engine Swapped Build

Dirty30 Dillon /

Wow, this is really well done, and properly executed.

The cradle mount is an excellent idea.

Re: MB5 Engine Swapped Build

Awesome, welds lookin' great.

Could you use an offset front sprocket or rear sprocket to help with your chain alignment problem?

Re: MB5 Engine Swapped Build

Dirty30 Dillon /

> Cameron Cate Wrote:

> -------------------------------------------------------

> The only "problem" so far is that the engine needs to be offset about

> 20mm toward the clutch side in order to get a straight chain line.

> Hardly a noticable amount while sitting on it but I bet I'll feel it

> riding.

I bet you won't even notice it. Vespa scoots have like 30 lbs of motor hanging 6 inches off centerline and it's not horrible.

Re: MB5 Engine Swapped Build

Oh yeah that's true. I forgot those vespa motors are almost entirely offset. I also ride a Sportster too and there's a crazy amount of clutch hanging off the side of that thing.

Re: MB5 Engine Swapped Build

Of course the gas tank was rusty from sitting for who knows how long in a shed in western WA. I decide to go the vinegar route first, I've had pretty good luck de-rusting old dirtbike parts in the past and vinegar is so cheap compared to stronger acids. This is the general procedure I followed if you're unfamiliar.

- Fill tank with vinegar and let sit for a few days periodical shaking it to death.

- Drain tank, strain the chunkies out, refill and repeat.

- If its still rusty/crusty, drop a handfull of small metal parts i.e. washers, nuts, BBs.

- Refill with vinegar and continue shaking the shit out of it.

- Repeat until the tank is more or less clean.'

- Rinse with water repeatedly until it comes out clean.

- Drain and rinse with alcohol to get rid of the remaining water.

- Drain and rinse/fill with premix. You want to do this quick because the alcohol will flash rust the bare metal once it dries.

I decided not to coat the tank at this point, seemed like more hassle than its worth for a tank that I intend to keep topped off.

I couldnt get a decent pic of the inside but it looks very clean from what I can see. most importantly, the outlet screen is clear and intact.


Re: MB5 Engine Swapped Build

There were 3 spots where it had rusted through under the seat because of spots where the seat wore through the paint. I've never patched a tank before so I looked into a lot of different methods. The holes were way too big to rely on a coating from the inside and I didn't have much confidence in any of the exterior epoxy type patch products. Gasoline is pretty nasty and it seems to me like anything non metallic would be a temporary fix.

I thought about welding it but I didn't want to ruin the surrounding paint with heat. I watched a few videos and decide to solder it up. I have only ever soldered wire connections so I was a bit skeptical about how it would stick to old sheet steel. I used flux paste, regular electrical solder, and a small butane torch to get these results. A bit of a learning curve here but I think it turned out really nice. I used a small piece of sheet metal to fill in the majority of the biggest hole and then sealed around it with solder.

It's been sitting full of premix about a week now with no signs of leakage.


Re: MB5 Engine Swapped Build

Looks good, that’s gonna be a fun bike to ride.

Re: MB5 Engine Swapped Build

Brandon Weiss (Detective brandon to you) /

Now I'm wishing I had soldered my Laguna tank instead of coating it! Had like 12 pinholes all over the bottom. I used Caswell though, so it'll last forever anyway

Re: MB5 Engine Swapped Build

I was leaning toward Caswell but the "pinholes" I found turned out to just be holes holes.

Re: MB5 Engine Swapped Build

Damn son, this is cool as shit. Gotta say I was pretty skeptical before you started posting pics, but now this is RAD

Re: MB5 Engine Swapped Build

> Cameron Cate Wrote:

> -------------------------------------------------------

> I was leaning toward Caswell but the "pinholes" I found turned out to

> just be holes holes.

Nice solder job . From what I see , you'll be doing more if you don't coat the inside .

Re: MB5 Engine Swapped Build

Gotta tease a little bit haha. I'm also pretty skeptical of plans that never happen, some talk big with no action around here.

Here is where we're sitting currently. Everything is bolted down and it's a solid roller.

The OG front master cylinder is pretty crusty and stuck so I opted to just replace it with a spare Nissin I had sitting around from an old yz125. The caliper sticks and probably should be rebuilt.

The chain/sprockets are together currently. Nice straight chain line and doesn't rub anywhere. Its sitting with 14/43 gearing right now but I did some maths taking into account stock final drive and tire diameter of both machines and I decided to bump that up to 15/50. New chain/sprockets on the way.

The sprocket situation was another thing that worked out surprisingly well in my favor. The Blaster runs a 520 and the MB5 a 420. One was going to need swapped. I looked into a 520 rear but the MB5 has a weird pattern and I had no luck. Looking through my junk pile I found an old front 420 sprocket from rebuilding my kx80, wouldn't you know the countershafts are the same spline. Turns out there are tons of options here because a most of the old Jap mini motocrossers used that same spline.

Still to do

-Fab up a rear brake lever

-Mount pegs donated from a kx250f

-Sort out the electrical system

-Clutch and throttle cables

-Figure out an exhaust system


Re: MB5 Engine Swapped Build

That's fucking wild

Re: MB5 Engine Swapped Build

This is sick, Cameron. Come show it off at a Moped Monday when you're done and life resumes.

Re: MB5 Engine Swapped Build

it really doesn't even look that crazy. If the cradle was painted, I feel like 80% of people who don't know MB5s pretty well probably wouldn't even notice.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account