Topic: Piston pin wear/clearance

[andrewk]

Hi all-

I'm in the process of doing a "budget" (there's no such animal!) rebuild on a K5 engine I've got.  The bores were pretty good, depsite a very small amount of water damage, so I decided to deglaze/hone the cylinder and install new rings. 

When I removed the pistons from the rods, all four piston pins were galled.  ^%$#!#.  The small ends of the rods all look to be ok, but I'm a bit concerned about the rod on cylinder #1-  It was galled the worst, and it has the most pin clearance of the four rods. 

My question is this-  In a practical sense, how much pin clearance is too much?  I have HondaMan's book, and his number is .02mm.  I think I'm going to be around .035mm, with my best used piston pin.  I've ridden alot of bikes that ran damn hard with some outrageous clearances, but I've never had piston pin clearance issues before.  Should I be trying to replace ONE rod in this thing?  Or am I never going to be able to tell the difference?

Thanks for any thoughts...

[lucky]

We would have to see the pistons.
All pistons end up with some galling or scratches.

Is the piston pin difference with the rod end,  OR is it with the piston???
Did you measure the piston to wall clearance on the thrust face of the pistons and cylinders?


How many miles on this engine?

The piston pin difference is .0006 or.0003 on each side. Piston or rod?
Otherwise about 1/2 thousandths.

[wrenchmuch]

If you have galling on the pin the rod is likely damaged as well. You could have a bronze bushing installed in the rods to correct this or search for different rods. If you replace one rod be sure it has a weight stamp the same as the others. This is an ink stamp on the rod big end. Numbers are big end tolerance and letters are the weight code.

[lucky]

If you have galling on the pin the rod is likely damaged as well. You could have a bronze bushing installed in the rods to correct this or search for different rods. If you replace one rod be sure it has a weight stamp the same as the others. This is an ink stamp on the rod big end. Numbers are big end tolerance and letters are the weight code.

He said the galling was on the piston.

[wrenchmuch]

Read again. He was talking about the piston pins.

[lucky]

Sorry, YOU ARE RIGHT wrench much.

He said all four piston pins were galled!

That is very unusual. I would like to see a photo of that.

I would have to see a photo of that.

If all of the wrist pins are galled how can the rod ends be ok??
Those steel wrist pins are much harder than the bronze bushings.
Makes no sense.

[andrewk]

Here's a picture of the pins.

Stock 750 rods have no bushing in the small end of the rod- it's steel on steel.  Not trying to dig on you, but as a proclaimed "worked in a honda shop" guy, how do you not know this?

This situation isn't that uncommon on an engine that has been left to sit unprotected.  This galling is almost certainly the result of moisture accumulation.

My measurements were off a bit, I'm about .0005 inches out of spec, but my measuring equipment is less than precision- I've only got a good caliper to work with, not the ideal tool to make the inside measurement.  I cleaned up the small ends with some 400 grit, and it will live just fine with a different set of piston pins.

[lucky]

Very unusual. Can you feel those scratches with your finger?
Something must have gotten into the oil.
OR check that plug in the crankshaft and see if there are metal filings and debris inside the crank oil journals.

Those dark bands are where the wrist pin has got very hot.
Maybe it was run low on oil?
As far as forgetting about steel rods and steel wrist pins It has been a long time.
You are right.
Usually there are bronze bushing in the small end.
When I was working on those CB750's none of them had a major failure yet.
Most dealerships that sell new vehicles also do not get into major problems the first few years of a new model.
When the vehicle gets older the smaller shops run into these serious problems and serious repairs. That is why I liked working at the smaller shops. I learned more and sometimes knew more than a dealership mechanic about certain problems.
I guess only good rods have replaceable bushings.

[wrenchmuch]

http://item.mobileweb.ebay.com/viewitem?itemId=230848822896&index=24&nav=SEARCH&nid=75472726693

http://item.mobileweb.ebay.com/viewitem?itemId=130657865555&index=3&nav=SEARCH&nid=33048366826
Here are a couple listings for pins. Lots of these motors go a long way with these rods. Moisture is the usual culprit for this kind of damage. Try and measure the small end properly before putting them back into service.