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Author Topic: 1982 CB750F... The resto-not  (Read 33457 times)

Offline Jimbonaut

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Re: 1982 CB750F... The resto-not
« Reply #450 on: Mar 18, 2019, 12:53:31 »
Uncovered a bit of a mystery too yesterday.  After measuring the cylinder bores it looked like all of them were out of spec, and larger than the wear limit specified in the Clymers manual.  Although my first reaction was to get the hump knowing I was going to have to shell out on an overbore and new pistons, I then thought to check the piston diameter.  I hadn't removed the pistons from the con rods so that was the first job (no circlips took a dive into the crankcase, but boy did they try) - no drama. 

Then measured them.  Here's the table showing the piston diameter, and the corresponding bore diameter -



All the ring gaps were within spec (at least they were when referencing the stock ring gap specs in Clymers).  Could it be that the cylinders have already been bored and new, larger pistons installed?  I mean, I can't think of any other scenario really.  I don't know how stock pistons are marked, but the ones in my engine are marked "25" (albeit faintly) on the top -



and "425 8" on the side -



I know they're OEM as they're marked Honda on the other side -



Any ideas?

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Offline advCo

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Re: 1982 CB750F... The resto-not
« Reply #451 on: Mar 18, 2019, 13:10:08 »
I'd think that the "25" on the crown could possibly mean .25mm overbore which is first over. Can you check your measurements with the manual and compare? If you add .25 to the specs in the manual do the numbers make more sense? There should also be a piston to cylinder clearance value that should hold true even in the case of an overbore.

Offline Maritime

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Re: 1982 CB750F... The resto-not
« Reply #452 on: Mar 18, 2019, 13:14:29 »
You're consistant with .003 on each piston for clearance so they very well could be overbore pistons and the machine shop matches the bore to each piston when they do it. the ring gap is the ring gap no matter the piston size, the ring Diameter however would be larger than stock so see if you can find the 1 over sizes and that may be what you have already.  It would make sense that you got good ring gap measurements on the rings if this was a recent overbore.
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Offline Jimbonaut

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Re: 1982 CB750F... The resto-not
« Reply #453 on: Mar 18, 2019, 13:28:27 »
Thanks fellas - I've dug up what I can on the internet and it looks like the "25" marking on the top of the piston may well be the indicator that a .25mm overbore has been performed on the engine.  Haven't found anything conclusive yet, but things are pointing in that direction. 

I'll check out the specs again and re-jig the numbers allowing for the extra .25mm.  Hopefully all will point to these being over-size.  The compression on the engine was excellent (165psi across all cylinders) so I guess this could be part of the reason - the thing is I'm just not experienced enough to know (or look out for) the tells that the engine has been opened up and worked on. 
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Offline doc_rot

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Re: 1982 CB750F... The resto-not
« Reply #454 on: Mar 18, 2019, 18:24:01 »
the easiest tell-tale sign that an engine has been opened up is the gasket surfaces have already been scraped. I would run those pistons with zero worry, however new rings may not be a bad idea as you can see that carbon has been getting past the first ring. Not the end of the world, but not ideal either. And definitely get new circlips.
« Last Edit: Mar 18, 2019, 18:27:05 by doc_rot »

Offline Jimbonaut

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Re: 1982 CB750F... The resto-not
« Reply #455 on: Mar 18, 2019, 18:38:22 »
Cheers Doc, gotta say that's a huge comfort.  If you'd run these pistons with zero worry that's more than enough for me.  I'll look into a new set of rings - when you say circlips are you talking about the circlips on the wrist pins?  Why would they need to be replaced?  Metal fatigue?
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Offline doc_rot

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Re: 1982 CB750F... The resto-not
« Reply #456 on: Mar 19, 2019, 01:58:55 »
Cheers Doc, gotta say that's a huge comfort.  If you'd run these pistons with zero worry that's more than enough for me.  I'll look into a new set of rings - when you say circlips are you talking about the circlips on the wrist pins?  Why would they need to be replaced?  Metal fatigue?
yeah the wrist pin circlips. getting them out can cause them to loose spring tension. its not something you want to pop out while the engine is running. Its cheap insurance.

Offline Jimbonaut

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Re: 1982 CB750F... The resto-not
« Reply #457 on: Mar 19, 2019, 10:57:07 »
Was having a late night look at some photos of the head and cylinder block (before I honed them) and noticed something I'd missed before.  There's what looks like an 'overspill' pattern of carbon that is exactly the same on the top of each cylinder -



close up -



What would cause that?  Knowing now that the cylinders have - at some point, not very recently judging by what little info I got from the PO when I bought the bike - been overbored and the pistons switched out with .25mm larger ones, would this have happened after the piston switch?  I'm assuming yes of course, in which case what would have caused it?  Poorly fitted gasket?  Poorly honed mating surfaces?  Is it normal perhaps?  Very interested to learn more about this.
« Last Edit: Mar 19, 2019, 15:14:56 by Jimbonaut »
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Offline advCo

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Re: 1982 CB750F... The resto-not
« Reply #458 on: Mar 19, 2019, 10:59:11 »
Looks like it could be just an abnormally shaped aftermarket head gasket. Since there is no blowby it wouldn't be caused by a leaky head gasket or bad mating surfaces.

Offline Jimbonaut

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Re: 1982 CB750F... The resto-not
« Reply #459 on: Mar 19, 2019, 11:05:33 »
Blow-by...what exactly is that?  Oil that gets past the rings?
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