#6

[speedpro]

Pressed the good crank apart tonight, thanks Rob and ESE, and it still looks good. Obvious wear but nothing to worry about. I'll order some flat silver plated bearings and new thrust washers and put it back together.

I've pressed a couple of other MB100 cranks apart as well. As a minimum they'll get fresh bearings and thrust washers. If anyone is interested I could build up custom cranks in exchange for your old one. I can rebalance them so they're usable in a motor without the balance shaft just like in my 30hp engine, and/or make one with a longer rod. I've found someone I trust to do a good job making whatever crankpins I need as well so if there is enough demand I could get a batch made. On that note I still think using a rod kit with say a 22mm crankpin and offset boring the crankweb holes to increase the stroke 2mm is a good idea and then using kart pistons. You'll be stuck with aircooling and a 24mm carb but it doesn't seem like it would be too big an issue. Cranks could be supplied with new main bearings as well, C3 unless you have a preference.

It also looks like I will be making 2 watercooled heads. If anyone is interested let me know and I'll get the lot done together.

One of the bikes I'm building needs a good pipe which I'll probably go to Wobbly to design and lazer cut. Not too sure about the final spec yet but probably aiming for say 25hp at 11-12,000rpm. I've tried "my" Wobbly pipe for the 30hp engine on this bike in the early dyno runs and it made 2hp more through the mid range and maybe 1hp more at the top and it was completely mismatched to the motor so a more compatible design can only be better.

I'm feeling better after the GP now, getting busy in the shed, working on my scooter as well. Until the wife finds out anyway.

[F5 Dave]

So to get to 104cc with a 52mm piston you only need to destroke 0.6mm or offset 0.3, or is memory serving me badly? stroke is 49.5 stoke, so 48.9 is under 104cc.

Is it feasible to weld & rebore 0.3 off. Not taking much out of the centre if you don't put a 22 pin in it. But 20mm pin leaves you with a 100mm MB rod unless can find something else decent.

But yes let me know, I need to get something sorted for my next bottom end.

[kel]

I'm feeling better after the GP now

Me too. well almost

[bucketracer]

So to get to 104cc with a 52mm piston you only need to destroke 0.6mm or offset 0.3, or is memory serving me badly? stroke is 49.5 stoke, so 48.9 is under 104cc.

Is it feasible to weld & rebore 0.3 off. Not taking much out of the center if you don't put a 22 pin in it. But leaves you with a 100mm rod unless can find something else decent.

But yes let me know, I need to get something sorted for my next bottom end.

Volume = PiexR2xH ... volume equals pie times radius squared times height.

Volume = PiexD2/4xH ... because R2=D2/4

Bore = 52mm and Stroke = 49.5mm

To get the answer in cc use cm ie 52mm = 5.2cm

Pie x Bore x Bore x Stroke all Divided by 4 = Volume

(3.1416 x 5.2 x 5.2 x 4.95) / 4 = 105cc

Re arranging for stroke, volume (0.104L) in liters to get mm

(4 x 0.104) / (3.1416 x 5.2 x 5.2) = 48.97mm or 49mm

So 49mm stroke and 52mm bore gives you 104cc

And a 48.5mm stroke gives you 103cc

[F5 Dave]

Yes I can do the math, I couldn't reliably remember if it was 49.5 stroke as I was bashing away. [edit, yes it is]


The maximum capacity for rebored engines shall be:
F4 2 stroke 55-100cc - 104cc

so can run up to 104cc

52 bore & 49 stroke gives a knats bollock over 104 which is why I suggested 48.9mm

[speedpro]

My offset pin gives a 48mm stroke allowing pistons up to about 52.7mm to be used. .75mm offset = 1.5mm change in stroke. With it in so stroke is increased and with larger pistons that will fit capacity goes to about 115cc or a bit more if you want to push limits with the thickness of the cylinder

[F5 Dave]

Are there any KT pistons that go more than 52? Thought they were clamped on size bar tiny clearance versions

Has anyone tried weld & bore?

[jasonu]

Yes I can do the math, I couldn't reliably remember if it was 49.5 stroke as I was bashing away. [edit, yes it is]


The maximum capacity for rebored engines shall be:
F4 2 stroke 55-100cc - 104cc

so can run up to 104cc

52 bore & 49 stroke gives a knats bollock over 104 which is why I suggested 48.9mm

But if you do all that hard and expensive work and later need to rebore would you not be fucked as you will go over the allowable 104cc limit unless you get rid of the water cooling and get a 24mm carb?

[F5 Dave]

or you just plate it. Apparently the NZ place does steel, but US chrome did my 50 about 7 years ago.

[husaberg]

Volume = PiexR2xH ... volume equals pie times radius squared times height.

