Koban piston – conrod assembly

[sputnik16335]

Hi all-

The Koban project has taken a few lurches forward lately. With the help of the forum, I’ve managed to find a set of pistons. They are the correct size, and it looks like all of the enbloc Koban pistons interchange. These fit the bores, but they’re pretty snug. They seem to need a lot of force to move them in the bores of my power head. The bores in my cylinders are very nice, and I honed them a bit to slick them up. The gap on the rings seems to be OK, too.

My question is, can anyone steer me to someone who can resolve the piston fit? At the same time, I should probably have the conrod-to-crankshaft fit checked. I’m looking for someone who knows about old motors (I’ve had some really bad experiences with machinists, one of which really made a mess of a nice old upright Maytag).

I’ve managed to find most of the other parts, but I still need some details and parts for the linkage from the tiller to the rudder…

Moving forward at glacial speed-
Thanks
Rich R

[pm-t2]

regarding the piston fit – I’d suggest measuring the bore with a telescoping gauge and micrometer, then measure the piston skirt to verify the clearance. Using a rule of smashed thumb, I usually suggest it should be in the vicinity of 0.002" of clearance per inch of bore diameter.

if the piston to bore clearance checks out, look at the radial thickness of the rings next. The gap might be correct, but if the radial thickness is excessive, the sealing surface of the ring will be proud of the ring groove surface.

Checking the fit of the rods is a simple enough thing to do. With the rod removed from the piston, clamp the rod onto the crank and tighten to torque spec. With the crank laid into V-blocks (or held with the crank axis parallel to the floor) put the rod at the one o’clock position. If it falls quickly to the bottom and swings back and forth, its too loose. If it doesn’t fall at all, its too tight. Proper fit for operational purposes is attained when the rod swings from the one o’clock position down to six o’clock at a controlled rate, and doesn’t flop around loosely at the bottom.

Hope this helps.

best,
PM T2

[The Boat House]

• This reply was modified 5 years, 5 months ago by The Boat House.
• This reply was modified 3 years, 2 months ago by The Boat House.

[garry-in-michigan]

If the wrist pin were pushed in the wrong way. the piston may be out of round ?

Evinrude pistons have one side marked ‘loose’ – that is the side to push on to keep from distorting the piston. Full floating pins are ambidextrous. . . 😕

[The Boat House]

quote Garry in Tampa:

Full floating pins are ambidextrous. . . 😕

Good one Garry. Made laugh again. 😆 😆 😆 😆

[sputnik16335]

quote PM T2:

regarding the piston fit – I’d suggest measuring the bore with a telescoping gauge and micrometer, then measure the piston skirt to verify the clearance. Using a rule of smashed thumb, I usually suggest it should be in the vicinity of 0.002″ of clearance per inch of bore diameter.

if the piston to bore clearance checks out, look at the radial thickness of the rings next. The gap might be correct, but if the radial thickness is excessive, the sealing surface of the ring will be proud of the ring groove surface.

Checking the fit of the rods is a simple enough thing to do. With the rod removed from the piston, clamp the rod onto the crank and tighten to torque spec. With the crank laid into V-blocks (or held with the crank axis parallel to the floor) put the rod at the one o’clock position. If it falls quickly to the bottom and swings back and forth, its too loose. If it doesn’t fall at all, its too tight. Proper fit for operational purposes is attained when the rod swings from the one o’clock position down to six o’clock at a controlled rate, and doesn’t flop around loosely at the bottom.

Hope this helps.

best,
PM T2

Thanks for the advice. Relieved to report that everything checks out OK dimensionally using your recommendations. I also checked the dimensions on the donor block’s cylinders and crank journals and they match my project motor (I believe the donor power head came off a running motor).

I did find some carbon buildup in the ring grooves under the rings. I think that might be a big part of the problem considering the width of the rings. I think cleaning up the grooves and some more bore polishing might help a lot. I’m sure I got all of the grit from the honing stones out the last time, but I’ll make sure I really scrub it this time.

Thanks!
Rich R

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