new piston wont fit

[dansofield550]

got a new piston from amoc spares and tried fitting it today, all went fine until i tried to turn the engine over, it locked solid at bottom dead center , it would seem to be not machined on the scoops on the underside that would clear the crank flywheels, not too happy about this thats for sure, anyone else had this? ,

it would also seem that the scoops are set offset in the underside in the wrong direction , i know you'll most likely think i have the piston in the wrong way but no, the valve cut outs are both the same on the crown, in the picture the pin is holding the two pistons together

[aobp11]

I have a GPM 6916 70 mm here. The scoops extend exactly symmetrically in front and rear direction.
I know nothing about G2 engines. Has the crank shaft some offset from the cylinder center line? In a 350/500 HW engine the piston orientation would't matter.
Albert

[dansofield550]

quick annswer is it can all only go on one way round (cylinder head etc, the piston can be fitted backwards but was fitted correctly ), the piston has cutouts for the valves , and even this looks different , the exhaust side looks to be no where near the right place compared to the old piston.

[aobp11]

I downloaded the 1959 G2 Exploded Engine View. As far as I can see the crank shaft is exactly mid below the cylinder bore. Hence I don't see a mechanical reason for having the scoops more to the front or more to the rear.
Albert

[spitfire]

on the 250s the cylinder centre line is offset forwards from the crankshaft centre line, so it is vital that the scoops in the bottom of the piston are machined in the correct position. the piston comes very close to the flywheels and if the scoops are in the wrong place they will indeed hit them. looking at your pictures the new one does not look right at all, the scoops appear to be in the wrong place and the valve cutouts look to be slightly out, but it is a bit hard to tell for certain how badly out they are from the photos.

[SideValve]

Looking at your pictures someone has boobed. Presuming you kept the two pistons together when you turned them over between photos, the valve cut-outs are one way round and the crank cut-outs on the bottom are the other way round.
If you put the piston in with the crank cut-outs the other way will the crank rotate?
If so you just need to check the valve cut-outs clear the valves (a thin layer of plasticine is good for this).

Or just point out their mistake & get another one sent?

[Ozmadman]

Well spotted!

[dansofield550]

The pistons were indeed kept together for the photos Peter, I can't stand the whole sending things back lark! I may take it into work and machine the piston correctly, definitely seems top or bottom is arse about face, the valve cut out though may stop me , I don't know how fussy this has to be but when the engine was all back together it would pass over Tdc no problem so may as well leave that part alone I guess

[aobp11]

on the 250s the cylinder centre line is offset forwards from the crankshaft centre line

Thanks, didn't expect to learn something new about AJS/M in the remaining hours of 2013.
Albert

[dansofield550]

Desaxe cylinder is the term I think,