Volume = PiexD2/4xH ... because R2=D2/4

Bore = 52mm and Stroke = 49.5mm

To get the answer in cc use cm ie 52mm = 5.2cm

Pie x Bore x Bore x Stroke all Divided by 4 = Volume

(3.1416 x 5.2 x 5.2 x 4.95) / 4 = 105cc

Re arranging for stroke, volume (0.104L) in liters to get mm

(4 x 0.104) / (3.1416 x 5.2 x 5.2) = 48.97mm or 49mm

So 49mm stroke and 52mm bore gives you 104cc

And a 48.5mm stroke gives you 103cc

Gee that sounds like a lot of hard work.

Bore squared x.7854 x stroke/1000=cc

[speedpro]

Are there any KT pistons that go more than 52? Thought they were clamped on size bar tiny clearance versions

They go out to 53mm plus the local shop has other pistons I've seen up to 53.75, there may be others. The genuine Yamaha and the KSI brand pistons are available in 51.9?mm to 53mm in .05mm steps.

Turns out the kart guys change their KT100 pistons every 5-6 hours, the fast guys do anyway. There are other brand pistons they have as well including some lovely ones with Dykes rings and coated skirts.

With the small increments you just hone the cylinder and fit the next size up piston. Generally you fit the next size up again ring keeping an eye on the end gap. So a 52.15 piston would be installed with a 52.2 ring.

[husaberg]

Strike do these are the J pistons too BTW. Not the ugly "s" ones.
they seem to go to at least 53.45mm
http://www.strikeproducts.com.au/pistons.asp
PJ002 - Piston KT100J Big

To suit Yamaha KT100J for speedway racing applications.

Sizes are made to order for a range between Æ52.0 and Æ53.0

Ring to suit is p/n PRS001

[Bert]

They go out to 53mm plus the local shop has other pistons I've seen up to 53.75, there may be others. The genuine Yamaha and the KSI brand pistons are available in 51.9?mm to 53mm in .05mm steps.

I have some vague recollection (talking to kiwikarts years ago) that they make them out to 56mm or something to make a 125cc kt's... might be worth asking about.

Turns out the kart guys change their KT100 pistons every 5-6 hours, the fast guys do anyway. There are other brand pistons they have as well including some lovely ones with Dykes rings and coated skirts.

yip that were I sourced a few of mine when I was a poor student / poor worker / I'm still poor but not using KTs anymore... Dyke ringed piston are/were generally European; HPV, Komet & Corsair.. never really saw many of them here in NZ. that remind's me: http://www.kartsandparts.ca/store/home.php?cat=37 some interesting options worth considering.

With the small increments you just hone the cylinder and fit the next size up piston. Generally you fit the next size up again ring keeping an eye on the end gap. So a 52.15 piston would be installed with a 52.2 ring.

That real important for best performance; and also really important to check and adjust the end gap of the oversize ring; they tend to make a little bit of a mess of your exhaust port when they heat up and pop out

[F5 Dave]

Strike do these are the J pistons too BTW. Not the ugly "s" ones.
they seem to go to at least 53.45mm
http://www.strikeproducts.com.au/pistons.asp
PJ002 - Piston KT100J Big

To suit Yamaha KT100J for speedway racing applications.

Sizes are made to order for a range between Æ52.0 and Æ53.0

Ring to suit is p/n PRS001

That picture you have seems to have the pin about 4" from the crown.

[husaberg]

That picture you have seems to have the pin about 4" from the crown.

Its a picture Dave, from the strike page. The pistons they sell are to J and s spec maybe the pics are round the wrongway?FIIC
But a j piston is a j piston.(What ever the pic shows) An s piston and is a different beast. but yes it looks more j than s.
you could use them in your Non Honda Honda so it would have even less Yamaha parts

What the fuck shit how did i do that an s piston is the STD model kart isn't it . so f'ed that one up Good and proper whoops.

Start again whoops an a f-ing big way

this is the beast you want.

Looks like they will do custom to suit.

PS003 - Piston KT100S STD 53.75 and above


To suit Yamaha KT100S and ARC SPEC100 for AIDKA and Speedway racing applications

Sizes are made to order for a range between Æ53.75 and Æ55.0

Ring to suit is p/n PRS002

PS004 - Piston KT100S Special


For custom applications for AIDKA and Speedway racing applications

Custom options include:


High Ring

Rotated Anti Rotation Pin for reeds and rotaries

Dykes rings


Sizes are made to order

Rings to suit are dependant on application

PSS001 - Piston KT100S Smooth Finish


Smooth piston skirt finish

To suit Yamaha KT100S and ARC SPEC100 for AKA racing applications

Size increments 0.05mm between Æ52.0 and Æ52.75 then 0.01 increments above Æ52.75mm

Ring to suit is p/n PRS001

Drugs don't do them kids