First build... CPI Oliver

[waltinhawaii]

Thanks Rune, done that. But I have reservations about hitting the crank too hard, the puller gets around that issue without the potential for causing any damage to crankpin alignment. And my bearing to case fit in this instance appears to be super tight, so I'd have to hit kinda hard to get them to move.

[waltinhawaii]

I gave up on my idea of using the case separator and like a good friend, pirated John's idea of using a piece of threaded rod and whipped up a puller with 10mm on one end and 12mm on the other for easier future builds. WOW, one half twist of the puller (with a little black pipe in between as a spacer) and the crank eased right up and is turning freely: (note: the only 12mm nut I found in my box of old junk was a castellated nut, so it looks a little strange welded onto the threaded rod):


installed the oil seal on the flywheel side. Now comes decision time... there was a plastic gear, metal plate to hold it in place, and oil injection pump which I now don't need, and an external C clip to hold it in place:


Since I don't plan to run an injector, do I need to install any of this? I don't see any need to... if any of you have chucked the oil injector and run into problems with leaving the C clip out, pls advise! I also note that folks fabricate a cover to go over the injector hole; is that necessary? Looks like it would only be cosmetic. But, no problems with the oil seal 'walking' out or anything if I leave all this out?

Intake cutout:
I put the piston right at where it closes over the boost port, so at this point all gas flow into the cylinder above the piston stops, so holes in the cylinder or piston don't do any more good for the current charge (but notice the next charge is still picking up air as the piston rises)

This will be the limit of the piston skirt-to-intake cutout. More cutout won't do any good (I think... John, feel free to chime in here). However, it can be wider so that the charge can move in easier. I decided to widen it by .250" each side:


Next comes the piston windows, these were slightly moved toward each other to narrow the bridge in between, but narrowing it too much will decrease the bearing surface of the piston skirt against the back wall and promote quick wear and increasing 'rocker' of the piston so cannot be cut too much. this is why I limited the intake cutout a little lower down as inconsequential to intake charge. As the piston moves higher, case vacuum rises, but the space behind the piston can now be fed by the boost port through the piston windows, so does not need the intake cutout to be any higher.


I decided to remove about .050" or a tad more from each side (sorry, didn't measure the bridge but it will be about .100 narrower):

[waltinhawaii]

Oh, I also cut the starter ring on the left side of the case and was going to remove it; but if I only have a 'normal' hoca variator for now, do I have to leave it in? or can I remove the starter, starter clutch, and ring now?

[ryan]

walt id like to see what you come up wit hon the windows. you got a good point

[ryan]

also unless your running the OR kit you would have to mock up a spacer just to run the vari by itself

[190mech]

Junk all the oil injection stuff!The seal holder is good to keep though..
Modern cylinders hardly have a rear cylinder skirt,I think the thought is for better flow from the reed induction;
www.ebay.com/itm/Cylinder-Piston-Ring-Top-End-Yamaha-YZ125-YZ-125-05-12-/271269938455?pt=Motorcycles_Parts_Accessories&fits=Model%3AYZ125&hash=item3f28f2c517&vxp=mtr
I feel also the major thrust on the piston skirt is on the exhaust side,so a bunch of rear skirt is not really needed..
Something to look at is how shrouded the transfer tunnels are when the piston just opens the transfer ports,often those long cyl skirts on the tunnel areas need to be relived,rounding the edges of the piston in that area helps flow also..
Love your crank puller,much nicer than mine!!

[waltinhawaii]

John, yah; I set the piston to BDC and the cutouts on the sides were even with the transfers so I didn't bother to modify the piston skirts in that area at all. I'll check again, though. I used to cut the top of my piston skirts a bit, but I noticed a ring of very brightly polished metal on the end of the top of what remained of the piston skirt when I did that, so I know the piston was rocking a bit much. I'm gonna test it like this for now, can cut it later on if need be.

thanks for the idea of using a threaded rod! much easier than trying to drill and tap my stupid case puller, which is where I was before I asked you about yours... much better idea.

[waltinhawaii]

Random ramblings... this is the boost port.


It has 2 functions: to clear the bowl (in older designs like ours) AND to provide a port in back of the piston crown through which gas can flow into the cases via the windows in the piston to charge the case for the next cycle. This is important as it is available to fill the case when the case vacuum is highest, nearest TDC. My boost is a bit small for my liking, but I'm limited by this:


If you look carefully you can see the 2 little pins in the ring grooves located at about 10 o'clock and 2 o'clock. They measure about 1.200" apart, give or take (chord width). I then took a pair of calipers and set them to the width of the boost and compared the 2:

So technically I could widen the boost port to near this width, but as you get closer you run into greater risk of snagging the ends of the rings. Still, this does allow for widening the boost substantially. The only problem.... the boost is set at an acute angle to the cylinder, not like the transfers at all. It is a tough angle to duplicate. Not sure if I want to mess with that... will mull it over for a bit.

[190mech]

What about widen the B ports?They do more work than the C(boost) port..

[waltinhawaii]

Exactly right, John! ...but I didn't want to invest $300 in a cc specialties porting tool, I only figured on the 'quick and dirty' approach getting me enough grin for the $$ that I'd be content. My porting head is a little too big to get into the B ports at the correct angle to make them shoot straight, afraid to touch them too much.

I settled on widening the boost port just a touch, about .050-.060"; very conservative; and plan on adding a generous chamfer to make up about a .100" increase including chamfer; shouldn't give me any ring problems at all. Lots more real estate in that area to take advantage of, but I'm flying blind here. Until I actually set the piston into the cylinder and mount it, I have no idea, really, how close I'm getting to the ends of the rings. since the cylinder can sort of 'swim' back and forth on the studs, I suspect I'd better not cut too much.

[190mech]

I bought a 90 degree Dremel head in hopes of having a cheap transfer porting tool,too long,,so I decided to make it shorter..Tore it apart and looked at the design,nice spindle,gears and bearings,just oversized to give the plastic housing some help,a nice steel or aluminum head with the guts out of it would make a nice unit..I dug up an old fried Dremel and cut the shaft/chuck off and used it to shorten the unit so it could fit a 52mm bore with short stones;;

[waltinhawaii]

Nice save, John. I had heard about the dremel 90 degree head too, never saw one till now. Beefy! I had to cut the shafts of my bits, they stuck out too far to use in the cylinder. I gave up using stones, go through them too quick; but they do give a superior finish and don't 'grab' as much as the carbide bits. My head is smaller than yours but I still can't get the aux transfers to shoot toward the rear of the cylinder, the width of the tool means the port ends up shooting mostly straight into the middle.

[190mech]

Thanks Walt!What about a ball or football type cutter and sweeping the tool up and out to angle the port to the rear?We can never achieve this but we can sure try;


From PitLane:RSA125 thread

[waltinhawaii]

Hahahaa, just buy the rsa and be done with it! You know you won't rest until you get one. And it will probably be less labor intensive than cutting the sides of our cylinders off to get at those damn transfer tunnels to reshape them how we really want them to be. I smell another frankenstein build in John's future, if only he could find some cases with good metal!

I widened the boost very slightly, you can hardly tell the difference, I probably shouldn't even have bothered:

[waltinhawaii]

Sigh. can't find my (%**$($($!!! circlips. How stupid is that?? Well, might as well split the trans now since I might as well order that gasket and any needed bearings in the trans at the same time as the circlips. Also need a jetting kit. Does anyone have a link to GOOD circlips (the kind that don't break off and become shrapnel to shop peen your head later) and a link to trans gaskets? I have a long case.