Author Topic: Piston clearance  (Read 2048 times)

Online morris

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Piston clearance
« on: 09.05. 2014 23:05 »
Daft question maybe, but where do you measure piston clearance?
I figured out it must be on the front/back of the skirt, or do you measure it left/right?
We're talking about split skirt pistons here.
Thanks already
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Online Brian

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Re: Piston clearance
« Reply #1 on: 09.05. 2014 23:27 »
You measure them front to back. Pistons are oval in shape to compensate for expansion when hot. The amount of "ovality" depends on the make of piston.

A good quality piston also tapers as it goes down, the bottom of the piston is larger than the part just above the pin. The manufacturer should give you information as to just where you take a rebore measurement from but as rule of thumb about 3/4" from the bottom.

Some pistons have more taper than others and if the cylinder is not bored correctly for the piston they will "nip up".

I've added a scan from a Suzuki I'm currently working on. Even though its a japanese bike the pic will give you some idea of how the measurement is taken.
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Online muskrat

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Re: Piston clearance
« Reply #2 on: 10.05. 2014 00:33 »
Split skirt pistons require a lot less clearance than solids. 2 1/2 thou" is about right.
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Online orabanda

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Re: Piston clearance
« Reply #3 on: 10.05. 2014 00:47 »
Hey Brian,
How about a picture of your water bottle (not the one you take to bed)?
Richard

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Online Brian

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Re: Piston clearance
« Reply #4 on: 10.05. 2014 05:55 »
At the risk of being sent to hell (again) for putting a pic of a japanese bike on a BSA forum here it is.
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Online orabanda

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Re: Piston clearance
« Reply #5 on: 10.05. 2014 07:01 »
Noice!
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Offline cyclobutch

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Re: Piston clearance
« Reply #6 on: 10.05. 2014 10:41 »
Looking good from where I'm sitting.
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Online morris

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Re: Piston clearance
« Reply #7 on: 10.05. 2014 22:06 »
At the risk of being sent to hell (again) for putting a pic of a japanese bike on a BSA forum here it is.
Last of the proper bikes I'd say. After that, motorcycle designers went complete bananas......
Won't send you to hell for that Brian. A light spanking will do *smile*

Thanks for your replies lads.
I have taken the head and barrel from the plunger because it was handpainted at some stage and it looked like been done by a child so I had it ice cleaned to get the paint off. The advantage of ice cleaning is that there's no grit or sand used, but due to the cold from the ice, the surrounding air condenses and the bores start to rust immediately. For that reason I had to give them a light hone.
I then measured the clearance and got about .015-.020 so won't need a rebore then. There's still room for some though, as the bore is at +40, but for the moment I'll leave it like it is, give the pistons a clean and some new rings, and put it back together.
Ordered new valves and valve springs also. One of the inlet valves had a lot of carbon deposit on the upper face. It seems to be bent and not seating properly, and also it's inner spring was broken.
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Online chaterlea25

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Re: Piston clearance
« Reply #8 on: 11.05. 2014 22:13 »
Hi Morris,
Quote
I then measured the clearance and got about .015-.020 so won't need a rebore then. There's still room for some though, as the bore is at +40, but for the moment I'll leave it like it is, give the pistons a clean and some new rings, and put it back together.

Are you measuring in inches or mm ?
15 to 20 thou is huge ???
.15 - .2mm = 6 to 8 thou which is still a lot more than Muskys 2 1/2 thou =.06-.07mm

More worryingly than this confusion your pistons are showing rub marks above the top ring
this usually indicates that the piston is rocking in the bore at tdc and bdc
this causes a tapping sound at low revs
Excessive clearance or bent pistons ????

John
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Online morris

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Re: Piston clearance
« Reply #9 on: 11.05. 2014 22:49 »
Sorry John, forgot an important 0, one that comes right after the dot *smile*
So it should read .0015" - .002"
I measured it with feeler gauges.
.0025" didn't pass so the measurement seems correct to me. (measured it at various heights, and all where the same)
The rub marks you see are more of carbon traces in reality I think (picture's not so good quality). The PO mostly made short runs on it, and there's a lot of carbon deposit in the head and on the piston surface.
The carb hasn't got a choke, but according to the PO it always started well from cold so I think carburettor settings are a bit rich to.
I have taken the pistons off today, and got them on the bench for cleaning. I am going to clean them gently to see if the edges are scored. There where no markings inside the barrels.
I'll try to post a better picture soon for judgement
Thanks
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Online morris

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Re: Piston clearance
« Reply #10 on: 20.05. 2014 22:15 »
As promised some pics from the pistons after a clean up.
They look reusable to me. The pistons are stamped +60 so they're not +40 as mentioned earlier.
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Online RichardL

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Re: Piston clearance
« Reply #11 on: 08.07. 2018 18:36 »
Playing with the A7 and trying to decide if my bores are good enough to experiment with about 0.007" clearance. I'd really like to put off new +60 pistons until some time after the restoration (not concourse) is complete. Everything seemed heading in the right direction for this until I saw the vertical scoring shown in the photo. I can feel these with my fingernail. If I was giving myself good advice I'd probably say, "game over, go for the +60s," but I want to hear some one say, "ah, that's nothin' Richard, put 'er together and let the good times roll."

Anyone willing to be that optimistic?
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Online Greybeard

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Re: Piston clearance
« Reply #12 on: 08.07. 2018 21:01 »
I recently had some shallower tramlines honed out of one bore. Might be worth trying.
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Online muskrat

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Re: Piston clearance
« Reply #13 on: 08.07. 2018 21:06 »
G'day Richard.
A quick hone will take a tad more. I ran 8 thou" clearance in the A7SS racer with standard Hepolite T160 pistons. The scratch isn't too bad, she might puff a bit of weed out the left. Most important is remove any ridge top and bottom to avoid broken top and oil rings.
Cheers
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Offline ellis

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Re: Piston clearance
« Reply #14 on: 08.07. 2018 22:07 »
Hi Morris.

As Greybeard said I would also go down the honing route and see how the cylinders come out. If they wont clean up then your only option is to go for a re bore.

ELLIS
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