720cc ?

Like the sound of that , is it a good upgrade to do ?

Ok ! stand down - just seen that it needs to be an unlinered barrel to achieve 720 .

You can do two possible 'enlargements' to your M21...

Using an 85mm 'Long Rod' B33 piston will take it to 635cc..or using a 90mm piston will take it to 712cc...

I used a 90.5mm piston to achieve 720cc as I happened to have one at the time....

In all cases, ideally, you will need a late 'unlinered' M20/21 cylinder, not one with a liner fitted...

An alternative liner could be used in a linered barrel for the 85mm piston but I'm not sure about that with the 90mm piston as I've never done the measurements to establish whether there is enough material in the barrel...

I get the 90mm pistons made in batches periodically when there is sufficient demand...At the moment I don't have any...

Original B33 'long rod' pistons are pretty scarce but do come up from time to time or alternatively a pattern piston can be used...

Although the capacity sounds impressive the gains are not as great as those that would be achieved in an OHV engine such as the B33..The sidevalve design is inherently inefficient and remains so without further alteration..

However, cruising speed is improved as is fuel consumption and performance up hills is substantially better.
The greater part of any gain is in the lower to mid range...

Personally I think it is worth it as I use my M20 for longer runs when touring (and I want to do that on a WD bike).

If you only intend to run about locally I would think twice about whether it's worth the cost...and unless you specifically want a sidevalve you can do more, more easily and more economically with a B33...Ian

email (option): ian@wright52.plus.com

Thanks Ian , very informative , think i'll stay at 600 at the moment

A slightly cheaper alternative is to use a VW pancake piston.
Being an aircooled engine the pistons are much more beefier than watercooled car piatons so will take the punishment.
Again you will need the sleeveless barrel but you then have a lot of alternatives sizes up to 978 cc if used with the long crank

email (option): wariron@tpg,com,au

I used a Triumph car piston in an oversize engine years ago...It fitted but was ridiculously overweight...You have to do a lot of work on the crank to balance very heavy pistons and even then you have a lot of inertia in the assembly....

Personally I prefer lighter bike pistons..These days with most bikes being well 'over square' there are quite a few large but lighter pistons...Ducati for example....

Bike pistons also tend to have smaller gudgeon pins...This often rules out car pistons unless you want to get into sleeving the gudgeon pin bosses in the piston etc.

978cc works out to approx. a 105mm bore when used with an M21s 112mm stroke crank...I've never looked at going to that size...Can you get that into an M20 barrel?.... Ian

email (option): ian@wright52.plus.com

those in the know : when you do the overbore - do you end up removing the spigot that locates in the crankcase mouth ?

I was measuring up a barrel and if I went to 90mm the spigot would be machined into thin air.- I think

do you dowel the barrel ? or just use the flange bolts for location?

Yes the spigot goes but it doesn't appear to be a problem and the bolts hold the barrel in place OK.

Cheers
Pete

email (option): petercomley@web.de

thanks

thanks

EDit - so appreciative i said it twice

I'm planning to fit my 85mm piston sometime. I can't remember if my barrel has a liner or not. I have a new liner in stock to measure and I calculate that boring out a liner to 85mm will leave a wall thickness of 2.25mm. Do the enlightened think this is acceptable or not? Ron

email (option): ronpier@talk21.com

That will be plenty Ron..I've done them down to 1.5mm wall thickness without any problems...In fact that's what's in my 720cc B33 at the moment....Ian

email (option): ian@wright52.plus.com

Great thanks Ian. I'm sure that's what you told me a while ago. It was just your comment earlier in this thread that gave me some doubts.

"In all cases, ideally, you will need a late 'unlinered' M20/21 cylinder, not one with a liner fitted..."

Cheers Ron

email (option): ronpier@talk21.com

Yes, not strictly correct there...I'm shortly going to experiment with fitting an oversize liner to a 'linered' M20 barrel to get the 90mm piston in...Ian

email (option): ian@wright52.plus.com

Cheers Ian. If you're going to order any more pistons? Put me down for one as my engineer is in the process of putting an M21 crank together for John A who wants another 720 engine for his other M20. Ron

email (option): ronpier@talk21.com

Wow, 1,5 mm wall thickness..?

And I was wondering if I can go to plus 0.60 oversize in my '55 Triumph Speed Twin ! (facory figures stop at + 0.40 if I'm right)

ian - do you have any of the 90mm pistons left ? are you likely to get another batch ?


I located a linerless barrel here in australia and am contemplating an over bore.

thanks

I have one BSA M21- 1960 with one of Ian special pistons.
(The project I showed here some time ago with the Suzuki fork)


https://www.youtube.com/watch?v=m5BqKlRLego
https://www.youtube.com/watch?v=25LYckPDZ_Q
https://www.youtube.com/watch?v=QcXwYYtxk0U

I did not finish the run-in yet, but it does pull like a train...
I took my son with me for a ride, when both of us weigh about 150kg /24Stone.
We climbed up a bridge, about 10 Deg incline, I guess, in 4th gear,
Easy, rolling at about 65Kph/40Mph, and then I tried to accelerate for few sec,
Just to see if it can, she immediately started pulling significantly... ((-:
What a difference from my WM20/500cc...

I use the M21 376 Carb with standard jetting, A 20T Crankshaft sprocket and 19T final drive sprocket.
Thinking about gearing it up later for a lower rev cruising...

