Piston question

[Phavas]

I've got a Franken-motor: Two halves from different engine - both 1100's.
Smokes like hell from the left side. Took off the head and looked at the bores - all good but the piston seemed small in the cylinder; I could see the rings. I checked the pistons that came out of that engine and they're stamped 25 on the crown and measure exeactly 75mm. My assumption is that these are 0.025 oversized and my oil burning is coming from standard pistons in an overbored block. Does Honda mark piston weights with letters like everybody else? Or does the 25 signify a weight grade?
Hard to discern the piston slap from all the other monkey motion noise in the rig.
Standard bores are 75 so pistons must be smaller; Yes? No? Maybe?
Can anyone shoot this theory down?

Conventional wisdom?

Bores should mic out at 0.026 / 0.030 over? I'm used to running .001-.005 piston to bore clearance on German and British bikes, 0.002 on Jag engines. Anyone had any experience with mis-matched pistons on Goldwings? I don't think the overbore is going to cause any unbalance in the engine so I'm just going to slap in the bigger pistons on one side (I know, I know I can already hear the howling from the "crankshaft out of balance" bunch) but this engine is old and I'm not increasing compression. (165 all around).

Any input is appreciated including advice to run for cover when it starts.

Cheers,
Zaphod

[otto1]

You didn't say if you measured the piston and bore before installation or if the ring gap is with in acceptable range.. IF piston and bore and ring gap measured within spec then your oil burning may be from the oil rings. I had a thumper I rebuild and when I started it it smoke like hell. I tore it down and found that the spring between the two oil rings had overlapped on the ends so there was no tension there once in the bore. I fitted it correctly and put it all back together and oil burning was gone. I still had good compression evn with the compromised oil rings. If you have compression as you stated then oil burning may be from a valve guide seal improperly installed.

let us know what you find out.

Paul

[Fatwing Chris]

I was told(by Honda) that an oversized OEM piston was exactly the same weight as stock.I had both RH cylinders bored and installed oversized pistons with no ill effects on my 81 1100 Int.

[Phavas]

Thank you both.
I mixed too much wine with the rebuild lube I guess.
No. I didn't measure anything when I slapped the beast back together which is why I suspect the right cylinder block is bored out more. The only thing I remember was that there was a broken scraper ring, which I replaced. No, I didn't gap anything: it fit together so I assumed (yeah, I know) it was good. I didn't notice the numbers on the crowns until I cleaned the pistons afterward.
Interesting comment about the piggy-backed oil ring retainer. I will check that when I pull it apart again.
Any ideas on the "25" stamped on the piston crown? I suppose Honda were better at casting than the Brits so they didn't have to grind too much slag off and make "matched" sets (we found some about the same weight) ergo I will also assume the pistons weigh the same regardless of their oversize.
Cheers,
Zaphod

[otto1]

Education is never cheap

[Phavas]

Duh....
Zaphod Ph.D.