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Piston Clearances
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Sponge
Lancashire (A chip shop somewhere near Preston)
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January 13, 2021 - 10:35 am
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Can anybody out there tell me the correct way to measure piston clearances.

I have workshop data for clearances top and bottom of the piston measured on the 'major axis" and some vernier calipers and a set of feeler guages but am not sure about the exact procedure. Can any tecnical superhero out there enlighten me please ?

Sponge

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SpacedMarine
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January 13, 2021 - 7:32 pm
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Just push the piston into the bore.You want it sat where it would normally run when the engine is running and in the correct orientation.try and squeeze the feelers down the front and back of the piston as this is where the piston "rocks" in the bore on the gudgeon pin.Do the same top and bottom. That's how I'd do it anyway Sponge 👍🏼

What's 7/16 in mm again?

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Sponge
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January 13, 2021 - 8:38 pm
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SpacedMarine said
Just push the piston into the bore.You want it sat where it would normally run when the engine is running and in the correct orientation.try and squeeze the feelers down the front and back of the piston as this is where the piston "rocks" in the bore on the gudgeon pin.Do the same top and bottom. That's how I'd do it anyway Sponge 👍🏼  

Many thanks for the reply mate. My confusion is ....when the clearance is listed as say .040" does that imply .040 front and .040 back or split down to .020/.020 .....? 

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Nick_S
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January 13, 2021 - 9:03 pm
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To really do it accurately, you need a bore gauge & a micrometer.

Bore measurements minus piston measurements equals clearance.

The clearance measurement is not divided either.

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Sponge
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January 13, 2021 - 10:24 pm
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Nick_S said
To really do it accurately, you need a bore gauge & a micrometer.

Bore measurements minus piston measurements equals clearance.

The clearance measurement is not divided either.  

Many thanks for that mate. Very clear. I have a micrometer....not sure about the bore guage but given the current lockdown I will see what I can improvise. Something to think about overnight. Much appreciated.

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SpacedMarine
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January 13, 2021 - 11:58 pm
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Sponge said

Many thanks for the reply mate. My confusion is ....when the clearance is listed as say .040" does that imply .040 front and .040 back or split down to .020/.020 .....?   

Provided the piston and bore are round if you put a 0.040 feeler down one side there will be no clearance the other side as it'll push the piston to one side.

If you can fit the correct size feeler all the way round with the piston in the correct orientation you should be good.

Again just the way I'd do it.

What's 7/16 in mm again?

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mike p5xbx
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January 14, 2021 - 1:07 am
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The trouble with internal screw gauges are the bore wears oval ( ) front to back
and you have to remove them to measure with a vernier or micrometer
you can measure the piston skirt quite accurately with a vernier and then place piston in the bore and measure with feeler gauge gap at front of bore just below the exhaust port
new piston and new rebore clearance at bottom of skirt is 0.004” so that and more then 0.015” is not good

D? - D10- D14 Bantams 350 AJS -500 Triumph http://bsanotru.....lfire.com/

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Nick_S
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January 14, 2021 - 3:02 am
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mike p5xbx said
The trouble with internal screw gauges are the bore wears oval ( ) front to back
and you have to remove them to measure with a vernier or micrometer
you can measure the piston skirt quite accurately with a vernier and then place piston in the bore and measure with feeler gauge gap at front of bore just below the exhaust port
new piston and new rebore clearance at bottom of skirt is 0.004” so that and more then 0.015” is not good  

Exactly mike p5xbx & that is how a professional with the correct tools would do it.

As not many people have access to bore gauges/micrometers that are suitable, your suggestion on how to measure is ideal & I have no issue with doing it that way either.

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SpacedMarine
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January 14, 2021 - 8:11 am
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Nick_S said

Exactly mike p5xbx & that is how a professional with the correct tools would do it.

As not many people have access to bore gauges/micrometers that are suitable, your suggestion on how to measure is ideal & I have no issue with doing it that way either.  

I would imagine even less had access to them in their shed in the 50s and 60s unless you were an engineer by trade.I wonder how much just a micrometer was back then never mind a bore gauge I dare say a lot more than they are now they're all made in China.Agreed if you took it to shop they probably would but your average Joe had a set of feeler gauges for doing spark plugs etc.

If your just checking whether the piston/bore is worn out surely you could just check the clearance with a feeler gauge and wouldnt even need to measure the piston?If it's the one that's been running in it already?If that makes sense????

What's 7/16 in mm again?

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Sponge
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January 14, 2021 - 8:23 am
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SpacedMarine said

I would imagine even less had access to them in their shed in the 50s and 60s unless you were an engineer by trade.I wonder how much just a micrometer was back then never mind a bore gauge I dare say a lot more than they are now they're all made in China.Agreed if you took it to shop they probably would but your average Joe had a set of feeler gauges for doing spark plugs etc.

If your just checking whether the piston/bore is worn out surely you could just check the clearance with a feeler gauge and wouldnt even need to measure the piston?If it's the one that's been running in it already?If that makes sense????  

It is the other way around in this case......I have a good piston and a good barrel but both originating from different sources. The piston seems very 'snug' in the bore, perhaps a little too snug so I wanted to measure it up before I fit it to a bike. Looks like I can do this with my feeler guages and verniers.

Many thanks for all the advice and tech knowledge guys. It's good to learn new stuff. 

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not henpecked
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January 14, 2021 - 9:21 am
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Hi All

When I stripped my B175 down the big end cage had broken up and the piston was scrap. Living in Leicestershire I went buy a new piston from Rex Caunt and asked his opinion on the condition of the bore. He said it was OK got out the appropriate new pistion and did a feeler gauge check around it and again pronounced OK. If Rex did it with feeler gauges, and he knows his Bantams, what has been said in this thread already must be OK.

B175, on the road. Honda XBR 500, and ....... Suzuki Burgman 200 scoot! Nicknamed "The hair dryer" - by me I hasten to add; & great storage under the seat when you get to your destination.

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