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Largest Sleeve 809 and Ultra Cylinders

E
Nov 26, 2007
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A buddy of mine owes me some shop time so I'm thinking this may be a good opertunity to get the sleds set up for next year.

What is the largest a 97 Mach Z 809 cylinder can be sleeved to?...and what has proven to be a reliable size upgrade for these cylinders?.

Also what is the largest a 96 Ultra 680 cylinder can be sleeved to?
(This top end is already sleeved to a "800"...HTG kit)...can it go bigger?.

Also looking at the pros and cons of a steel bore vs Nikasil bore?.
 
B
Jan 3, 2008
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I think that 900 is the biggest

you can go on the ultras, aftermarket jugs went clear to 1029 with Price performance hardware, things ripped.
 
B
Nov 26, 2007
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I know you can go to 76mm on both. The ultra cylinders WILL come apart. HTG was doing the ultra "900". CC'd to 884 and they told they've never had anyone break a cylinder other than me. I talked to 4 others that heard the same story. Five guys accounted for at least 20 broken cylinders. The mach's are fine, but don't go any bigger than 72mm on the poo. mike
 

Norway

Well-known member
Lifetime Membership
Nov 29, 2007
1,978
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On the Ultra I would recomend 800. Many people have done this and I've yet to hear of any trobles from just the boring.
You can run stock Polaris pistons if you want to.

Mach Z:
This motor is stroked to 68mm vs. 65 on other 800 engines. This is just nice since it gives you a better bottom end torque.
Not sure if that was the reason, but Rotax made this engine quite narrow. The polaris triples are wider, the T-cat even wider!
This makes a difference when big-boring since you need room for tranfer ports to feed the new displacement.
This can also be a reason to sleeve the biggest bores so you get a tough steel sleeve as cylinder wall instead of a tin-can alu wall with nicasil.

I've heard here long ago that 925 was the biggest recomended for the Z.
Crank Shop would be a good place to call, and Big-John on the forum here!! He's the greatest most helpful guy in the world and runs one himself for the n'th year. Most importantly, he can help you get clutched!!!

Luck.

RS
 
D
Oct 19, 2003
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Rapid City, South Dakota
The common big bores with stock sleeved cylinders on the Rotax 779/809 were 889, 925 and 975. You can build 992's and 1025's useing stock sleeved cylinders, but you have to stroke the crank and use 670 rods for the 992 and 1025 on stock bored cylinders. The 1025's useing aftermarket cylinders are not stroked, but do have 670 rods. I built a 950 last year, which is just an overbored 925. This summer I'm building a 975 for fun. The LA Sleeve cast sleeves come in overbore form for the 925 and then you can get +.040'' O.D. sleeve too. Doing that does use up some transfer volume, but does allow a thicker sleeve on the larger bores. Just don't go much thinner than 1/6'' on the skirts.
 
B
Jan 3, 2008
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I had an 800 engine tech.

The ET motor coupled with PSI tripple pipes (85-8600 rpm) and a rack of flatslides, no twin under 1000cc's could touch it in a drag race. On the hills if you kept the clutching right it would do the same. 800 twins with nitrous could beat them but not by much.
 
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