Piston Height

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Piston Height

Postby MacL » Tue Apr 21, 2020 1:05 pm

Can anybody tell me how much higher up the bore a high comp[10.25:1]piston will show on a G12 CSR as measured with indicator down the plug hole. My LH goes up higher than the RH[off side] quite some way. I am sure it just can't be down to diff in carbon build up.

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Re: Piston Height

Postby SPRIDDLER » Tue Apr 21, 2020 1:20 pm

MacL wrote:Can anybody tell me how much higher up the bore a high comp[10.25:1]piston will show on a G12 CSR as measured with indicator down the plug hole. My LH goes up higher than the RH[off side] quite some way. I am sure it just can't be down to diff in carbon build up.


Is this any help?

http://archives.jampot.dk/technical/Pis ... istons.pdf
I poke badgers with spoons.

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Re: Piston Height

Postby MacL » Tue Apr 21, 2020 1:59 pm

Spriddler, thanks for that info. Helpful insofar as indicating there are plenty of options to chose from that could account for the difference but no Height data. Just for the moment don't want to strip off the heads to find out.

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Re: Piston Height

Postby SPRIDDLER » Tue Apr 21, 2020 2:46 pm

Ah yes, I see your point. I didn't twig there's no overall height shown nor is there a pin centre to crown height shown.
There are a couple of Hepolite catalogues/guides in the Archive which may have overall dimensions.

http://archives.jampot.dk/technical/Pis ... gs_Valves/

Assuming that both conrods are the same length ( :rofl: ) you may find pin centre to crown dimensions in those catalogues/guides which would enable you pin down the height differential with your pistons but I don't know whether pistons would vary in height by as little as the amount of a carbon deposit :?
I poke badgers with spoons.

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Re: Piston Height

Postby Mick D » Tue Apr 21, 2020 3:24 pm

MacL wrote:Just for the moment don't want to strip off the heads to find out.


Hi

If I found a significant difference in the TDC height of the pistons on a twin I'd have the head off before riding it - differing pistons will probably be out of balance and stressing the crank shaft unduly.

You can buy miniature cameras that link to a mobile phone and can be passed into the plug hole which will give you a good view of the crowns:

https://www.amazon.co.uk/Pancellent-End ... 5136303031

What difference in TDC are you seeing?

Regards Mick

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Re: Piston Height

Postby MacL » Tue Apr 21, 2020 6:28 pm

Mick and Spriddler , Thanks for comment. I got this machine beginning of last year. Been working my way slowly to recommission it. Doing all the routine oils;grease and adjustments. Iv'e come across some real poor care and work practice needing new parts. I have not ridden it and only run it briefly to set up carb and ignition. It will not run steadily at slow running. Checked through all the fuel system first. New jets, and, new air slide which was really worn. Then the ignition timing - OK on the RH pot but considerable advanced on LH. LH 11 O'clock mag cam OK but RH 5 O'clock cam quite badly warn - so mag armature running out of true. Got a new cam ring from Tony Cooper. Old ring was the devil to remove - free when fully in, but as soon as moved out it siezed up. Tony's new ring would not go in. Checking with vernier showed points end plate aperture oval. Looks like mag has been banged/dropped. It was during checking the points timing that I noticed on moving my timing plug adapter that tdc on LH was higher than on RH. I checked that the adapter was screwing into the head the same depth measured against the fins so became very worried. Last time I was confronted by this scenario was 1963 when earlier G12CSR broke a crankshaft. When I have had it running briefly there is no unusual noises or exhaust smoke. I does though seem out of balance / vibrates and will travel at idle on the centre stand. Could that be a clue? So Piston height or conrod length. Will not run again till I find out.

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Re: Piston Height

Postby MacL » Tue Apr 21, 2020 6:42 pm

Mick, Further to above. Can't see anything amiss with boroscope. Diff was just short 6/32 [ by memory as I removed the mark to check the points on Tony's new ring once I got it in]

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Re: Piston Height

Postby Mick D » Wed Apr 22, 2020 5:17 am

Hi Mac

Why not re-measure as accurately as you can? 3/16" difference would indicate to me that the pistons are not a pair - I'd be looking to resolve this before the lockdown is over.

Thinking a little outside of the box, your barrels and heads are matched pairs aren't they?

Regards Mick

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Re: Piston Height

Postby robcurrie » Wed Apr 22, 2020 5:42 am

Why don't you take the heads off and have a look?

Rob C

PS here is a link to my compression ratio calculation, numbers often given in spcifications seem to be overly optimistic.
viewtopic.php?f=21&t=24078&start=10#p198860

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Re: Piston Height

Postby MacL » Wed Apr 22, 2020 7:56 am

Hi, Thank you everyone for comments/advice. In current situation;as I am well into the vulnerable classification, I am sitting on my hands and keeping bike in one bit. Unsteady idling of course can also be subject of Mag poor capacitor: and I don't suppose that has ever been checked/replaced since 62.
I've yet to check/set the tappets;which was next on my list of trouble shooting before being brought up short by piston revelation. Yes. Whatever, the heads will have to come off. Regards All ,Mike.

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