Piston skirt to cylinder wall clearance

[kabadi GF 1961]

hello all
    I want to know what is the correct piston skirt to cylinder wall clearance.??
I have the current  040 oversize piston with  skirt to wall clearance of .008"( feeler gauge going between cylinder wall and piston skirt ) is this excessive?
thank you
 

[RichardL]

Kabadi,

The 0.008" clearance is excessive, but if all else was in good condition you could try it and see how it goes. Unfortunately, all else is not in good condition. The bores look like they got well rusted and pitted sitting for five years. I trust it was the previous owner who gouged "L" and "R" and the arrows in the tops of the pistons. Not a good idea. If this were mine, this would be the time to go to +60 pistons. The ones you have now are 9:1 compression. Your new bores should be machined/honed for the pistons to have right around 0.005" cleaerance, but I look to others to comment on the acceptable range.

Richard L.

[muskrat]

G'day kabadi. It also looks like the left piston might have been rocking. 0.008" is at the end of its tether for the road. If it was a race motor with forged pistons I'd run it . Some of that clearance could be from piston wear so new pistons will bring it back a bit. But I wouldn't put new ones in those bores.
Cheers

[chaterlea25]

HI,
Are you measuring at the bottom of the pistons at 90 degrees to the pin?
take measurements with tha pistons at the bottom of the bore, and then measure again with the pistons near the top ?

John

[edboy]

according to bsa max wear is 0.0055 wear with 9-1pistons . personally i would try a hone first and see how they clean up at the top and try it first with new rings. nothing to lose. its a shame you can t buy cord rings anymore which expanded and took up wear in the top part of the bore.

[bsa-bill]

Are you measuring at the bottom of the pistons at 90 degrees to the pin?

 scuse what is maybe a dumb question  John - is this to do with pistons being slightly oval, never could get me old head around where the ovality is about and how they machine it

[chaterlea25]

HI Bill,
Yes its to do with the ovality thats manufactured into the piston design
To make matters more complicated they are also tapered towards the top
The ringlands are also stepped to smaller diamaters
There are lots of reasons why they are like this
The top of the piston runs hotter than the bottom so expands more,
the ovality is to lessen the rubbing areas and reduce friction, the piston thrust is at  the front and rear so this is the largest size
There are several videos on youtube on piston manufacture to amuse you LOL
I think that there were specially made machines to machine the ovality in vintage times where the tooling moved in relation to the rotation of the piston being machined
With modern CNC machining centres almost anything is possible??????

Cheers
John

[kabadi GF 1961]

thank you !
 yes I measured the piston clearance on top and bottom of the bore , and 90 degree opposite from the  pin holes. no luck it is still same ,
question is there a .050 oversize piston available ??
I believed the were made in even number increments like .020  .040  .060

but i still have a spare set of hepolite  .030  that makes me believe that someone may know if .050 pistons oversize were made

I want to have the clearance as per specs , I dont want to go through the trouble and have both exhaust smoking due to larger clearance.
thank you again 

[RichardL]

Kabadi,

Found the following at British Only. They also had 10.5:1, which I think should be avoided, unless you are planning on racing. I'm not sure what a 9.5:1 profile looks like, but suspect they are really 9:1. I would want to know before I bought them. I also don't know anything about Mondial pistons. Others might. Finally, are you sure that machining/honing out maybe less than 0.010 will get rid of all the pits?

Richard L.

Description   Suggested Price   Web Price   Used
35-30605
BSA,A10,9.5:1,.050,MONDIAL

SPECIAL NOTE: LIMITED QUANTITY. SLIPPER PISTONS. ONLY 1 PAIR AVAILABLE.

(Approximate Weight 1.81 Pounds)    $363.85    $272.89    NA
Your 25% Online Discount For This Part Is $90.96
Web Price Reflects Internet E-Sale 25% Discount    Sold As: Set
Condition Requested:    New

Lowest Retail Price Displayed - Currency Rates As Of December 11th, 2015
AUD $378.41   CAD $364.78   EUR €257.91   GBP £181.79   JPY ¥33589   NZD $415.72

[RichardL]

Thing I don't know #345,768,996,237,004: Is it a good idea to run slipper pistons in an A10?

Richard L.

[kabadi GF 1961]

thank you Richard

 I will call them tomorrow and find out . those marks on the bores are watermarks , they are not deep , but the piston rattles at .8 thou clearance being on .030 oversize. 
I am going to get a the bore honed tomorrow and post the final finding. I have no problems going .060 over but still want to preserve whatever is left.

[BSA_54A10]

according to bsa max wear is 0.0055 wear with 9-1pistons . personally i would try a hone first and see how they clean up at the top and try it first with new rings. nothing to lose. its a shame you can t buy cord rings anymore which expanded and took up wear in the top part of the bore.

Plesae remember that anything BSA published is specific to what they were using at the time.
Piston clearences are set by the piston makers so the BSA figures are for correct Hepolite's and a guide for anything else.
If you use the feeler gauge between the piston & bore, which is a very bad practice remember you need 2 feeler gauges, opposite each other because motorcycle pistons float on the pin.
Measuring the bore at 3 places then measuring the piston is a much better idea

[Triton Thrasher]

How does the bike run now?

[bsa-bill]

HI Bill,
Yes its to do with the ovality thats manufactured into the piston design..................................

Thanks John, guess working in my shed amongst 50 year old stuff leaves the modern world and methods behind sometimes

[kabadi GF 1961]

Hello triton thrasher
It worked fine ( there were new rings ) did not  smoked either
But there was a piston rattle for sure
-----
I Got another cylinder all complete (bored and honed) and  used good condition . 040 oversize piston
That measures .006" in new cylinder
I am planning to get it moly coated. On skirt and ceramic on the dome
http://swaintech.com/race-coatings/
They can coat a thick moly on skirt about .002 for about 45$ each piston
This should bring the clearance to a acceptable.004"
Has anyone tried this kind of coating on the skirts?