Mag Housings

[JETEX]

I’m building a GP 200 engine. I have a mag housing but I’m not sure if it’s off a small block 125 / 150 engine or a large block 200. Are they different ?. Somewhere at the back of my mind, I think there’s a difference in the shape of the transfer port on the mag housing. Your opinion appreciated. Thanks.

[Knowledge]

There is a small difference in the 125/150 and the 200 mag housings on the flywheel side transfer port but it is barely worth worrying about. It is detailed in the Spanner’s Guide (all editions).

In fact, at some point in one of the production runs the 125/150 mag flange was standard figment in the 200 case because of supply issues with the 200 mag flanges.

If you are really keen, Dremmel out a bit of ali to match them up but you will notice no change in performance for undertaking this work.

[AL BARKER]

I've got mag housing from a LIs engine that's never been apart until I got my hands on it and the scooped area is the same as on new uni housing...supposed to be different but as Knowledge said I wouldn't worry about it too much.

[JETEX]

Thanks for your replies, I’ll go with the one I have then.

[Warkton Tornado No.1]

There can be an issue caused by the spigot of the cylinder contacting the magneto housing preventing the gasket faces meeting, so watch out for that. It's certainly something that if unaddressed can deflect the spigot inwards causing a reduction of piston to bore clearance

Rectification involves removal of material either from the spigot or from the housing. If proper port matching of the lower transfers is done, that in itself will effect a cure.

[JETEX]

Warkton Tornado, great advice. That’s appreciated mate, thanks.