Aero 250 piston/bore

[Tazmantic]

Really sorry guys as I’m sure I see this info the other day but try as I might today I can’t find it...

What size bore and pistons did the 250 have, found the head was leaking very slightly so thought I’d check the bore while I had the head off.

Thanks

[EW-Ron]

51 x 60 mm bore and stroke gets a mention there, although a bit convoluted in the logic to be sure ?

https://www.douglasmotorcycles.net/aa-files/html/identify-part2/vintage4.6.htm

[Tazmantic]

Thanks Ron,

That was probably where I saw it but that's very interesting as my bore does have a slight lip at the top but below that lip appears to measure 50.8mm.

Added pic is this lip normal or due to wear as I would expect..

Cheers

[EW-Ron]

The lip is due to wear - but its very likely you also already have oversize pistons,
or those pistons would be rattling around.

Measuring the actual clearance is a finicky lot of work, you could just repair the leak and if the
bore looks good, clean it all up and bolt it all back together. ?

P.S. Spraying the head gasket both sides with silver paint will often make it like new again, if it came off undamaged.
Let the paint dry, and when fitted the paint lightly bonds to the surfaces, and generally seals quite well.
If it has any tears/rips though, its replacement time.

[Tazmantic]

The piston appears to have STD stamped on it, And my bore is smaller than listed?? The gasket is copper I’ve annealed it but still won’t seal and the thread on one of the bolts was looking bit dodgy and has just stripped 

[EW-Ron]

Ooops, that seems to have slipped beneath the radar.
I have no explanation for that, unless the mm numbers quoted are only approx ?

Until you get to the bottom of that, it can still be cleaned and reassembled and used ?

P.S. Did you try spraying the copper gasket with silver paint ?  Both sides.

And did a bolt strip, or the thread it goes in to ?
Easy enough ? to find another bolt.
A helicoil may be needed ...

[Tazmantic]

The bolt had stripped but as the heads on the bolts are quite long I managed to turn the head down and cut off the stripped bit..as I dont have any 3/16 x 25 tpi bolts.....

So I then tried the spray paint, only had black but sure colour didn't matter lol and it did seem to work on where it was leaking but then it started to leak from another place as the barrel thread was also dodgy.... so I ended up using a helicoil and by then had some blue hylomar (waiting for all the horror stories) and that all seemed to work fine.

So next question is, are 350 pistons available? if not does anyone know of any after market pistons that are the right size as I'm thinking I might just bore the barrels out to 350 size BUT are the heads the same??

Cheers

[EW-Ron]

I might just bore the barrels out to 350 size

I'd be very wary of doing that, until you verify that there is enough thickness in the cylinders to allow that.

If it was that easy, everyone would have bought a 250 and bored it to 350 size. ?
You may not be the first to find you can see daylight between the fins !

[Tazmantic]

Will most defiantly check but picked up a few bits yesterday one of which was a 350 barrel and on first look they seem the same outside dimensions its just the bore that's bigger.

[Tazmantic]

Ok so looking at the bits I have the valves in the 350 barrel are bigger than the 250 and also are straight and not at an angle so the spare barrels I have deff couldn't be bored to 350 ...

BUT...

The 250 barrels and heads I got as spares with my latest buy (and also fitted to the engine in the bike) are different to the 250 barrels and heads one my 250 aero... I didn't measure the valves on my aero when the head was off but I'm positive they are bigger than the other 250 barrels and also straight like the 350 so does any one know if the 250 comet engine has smaller valves than the 250 aero engine???

[Tazmantic]

up loaded some pics to show the differences....