1963 G80CS Top End Woes

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jack clegg
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1963 G80CS Top End Woes

Post by jack clegg »

Hello folks. Motor exhibited a rattle at low revs under load, so I thought it best to have a look inside. Good job I did. A myriad of problems & questions have arisen. Starting from the top:- Valve side Rocker arm ends a bit worn, a step formed on the inlet side. Don't seem to be able to find replacements with AMOC Spares or elsewhere.
Surprised to find coil valve springs fitted. Thought it would have hairpins. Not a real issue, but the inlet valve bottom collar appears to be sitting on the valve retaining circlip & is slightly askew. I presume it should really sit over the circlip & seat on the head, as the exhaust collar seems to do?
Piston, appears a bit sloppy in the bore. No real step or wear apparent in bore though. Measurements to be taken by a professional tomorrow, but I found ring gaps are over .030 thou so they are scrap.
Now for the real problem.
Gudgeon pin appeared stiff in the little end bush but loose in the piston bosses. I removed the circlips but the pin refused to move out, even with piston heated with a blowtorch. It appears a small burr had formed on both ends of the circlip grooves, more so on one side. I tapped it gently out of the lesser burred side. Little end bush appears scored/galled. Gudgeon pin is a tight push fit in the bush. Certainly not a running fit & will require reaming. The little end bush oil hole is at five o'clock, completely out of line with the top oil hole. I am unsure as to whether the bush has seized & moved in the rod, or just been fitted incorrectly by a goon. The piston (large domed HC type) also shows signs of running on the tilt, with a polished bottom edge on the nearside skirt & at the top edge of the crown on the offside. I would say that the only movement in the little end was in the piston bosses. I can push the pin back into the piston by hand, but as soon as it passes the circlip groove it suddenly loosens & is quite free to slop from side to side between the circlip grooves. Scrap piston too I think. Hope the rod is not bent.
There was no base gasket fitted, just blue Hylomar. No bottom seals on pushrod tunnels. I assume the two thinner O-rings in my gasket set are supposed to go over the tappet bosses at the barrel base?
That's all for now. Apart from the rattle, the bike went very well before all this. All advice gratefully received. TTFN Jack
John Marshall
jack clegg
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Re: 1963 G80CS Top End Woes

Post by jack clegg »

Minor update. The inlet valve bottom spring collar was fitted upsidedown, so now seats correctly with the recess over the circlip & not now an issue.
John Marshall
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robcurrie
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Re: 1963 G80CS Top End Woes

Post by robcurrie »

From what you described, I think the rod is bent, causing the gudgeon pin to move from side to side and hitting the circlips.

Rob C
Mick D
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Re: 1963 G80CS Top End Woes

Post by Mick D »

Hi

My take would be that the oil way being out of line has starved the little end of lubrication and caused it to seize, as a result of this the piston has been rotating around the gudgeon pin causing the wear you have.

New little end bush and piston / gudgeon pin should sort it.

Regards Mick
jack clegg
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Re: 1963 G80CS Top End Woes

Post by jack clegg »

robcurrie wrote:From what you described, I think the rod is bent, causing the gudgeon pin to move from side to side and hitting the circlips.

Rob C
Oh dear Rob. I think you may be right. Doubtless the pin has been bashing the circlips. I'm just struggling to understand the actual mechanics which would cause the pin to move from side to side. Is it the up & down throw which would move an angled pin from side to side?

One more dire symptom I missed yesterday, due to a dark shed & good access to right of bike only. The left inner piston boss shows slight contact wear from the little end bush. The little end bush itself protrudes on the right by a normal looking amount, but the left side shows heavy contact wear & is worn so badly it has almost no protrusion remaining, worn unevenly on a taper leaving the ID bigger than the OD. Looks 'orrible.

Mick, thanks for your take on this. I find it hard to believe the bush rotated in the little end eye, so assume an idiot fitted a replacement bush & this has seized at some point. The piston moves together with the little end bearing on the pin under normal circumstances, but stiffness or seizure of the little end puts all bearing movement in the piston alone. There is no rotation as such, oscillation I think might be the right word? Regardless, hope of a reasonably easy fix is fast receding. TTFN Jack
Last edited by jack clegg on Sun Jun 11, 2017 5:48 pm, edited 1 time in total.
John Marshall
Mick D
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Re: 1963 G80CS Top End Woes

Post by Mick D »

Hi Jack

The piston rotates around the axis of the small end as it oscillates - only a few degrees either side of zero but definitely rotation.

The piston / gudgeon pin interface is not designed to be a bearing and hence it wears if the two move relative to each other - the bearing surfaces are the gudgeon pin / little end bush which in your case appear to have seized.

I too am not sure how a bent con rod would induce sideways oscillation of the gudgeon pin - Rob what are your thoughts on this please?.

Regards Mick
jack clegg
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Re: 1963 G80CS Top End Woes

Post by jack clegg »

Hi Mick,

I thought the Yanks called it a wrist-pin for a most agreeably descriptive reason. Do they call the little-end a wiggle-bush one wonders?

I'm afraid I must beg to differ about the piston forming part of the bearing. I've always been used to heating pistons in order to remove the pin, so when Les Emery supplied me Atlas pistons in which the pins were slip fit cold, I enquired. His response was that with modern low expansion alloys, this was now normal & that the pin moves in all pistons once hot anyway, referring to the oilholes beneath the piston bosses, plus the fact that even with the old type pistons, once they got hot the pin lets go in the piston anyway. I found it hard to disagree, bunged the new pistons in (1964 N15CS) & no bother. Makes assembly a tad easier without the blowtorch too. Also, Alan Hitchcock's RE Interceptor pistons are, Indian type heat to fit pin, & forged expensive type slip fit cold.

TTFN Jack
John Marshall
Mick D
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Re: 1963 G80CS Top End Woes

Post by Mick D »

jack clegg wrote:I'm afraid I must beg to differ about the piston forming part of the bearing.
Hi Jack

Don't think I said that did I?

Regards Mick
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robcurrie
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Re: 1963 G80CS Top End Woes

Post by robcurrie »

Mick D wrote:I too am not sure how a bent con rod would induce sideways oscillation of the gudgeon pin - Rob what are your thoughts on this please?.
The gudgeon pin will not be horizontal, so the effect is that the piston will thrust to one side, at the end of the stroke the piston will then move to the other side. At speed, this sets up an oscillation in the 'rod causing the damage you have seen.
Just imagine the tremendous force at the end of the stroke when the piston's mass must be stopped a hundred times a second
Rob C
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Re: 1963 G80CS Top End Woes

Post by bjorn »

Rob Currie wrote:

“From what you described, I think the rod is bent, causing the gudgeon pin to move from side to side and hitting the circlips.”

I agree this is a likely explanation, but there are some other possibilities worth checking:

1. The cylinder bore can be out of alignment with the “vertical” centreline due to sloppy work from the guy doing a rebore or installing a new liner. The liner thickness should be equal all the way round the bore, both at cylinder top and cylinder bottom. (bought one like this once, rebored by an “expert”)

2. A small end bush with a bore that is not in alignment with the rod eye centerline will have the same effect as a bent rod.

3. If the crankcases are not the original “matched pair”, there may be a step at the seating for the cylinder base. The cylinder will not then be parallel to the crankshaft axis. (had one of these also, step of 0.3mm)

Regards
Bjørn
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