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Motorcycle Discussions => British Bikes => Topic started by: mark2 on April 24, 2014, 11:04:19 AM

Title: pistons AJS
Post by: mark2 on April 24, 2014, 11:04:19 AM

has anyone used one of these and was it ok , I have bought one and measured it , its 18 tho oval , this seams a bit high to me
http://www.ebay.co.uk/itm/331062239006?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Title: Re: pistons AJS
Post by: rogerwilko on April 24, 2014, 11:49:42 PM

Most pistons are cam ground. So the diameter from front to back is bigger than the sides. With JP pistons use 4 thou clearance or it will nip up. There are long rod and short rods with AMC singles. By the position of the gudgeon pin on the ebay one it is a short rod piston.

Title: Re: pistons AJS
Post by: CaptonZap on April 25, 2014, 01:35:30 AM

Quote from: mark2 on April 24, 2014, 11:04:19 AM

has anyone used one of these and was it ok , I have bought one and measured it , its 18 tho oval , this seams a bit high to me
http://www.ebay.co.uk/itm/331062239006?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

It isn't as high as these. http://www.motosport.com/dirtbike/Athena-Big-Bore-Piston    (http://www.motosport.com/dirtbike/Athena-Big-Bore-Piston) :o
As long as you have enough clearance on the widest dimension, you are good to go.

Title: Re: pistons AJS
Post by: mark2 on April 25, 2014, 09:33:31 AM

I have taken the piston and barrel to the engineering company I use (since the 1960s and have always had good service) , and they are not happy with the shape of the piston , and have ask if it could be a bad ground one , I have contacted JP but so far they have not sent the technical spec . they will boar to suit no problem but have recommended not to use it ,

Title: Re: pistons AJS
Post by: rogerwilko on April 26, 2014, 12:19:56 AM

I think you're being over cautious. JP have been in business a long time. Just use it. Look at the piston aformentioned

Title: Re: pistons AJS
Post by: CaptonZap on April 26, 2014, 01:25:52 AM

As a point of reference, VW Bug engine pistons, with a 3.3** diameter, have a .016 in difference in skirt diameter. Air cooled, iron sleeve, aluminum piston, pretty much the same technology.
Does your machine shop want to sell you the "correct" piston?

CZ

Title: Re: pistons AJS
Post by: mark2 on April 26, 2014, 08:17:47 AM

you could be right I will wait for the spec from JP , the only piston (new AE) I can compare it to is for a 500 Ariel , this measures 7 tho oval which is more what I would expect/like

Title: Re: pistons AJS
Post by: mark2 on April 29, 2014, 10:53:41 AM

got the spec from JP , 18 to 20 tho is correct , so will have board , see what it sounds like