ESE's works engine tuner

[adegnes]

A gas column with a smaller cross section has to accelerate more rapidly in order to transfer the same amount of air/fuel mass as a column with a large cross area.
But the rate of acceleration depends on the pressure difference between crankcase and cylinder, which in turn depends on the blowdown time.area and the efficiency of the pipe. At high revs, when the available time for transfer becomes shorter and shorter, small transfer cross sections may be less than desirable.

Sounds like a win for short and fat to me!

As always its impossible to say what would be a good mod if only half the info is available.
Grinding or epoxying would depend entirely on the case com ratio.
If its already way too small then grinding is needed as the case com ratio would have a way bigger positive effect than the cross sectional change may be a downside.
But that cylinder for sure needs a big radius on the duct/bore edge.

I know, I'll be back with the numbers when I've got the cylinder in hand. And yes, those sharp edges are ugly, I don't get why so many of the manufacturers do it like that.

[Frits Overmars]

A gas column with a smaller cross section has to accelerate more rapidly in order to transfer the same amount of air/fuel mass as a column with a large cross area.
But the rate of acceleration depends on the pressure difference between crankcase and cylinder, which in turn depends on the blowdown time.area and the efficiency of the pipe.
At high revs, when the available time for transfer becomes shorter and shorter, small transfer cross sections may be less than desirable.

Sounds like a win for short and fat to me!

That depends. Short and fat will fill the cylinder in a hurry. It also means the crankcase pressure will fall in a hurry. At revs that are too low for this setup, the transfer flow may come to a stop and then reverse before the transfer ports close.
Moreover, the positive exhaust return pulse that arrives too early at 2/3 of max.torque rpm, can do more harm because the lower inertia of short transfer columns offers less resistance to being shoved back into the crankcase.
Both these disadvantages can be balanced to some extent by lowering the transfer timing. But this is really a balancing act: lower the transfers too much and you loose the advantage of the short, fat transfer columns.
What you certainly don't need, is a combination of short, fat transfer ducts and a long transfer timing.

Between you and me: this may be the second reason (apart from upsetting the scavenging balance) that the Aprilia RSW and RSA cylinders respond so poorly to enlarging the entry of the B-transfers. Their inflow can certainly be improved, but that would reduce the column inertia and their timing is already 132°...

[adegnes]

Thanks to all of you! I'll shurely be back for more advice when I get the cylinder...

[husaberg]

hey guys has anyone attempted to move the piston rings locating pegs with any sucess ? i heard people do it but it failed. surely there must be a way

Well they aren't cast in there so the manufacturers manage it. Talk to a toolmaker.

Pete used to tap the original pin into the piston so it didn't interfere with the ring and then drill another hole in the new position and insert a pin he'd retrieved from an old piston. You just have to make a hole with the desired press fit for the pin, OR, do like others do and press it in from on top into a 2nd larger hole below the ring land and bend the end of the pin so it can't come out. Depending on the method you will need to get the right rings.

me at 17 was cutting pins out of old pistons and setting them up in rg50 pistons for my rd50 , never had a promblem , get a old piston and get some practice in

Pressing the pin in from above and then bending the lower end is the tried-and-trusted Yamaha method. But it requires a rather soft steel for the pin, or the bending force might cause loss of press fit in the vertical bore.
Attachment 310193Attachment 310194

Emot recently developed a variation on the theme: the pin is pressed in from above, after which the top of the vertical bore is closed by welding.

I have relocated pins without problems. I used the same method as EMOT. Just ensure that the top end of the pin is 1- 2 mm below the piston crown.

The pin was a needle 1.5mm dia out of a little end bearing into a 1.45mm ish hole. You may need to use a mill file to dress off the outside of the piston at the position of the pin after fitting the pin .

No need to demolish bearings or old pistons in order to obtain pins. One piano or guitar string will provide you with hundreds of pins in any diameter you want.

Attachment 311486
Posted By Ken in another thread

Our AR pins are 8 mm long, and Ø1.54, tapered at one end and have a longitudinal groove to prevent hydraulic locking should the drilled hole have coolant remaining. Using these, the hole should be blind to prevent any combustion pressure leakage via the groove.
.

[peewee]

that reminds me, i found a machine shop that will relocate the ring pegs for me. probly send the pistons out sometime this week. according to what wiseco told me the pin is just regular carbon steel with .002-.003" interference fit

[peewee]

wobb you got any advice about the roller bearing behind the sprocket. would a NJ style work fine or NUP be better ? im sure you know the difference but incase you dont, NUP has the seperate collar on one side, collar integrated into the bushing on the other side, so theres a collar on both sides . NJ just has the collar on one side as part of the bushing

[Frits Overmars]

wobb you got any advice about the roller bearing behind the sprocket. would a NJ style work fine or NUP be better ? im sure you know the difference but incase you dont, NUP has the seperate collar on one side, collar integrated into the bushing on the other side, so theres a collar on both sides . NJ just has the collar on one side as part of the bushing

The Aprilia RSW crankshaft bearings initially had bushings with integrated collars. These collars sometimes broke. After switching to separate collars the problem went away. I'm not saying that the same thing will happen with transmission bearings, but you never know until it happens.

