Yamaha or Honda piston

[nseagoon]

I have an old Honda MB50 bucket I've been rebuilding.
We blew up the main bearing, smashed the piston and setting up the carb had been a dog.
It was basically a barn find.

Most things I've rebuilt or found someone who could help

I'm having difficult finding a correct size piston for it.
None of the sources I've used have the factory diameter so i don't know how much oversize to get. Also prices are a little up there for somethingthat might not even run.

I'm trying a different route. The piston that was in it when it blew up is a Yamaha Izumi.
It has the number 17 inside the piston and 50 on one side of the piston face and a K and possibly a 2 on the other side.

Any ideas what yamaha this might have been made for?
Otherwise anyone know of another model bike that does a
40.5mm diameter
12mm pin
23.5mm from side of piston head to pin centre

Any good pointers appreciated thanks

[nseagoon]

A few more pictures of the wrecked piston.

[Yow Ling]

TZR piston maybe
http://www.ebay.ca/itm/Piston-Kit-Co...taW2gP&vxp=mtr

Minerali AM6 piston

RZ 50 piston, what about rg50 nsr50

just some ideas for you to research

[Grumph]

Check your compression height measurement - the DT/RD50 is 23.0mm and looks identical.
You've got a .50mm oversize piston which matches the Yamaha 40mm std bore.

[speedpro]

F5Dave is your man. Pretty sure he ran a Yamaha piston in his. don't take his advice on rebuilding crankshafts however

[mr bucketracer]

yes is yamaha whats in it , the rg50 piston would need a spacer made for under the barral , make sure to radius all sharp egdes on the piston , stops whats happen to it

[nseagoon]

cheers for the ideas and leads.
I had tried the Yammy RD50 previously but hadn't found the compression length.
the comment about the cylinder sleeve spacer was interesting. The difference between the easy to obtain pistons and the smashed yammy one is roughly 2.2mm

if I had a machinist mill out a 2.2mm sheet about the same shape as the sleeve gasket then RTV'd it to the sleeve would that affect anything, or risk damage to anything? I don't see there being any issues but I'm not a mechanic so don't have a clue.

in the mean time I noticed the piston in the ebay listing posted only has one ring, the one that blew up in my bike only had one ring, I've read it reduces piston drag. is the difference quite noticeable? is the main purpose of doing this to achieve higher RPM? or perhaps cycle through the rev range quicker?

[TALLIS]

Pretty much if you anit mixing gas you ain't slapping ass

[mr bucketracer]

Pretty much if you anit mixing gas you ain't slapping ass

if your mixing gas you are wasting cash

[speedpro]

cheers for the ideas and leads.
I had tried the Yammy RD50 previously but hadn't found the compression length.
the comment about the cylinder sleeve spacer was interesting. The difference between the easy to obtain pistons and the smashed yammy one is roughly 2.2mm

if I had a machinist mill out a 2.2mm sheet about the same shape as the sleeve gasket then RTV'd it to the sleeve would that affect anything, or risk damage to anything?

If using a different height piston some things you need to worry about are that each port opens at the right time and that the compression is about what you want. The only way to know these things is to measure them. A quick and dirty way to get in the ballpark is simply to get the squish back to where it was when you were using the original piston. That way the compression will be the same and the piston will be the same distance down the bore at the same amount of crankshaft rotation. If you go to a shorter piston make sure that at TDC the exhaust is not open under the piston into the crankcase.
The piston has jammed shrapnel between it and the squish band and not too big a bits by the look of it. This suggests squish clearance was not super loose but it also wasn't where it should be. On the MB100 you can run the squish down to about .7mm or you measure really carefully .65mm without problems. The 50 should be fine at that or probably a bit less.
There is no way to get it right without measuring.
Do NOT use RTV, use 3bond or Yamabond or something similar.

[nseagoon]

Well i managed to reinstall the mangled piston.
I measured 4 areas and had 4 significantly different results for squish. All over 1mm.

I also measured the length from cylinder wall down to side of piston at TDC.

I then did the results with a taller piston i have.
I couldn't measure squish but measured cylinder wall up to side of piston.

I then measured the height from top of pin hole to side of piston on both cylinders.

I reckon I might have two spacers made.

1-total length difference between cylinder wall height to piston

2-difference in height between pin and edge of cylinder.

Install one and measure squish
Then swap the other and repeat test.
Then see if i should have a different spacer manufactured all together.

If i shouldn't use RTV should i use a second cylinder gasket?
One above, one below the spacer at the bottom of the cylinder sleeve?

[speedpro]

I have used a gasket above and below a spacer and it worked fine. I did it to get the squish clearance where I wanted. If you are cutting out spacers start with a stack of various thickness alloy, including shim thickness in the middle. Bolt them all together with a base gasket then drill and cut and grind the whole stack at the same time. I find it useful to have a variety of thickness spacers when fitting cylinders to crankcases. There's enough variation in dimensions to make it useful when setting squish clearance. Of course it also alters port heights. Soft squishy paper gaskets aren't good if you are trying for tight clearances though used pre-squished ones are OK.
Over 1mm squish is rubbish. It needs to be a lot tighter.

[F5 Dave]

OK so yeah played this game.

I used RD50 piston. There were two types over the years. One had 1.2mm rings and could take the revs the mb50 piston never could without flicking off a ring tip. Power would drop off. With monotonous regularity. B
RD piston could take 12000 peak power + over rev no drama.

But 3 mods required. You had to run thrust washers either side of the pin as the Rd piston allows the mb bearing to travel sideways.
2 you have to trim the bottom so it doesn't hit the webs which means cutting the whole window area out. These days I'd rebuild with a longer rod. If you could use a yam one you might not need the thrust washers.
Next you need to move the ring peg. Send it to Pete at Total motorcycles in Palmy.

Set the barrel for 128 transfer and 196 exhaust and skim the top. 15:1 on 98 should be okay or 16 on Ave.

I welded on a jacket to the head and thermosyhnd (sorry spelling. Been drinking) with a rg radiator.


Much more from chassis modifications to make it actually behave.

Yeah and as Mike says I built a crank with Long brand rod. Not a good ending. Bit like the picture Mike uses for his avatar.

[cotswold]

[QUOTE=nseagoon;1131084519]I have an old Honda MB50 bucket I've been rebuilding.
We blew up the main bearing, smashed the piston and setting up the carb had been a dog.
It was basically a barn find.

Most things I've rebuilt or found someone who could help

I'm having difficult finding a correct size piston for it.

i used a KTM50 piston in my NSR50 engine, nice piston, 2008-2010 i think