Piston question


Technical information and Q&A applicable to all years and models of Goldwings
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Phavas
Posts: 60
Joined: Mon Feb 10, 2014 10:02 pm
Location: Maine
Motorcycle: 1976 GL1000 x 2, 1977 GL1000 x 3, 1981 GL1100 Sidecar, CB750 K8 CB 550 K8, 1954 Velocette MSS, 1937 Velocette KSS, 1932 Norton 16, 1946 AJS 18, 1948 Vincent Rapide, BMW R 100 RT1, BMW GS 80, BMW R60/7, 1936 Halcyon 50, 2003 Triumph Trophy 1200, 1994 Honda CB750 twin cam and still looking...

Piston question

Postby Phavas » Sat Dec 06, 2014 9:19 am



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


Writing roughshod over the English language

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otto1
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Location: rockville, md
Motorcycle: 2000 gl1500SE
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Re: Piston question

Postby otto1 » Sun Dec 07, 2014 1:03 pm

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

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Fatwing Chris
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Re: Piston question

Postby Fatwing Chris » Sun Dec 07, 2014 2:57 pm

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.
If I'da known it would last this long,I'da taken better care of it.
Chris
Double Dark
Darkside # 1602

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Phavas
Posts: 60
Joined: Mon Feb 10, 2014 10:02 pm
Location: Maine
Motorcycle: 1976 GL1000 x 2, 1977 GL1000 x 3, 1981 GL1100 Sidecar, CB750 K8 CB 550 K8, 1954 Velocette MSS, 1937 Velocette KSS, 1932 Norton 16, 1946 AJS 18, 1948 Vincent Rapide, BMW R 100 RT1, BMW GS 80, BMW R60/7, 1936 Halcyon 50, 2003 Triumph Trophy 1200, 1994 Honda CB750 twin cam and still looking...

Re: Piston question

Postby Phavas » Mon Dec 08, 2014 11:10 am

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
Writing roughshod over the English language

User avatar
otto1
Posts: 25
Joined: Thu Oct 30, 2014 9:01 am
Location: rockville, md
Motorcycle: 2000 gl1500SE
2003Yam wr250
2004 GasGas TXT125

Re: Piston question

Postby otto1 » Tue Dec 09, 2014 1:23 pm

Education is never cheap ;)

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Phavas
Posts: 60
Joined: Mon Feb 10, 2014 10:02 pm
Location: Maine
Motorcycle: 1976 GL1000 x 2, 1977 GL1000 x 3, 1981 GL1100 Sidecar, CB750 K8 CB 550 K8, 1954 Velocette MSS, 1937 Velocette KSS, 1932 Norton 16, 1946 AJS 18, 1948 Vincent Rapide, BMW R 100 RT1, BMW GS 80, BMW R60/7, 1936 Halcyon 50, 2003 Triumph Trophy 1200, 1994 Honda CB750 twin cam and still looking...

Re: Piston question

Postby Phavas » Tue Dec 09, 2014 5:11 pm

Duh....
Zaphod Ph.D.


Writing roughshod over the English language


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