[jonny quest]

Look at parts diagram of a 2013 KTM SXF250. They use rollers for crankshaft and output shaft bearing.

[husaberg]

Look at parts diagram of a 2013 KTM SXF250. They use rollers for crankshaft and output shaft bearing.

Many KTM's from the 85SX up have at least one Roller bearing on the crankshaft. The easiest way to see is look in the Pro X Catalogue.
Quite a few use the same bearing size as the Aprilias including quite a few karts.
On the original Honda 100/125 Four Strokes they had a Roller on the Ignition side they deleted it to save costs, when they went to the 185 and 200's they then had to run bigger bearings.
I guess it was still cheaper than rollers.

Here is the crankshaft of the week.
Any ideas to what it is from?
According to the author he had never seen one with worn bearings.
The rollers ran in steel sleeves in the cases.

[marsheng]

Finally got my Maico on the Ashburton Dyno.

It is lean at 6k and just slightly rich at 9k. Where do I start to fix this, Exhaust, transfer ports, crank case volume? Does ignition timing change this at all.

Or is it just typical 2 stroke.

What we did find was that there was not much difference between 3/4 and full throttle. It is running 36 and so a 34 may be better, however, could the limitation be the reed block ? 36 does not seem overly large for a 250.

[speedpro]

I would go bigger in the main jet to get the mixture right in the middle and then go bigger on the air jet to lean out the top, with a recheck of the middle afterwards. Rob is the guru with this stuff. Presuming tests done at full throttle only so the needle is non-effective.

[Muciek]

Many KTM's from the 85SX up have at least one Roller bearing on the crankshaft. The easiest way to see is look in the Pro X Catalogue.
Quite a few use the same bearing size as the Aprilias including quite a few karts.
On the original Honda 100/125 Four Strokes they had a Roller on the Ignition side they deleted it to save costs, when they went to the 185 and 200's they then had to run bigger bearings.
I guess it was still cheaper than rollers.

Here is the crankshaft of the week.
Any ideas to what it is from?
According to the author he had never seen one with worn bearings.
The rollers ran in steel sleeves in the cases.

Jawa enduro engines had bearing like this, it was almost impossible to get that bearing here in 1980s....

[TZ350]

Finally got my Maico on the Ashburton Dyno. What we did find was that there was not much difference between 3/4 and full throttle. It is running 36 and so a 34 may be better, however, could the limitation be the reed block ? 36 does not seem overly large for a 250.

100-90-80-70% throttle position runs.

Its been our experience too, not much difference over 75% throttle. And a bigger carb often did not add much.



Back to back comparison on my Suzuki GP125 of a 24mm OKO carb (Red line) vis a 30mm (Blue line) one.

It is lean at 6k and just slightly rich at 9k. Where do I start to fix this, Exhaust, transfer ports, crank case volume? Does ignition timing change this at all. Or is it just typical 2 stroke.

I would go bigger in the main jet to get the mixture right in the middle and then go bigger on the air jet to lean out the top, with a recheck of the middle afterwards. Presuming tests done at full throttle only so the needle is non-effective.

Yes, that is what I would try, bending your fueling curve down by drilling out the air correction jet to get the 9K area right and plump up the mid range with a bigger main jet.

Anytime you drill out the air correction jet you will need to compensate with a bigger main. On our 125's the air correction Jet is about 1.2mm and the range seems to be anywhere between 1 to 2mm. Smaller engines seem to start about 0.8.

[husaberg]

Jawa enduro engines had bearing like this, it was almost impossible to get that bearing here in 1980s....

Which explains why I can't find a picture of one on the net either LOL
But no the crankshaft is not Jawa.
Pretty sure the Jawas did have nice long rods though.

[husaberg]

Well I’ve managed to engineer myself into a corner with SE bearings again. I have a 50.75 barrel I am running in the interim on my MB so I ordered a ye olde YZ100 piston to check out. Good luck finding online info on a ‘76 piston.

Arrived on my desk this morning so yet to check it at home.

Single 1mm ring that is pegged looking like far enough away from boostport not to have to move. Bit heavy as 3^rd oversize & suitable for single port only. But heck it will keep this interim cylinder working ok & is far & away better than the MB one.


Whoops, 16mm pin. 14 is std. rod is 19mm ID. Yams usually run thin SE bearings, but 16x20 is their size (same as RD250 apparently so don’t yell foul yet).

I forgot to order a pin, it was a mission enough to find rings & piston from same seller who would ship. But that should match up with something else easy enough or be shortened.

So can someone point me in the direction of a reasonable resource (can’t seem to find a decent site, maybe I should try PeteSalesbearingcatalogue.com?) for finding if a 16x19x## bearing is indeed made? 1.5mm rollers? Mammy!

Looks like Jawa does Dave. not that it helps you now
Better late than never though.
http://www.jawashop.com/Crankshaft-and-Piston

http://www.jawashop.com/Detail/10999
16mm 19mm 20mm little end bearings