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I endorse what Jspooner has said. I use a heatgun it is slower to get to temp but there is no risk of melting the fins.

Do a google search on removing broken studs with Alum. Alum will erode the tap and spares the aluminium. It is slow needs warmth but does work.

Gill I did a quick google of GT250 cases and from a cursory inspection they share a lot of similarity with RD ones.

I don't think it would be too hard to make a four, a mate and I built a four cylinder Yamaha by cutting and welding cases and it really wasn't that problematic.

I would enjoy watching it being build so please  do start a thread.

If you have any questions about what we did ask and I will try to answer.

Have you looked at GT380 or 550 cases? Less complication than the watercooled 750.

It's a lottery with those cylinders, one I bought was not useable, the liners are pressed up from the bottom and can move down, and the porting is for the Indian market.
If you use them without fixing the porting the bike would be a complete slug and unreliable to boot.
They were a good option for us but I would not recommend them to anyone as an upgrade.

Love it !

There is a clip of its startup on Facebook. Link below.
Scroll down to the 20th of October.
https://www.facebook.com/pages/category/Personal-Blog/dowman_racing-1900237280084327/

The ignition is an Electrex world very similar to an HPI.
It does  sound nice but we have been told its a bit quiet.
Construction details are on XS650.com, in the lounge, new 700cc outfit.
Will try and post a link.
http://www.xs650.com/threads/new-700cc-outfit.52876/

More photos

Had its first races last weekend.

This may be of help with testing.

Machining Combustion Bowls

Hi I have been working at setting the squish and compression ratio of a couple Air Cooled RD 350 top ends. They are to be used on a 4 cylinder 700cc RD based sidecar.
I have been using the information from this forum and Chuck's photo's for guidance.

The cylinders have been O ringed, the heads have been centralised to the bore and dowelled, and squish has been set at 0.9mm. The angle of the squish band is 14 degrees.

Presently I am preparing to machine the bowls and have some questions.

From my workings to get a 43% squish band the inner diameter of   band should be 48mm (radius 24). However from Chuck's photos the radius cutter is set to a lot more than this.
What radius should the cutter be set to?

Regarding Compression ratios

It is intended to run on 98 octane or better premix. With Electrex world CDI ignition with advance and retard curve.

Initially   thinking to set the compression ratio of cylinders 1 and 4 to 6.7:1 and cylinders 2 & 3 to 6.5:1.
The lower ratio of the inner cylinders is an attempt to mitigate the lesser cooling.

The exhaust port is presently 28mm down from the top.

Once running and reliable it is planned to raise the EX port to 27mm. This will lower the CR of cylinders 1 & 4 to 6.5:1 and 2 & 3 to 6.3:1.

Do these numbers look right?
Thanks for reading all this.

Great read , thanks for posting.

Hi my first question on this site, I as lurking for  a long time on the old site and pleased to see it back.

Preparing to work on the heads for an air cooled RD 350 special race motor.

Peak power is targeted at the moment at 9000 RPM. Using Mitaka RZ 350 pistons (cast) with the rear tangs removed. Using Rods manufactured by Long.

The Heads will be pinned central to the bores.
The corrected compression ratio will be set 6.5:1. Running 95 or better octane.

The cylinders have been O ringed and looking to set the squish at 0.9mm (slightly more than 35 thou) at 14 degrees and 50% of bore area (9.4mm wide).

My best calculation/ measurement of the angle of divergence of the squish is 5 degrees.

Before anything is done I want to calculate the resulting MSV using TorqSoft.
I have in my notes a range of MSV of 15-22m/S for road racing but I didn't record he source.

Can anyone validate the range of MSV I have or more importantly give a more definitive MSV to go for?

Thanks

Well done and good luck for the future.

I read every post of yours and Lyn Garlands I see, you are  both very generous with your knowledge thank you.