Removal of surestart middle slit port

rex f. Thu Jul 25, 2019 4:36 am

Anyone suggest an easy method of removing the centre bar
On surestart cylinders have a lathe & most other modellers stuff
Thanks rex f.

balogh Thu Jul 25, 2019 5:41 am

COX International and Exmodelengines sell cylinders after such modifications, they or their subcontractors certainly know how to do this.

I assume a sort of saw disk like tool is used that also lowers the edge of the bottom slit for SPI.

rex f. Thu Jul 25, 2019 6:31 am

Thanks however there work varies by as mutch as ..015 in port height
Thereby increasing exhaust duration to as mutch as 155 degrees
Your motor will not pull pitch
Thanks rex f.

aspeed Thu Jul 25, 2019 7:53 am

An indexing fixture on a mill with a sharp cutter. If you set up a stop on the collet you can creep up on the height you want. I was really surprised how well one of my motors ran with the slit cylinder. I would not have believed it if someone told me, after reading about all the porting and polishing articles over the years.

crankbndr Thu Jul 25, 2019 10:29 am

IMO you are better off shorting the piston for 10 thousands SPI, I found some of the cut cylinders had too much SPI.
I experimented a lot with shortened, lightened piston in surestart cylinder and had some over 22K with 5/3 prop and 25% nitro.

rex f. Thu Jul 25, 2019 8:51 pm

Thanks crankbender have had the same result eith.025 spi& 35% nitro
Merlin drop in 3 shims rpm approaching 24.000
Rex f.

KariFS Fri Jul 26, 2019 4:27 pm

Interesting stuff. To my eye, the standard exhaust port of an SPI cylinder looks a bit large, and there are two of them. I would also rather go for shortening the piston skirt, but what would be the best way to do that?

I don’t have a lathe, the closest thing is an El Cheapo drill press Embarassed

Maybe I could take a well run-in piston, re-set the ball joint and start experimenting Huh...

some masking tape should protect the piston well enough when mounted in the drill press chuck... maybe?

10 thou is about one quarter of a millimeter, not a lot as such but considering the size of the engine, quite significant.

rex f. Sat Jul 27, 2019 2:02 am

Thanks karif f icut my pistons in the larhe but you can do it with a bench grinder
And a bit of fuel line to protect the rod you need to remove about .030 bot double check
As all engines are different 70 degrees of spi is good
Thanks rex f.

getback Sat Jul 27, 2019 5:13 am

https://www.coxengineforum.com/t4279-lightening-a-piston-using-common-tools?highlight=piston+shorten July-27

Cox International Fri Aug 02, 2019 9:00 am

rex f. wrote:Thanks however there work varies by as mutch as ..015 in port height
Thereby increasing exhaust duration to as mutch as 155 degrees
Your motor will not pull pitch
Thanks rex f.

Rex,

We deliberately have the exhaust ports cut lower to induce SPI. On a reed valve engine, this increases RPM by as much as 1,500.

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Removal of surestart middle slit port