Noam.

Have to agree about the benifits of Ian Wright's piston kit; the 3 piece oil control ring is worth the price of admission, much less blow by now. Pulls higher gearing and a bit easier cruising at lower rpm. Overall a very nice addition to the machine.

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email (option): teladelujo@msn.com

Noam and John - i'm sold!

just need ian to chime in with his piston availability

Good luck, buddy !

Just a bit ow work an good exhaust valve lifter,
As there is no way to kick it started without it...
Noam.

Hi

There are some other options to consider. This post from a few years ago, you were part of the thread too, so may have decided not to go down the Holden route, but there are some other options. Trevor mentions VW air-cooled. I'm doing a project for a bike for Lake Gairdner based on an M21, so I'd be interested in what you decide?

http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1348131&cmd=show

The VW mention is in this thread. Better rings like three part oil control and so on are possible if the piston matches something available from other applications I think!

http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1361174&cmd=show

You're in Melbourne too, do you go to the BSAMOA meets, the AGM is next Wednesday?

Cheers

Richard

email (option): Dickie.bobbie@hotmail.co.uk

Richard - yes I remember the holden piston thread - but a bit of digging on the web - came across the JP pistons catalogue.

says both m20/m21 = 82 mm bore -which we all know and 0.75" pin. (wrist or gudgeon- depending on your allegiance)

and notes :
M21 piston has a comp height of 1.221"
M20 piston comp height of 1.59"

the holden 6 pistons(173ci bores= 88.9mm) all have comp heights of approx. 1.78- 1.8" and pin dia of 0.8661"

so to fit one is a bit of an engineering exercise -and really only for a 500 crank .

id rather see if ian has the pistons ready to go with correct pin and comp height - less messing about .

aquick check of the VW psitons at 90mm shows they are about 42mm (1.65") comp height -and generally 22mm pin (0.866") agin really only for the 500cc M20 crank - plus the pin fix .

not a member of the bsa club -was , but let it slide - clashed d with my other love - early falcons- XK-XP club meeting . do go to the All Brit rally.

Way way back there was a site called "the Victory Library "
It was full of period and latter go fast modifications.
The VW piston came from there , along with putting the plug in the timming hole & filling in the head.
There is at least 2 M20's I know of the are running VW pistons and they are very fast .
One is a custom chopper that gets ridden with HD's and that bike flies the other was originally built for hill climbs but ended up becoming a club ride bike till the owner went to the dark side ( Notruns ).
I cast the heads for the latter and he machined a combustion space & inlet / exhaust ports .
He started out with about 5 heads and I got 3 back to return to the furnace.
At that time I was riding the B40 / B50 & A65 so was not really interested in working an M20.
This bike looked just like a normal M20 as I cast the head using an iron one as the pattern with about 1" of meat under the head for him to machine .
When painted black, it looked just like a normal M20 head with a thicker than usual head gasket and the plug in the wrong place.

email (option): bsansw1@tpg.com.au

Please put me down for a piston kit too as I've a spare barrel that no one seems to want so might as well up the oomphage.

If Ian can let us know where to send funds too and the pice that would be helpful too.

Cheers.

email (option): bo@whohareswins.com

Hi Ian. Can i have 2 piston kits when they come round again, Regards John.

email (option): rustytomm20@hotmail.com

I know a possible way to get a 90mm piston without much hassle. Ross racing pistons....they have on file a M23/24 piston...I called a while back as I need one oversized. They said minimum order of 2 pistons. I don't see why they can not scale it up from 82 to 90mm, or whatever you fancy!!! I believe they wanted 200 per. I'll look later when I'm in my shop and get the piston number if anyone is interested.

Adam

email (option): xeon25@hotmail.com

You'll require a 'flat top' piston for the M series engines unless you want to alter the cylinder head as well, so an M24 piston won't be suitable...

An oversize 'long rod' B33 piston would make a better pattern for pistons to use in a side valve with M21 flywheels fitted..As a bonus these can also be used to 'oversize' the B33 as well, thus increasing the possible demand...

That is exactly what I did when I had batches of pistons manufactured...

I also took the opportunity to employ a more 'modern' 3 piece oil scraper ring and narrower compression rings...Plus the pistons were forged, not cast, so also an improvement...

It should also be born in mind that with modern low expansion alloys and the increased diameter the piston, skirt clearance also has to be altered from standard (and varied slightly for different applications)...Ian

email (option): ian@wright52.plus.com

Hi, Phil here in Adelaide, S Aust. I also am starting a project of M20 with big bore/stroke for DLRA. 0438 935 958.

email (option): tilbrookphillip@gmail.com

I use the flange bolts for location...I've used that method on a number of oversize engines without any problem as the studs are a pretty snug fit.
You will also need to put a chamfer at the lower end of the bore to help in engaging the piston rings when the barrel is put on...I make a fairly shallow angle chamfer using a medium cut half round file and then smooth it over with some emery..That sounds a bit crude but cast iron files quite nicely....I've done plenty like that as well, in fact I've got one to do shortly for my 650cc B33 engine....Ian

email (option): ian@wright52.plus.com

Hmmm...Strange computer glitch...My nonsensical post above was a reply to a much earlier post made in 2017 which somehow appeared as the previous post when I came onto this thread...but then disappeared...Twilight Zone!!!.....Ian

email (option): ian@wright52.plus.com