# Rotax 670

Discussion in '2-Stroke Aircaft Engines' started by Phenomdriver, Jul 23, 2019.

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1. Jul 23, 2019

### Phenomdriver

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Anyone here have experience with the Rotax 670, such as reliability?

2. Jul 23, 2019

### PW_Plack

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I know two guys locally who have flown with a 670.

One is a PPC flight instructor. His 670 is a Rotax original, and it's been reliable for years. It matched a 914-powered pusher gyroplane in a static thrust test at a local event, a real beast, only thirstier than a 912/914 as you might expect.

The other is a gyro pilot who bought a "frankenstein" 670 from a non-Rotax-authorized rebuilder...let's just call him "Notax Nick." It was made from 618 and other parts, and had some interesting experiments such as sealants in places where where Rotax calls for gaskets. When it blew up in-flight at less than 50 hours - all three times - it was repaired under warranty, but it was put back together with severely pitted cylinder heads which were not airworthy.

The 670 has been out of production for a long time. There might be better choices now, such as the Yamaha four-strokes. But if you pursue the 670, IMHO, make sure you're getting a real 670.

3. Jul 23, 2019

### Armilite

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Any Engine is Only as Reliable as the Person who Builds it, Maintains it, Flys it, and Stores it! Most all Engine Failures are Human Related.

The Standard 670 using 11.5cr is probably one of the best Skidoo/Rotax Engines they ever built, made 1992-1999. Stock for a Sled their rated 115.7hp@7750rpm. There was also a 670HO using 12.5cr and better Cylinders in 1998-99 about 20hp more than the Standard Engine.

For Almost, All Rotax/Skidoo 2 Strokes, Detonation is the #1 Problem account they use High Compression Ratio's in most of their 2 Strokes. Minimum Fresh 91 Octane. Pump Gas can lose 2 Points in Octane in as little as 3 weeks. The Higher the HP/RPM you use, the more OIL I would use. Rotax Rick is probably the best Authority on 670's, he uses Amsoil Interceptor at 40:1, NON-OIL INJECTION only. There are a few other Oils he also recommends.

I have discussed using the Full Synthetic's with him, he has tried some, they Worked, but he is Old School, needs to see Oil Dripping off everything. Hirth has their Blue Max Full Synthetic they say 100:1, and Amsoil has their Saber Full Synthetic at 100:1. For a 670 using the Full Synthetics, I would probably start at 80:1 and probably go to 90:1 on a 670. I'm running 100:1 Amsoil Saber in my 292 TnT Race Sled with 93 Octane, and I like it. I'll be switching my 503UL over to it once I get my Airframe upgrades done.
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Atmospheric Pressure is 14.7 x 11.5cr = 169.05psi at Sea Level, and Good Fresh 91 Octane is Good for Max 175psi. So your Rotax Engine is on the Edge from suffering from Detonation. 100LL has a 5 Year Shelf Life. A 50/50 Blend of 100LL and 91 makes about 96 Octane. For a 670 92-95hp@6500rpm, a High HP Engine Pulling/Pushing a Heavier Airplane, I would use a minimum of 96+ Octane myself. Most Airports only carry 100LL, so that's what I would use. Your Air Density affects your psi, roughly about 3% for every +1000ft.

A 277UL, 14.7 x 11.8cr = 173.46psi.

A 670HO, 14.7 x 12.5cr = 183.75psi, some 532UL's also used 11.5cr and some used 12.5cr.

As Rotax or one of their Service Centers has No Control over what People use for Oil & Oil Ratio, or Octane Gas they use, or the many other Abuse's they do to an Engine, these Engine Builders don't have any control either.

Rotax 2 Strokes and most All 2 Strokes use a Sealant between Case Halves, there is a Gasket between the Cylinder & Case and Cylinder & Head only.

Pitting in the Combustion Chambers is a Visual Blemish, it doesn't affect Airworthiness! Cracks do!

4. Jul 24, 2019

### PW_Plack

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Fly whatever you're comfortable with, but these pits looked like they were made by metal fragments getting smashed into the head by what was left of the pistons. They were certainly severe enough to cause hot spots.

5. Jul 24, 2019

### Phenomdriver

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I talked to Rick for some time yesterday. He has a wealth of knowledge on the 670. Thank you all for your posts. I will probably go with a 670. It could be a great performer with a C box PRSU. Looking at just the recoil starter to keep weight down. Need to research radiators. Some info out there says a Honda Magna motorcycle radiator might work. Aiming for power and simplicity.

6. Jul 26, 2019

### Armilite

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Those Pits can be from Detonation, from a Broke or eroded Piston like this 583 piston, from a Broke Piston Ring, also caused by Detonation, Bearing Cages that Fail, and Rod Shims that Fail usually from not using a Good OIL. Using Upper Rod Cageless Bearings solved one of the issues.

Skidoo 583 11.5cr I took apart suffering from Detonation.

670HO Head 12.5cr. Cosmetic Blemishes.

7. Jul 26, 2019

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Not to take the thread off track.....but.....I have a 670 and will need to build it for aircraft use very soon,for my Stork project.
I know Rick builds these engines but I've wondered if anyone else also builds the 670 engines......
I have nothing against Rick what-so-ever.....but depending on one person to supply every 670 engine is the equivalent of putting all your eggs in one basket.
What happens if he decides to retire........???

Kevin

8. Jul 26, 2019

### Armilite

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Building a similar 462UL, 532UL, 582UL, 618UL, 670, Rotax is no different than the other. A 670 is the same as the 618UL, both use RAVE Valves is all. Just different Bore & Stroke used. Any Good Skidoo or Rotax Service Center can Rebuild one. But your Paying for his vast knowledge of what he has learned from Rebuilding over a 1000 Rotax's. You could learn to Rebuild it also. If you want another Big Name to Build it, Big John from Sled Head Racing in Mn is the one I would use, besides Rotax Rick. The Crank Shop, has a good reputation also. There are DVDs out on How to Rebuild a Rotax. The only Hard part to Rebuilding any of these Rotax's is if the Crank needs Rebuilding. Pigsaw on eBay out of Canada rebuilds a lot of Cranks, there are others. The Tools Needed to Rebuild a Crank is about the Cost to Rebuild (1) 670 Crank. A Rebuilt 670 Crank Averages $650. You have (2) different 670 Cranks 93-94, and 95-99. Only the 93-94 can use the Ducati Dual Ignition. NEW Parts to Rebuild a Skidoo 670 Crankshaft rebuild kit Crank Shaft repair Rotax twin-cylinder MXZ on eBay$289.99.

A Ski-Doo Formula Deluxe 670 1999 Wiseco Std Pistons & Gasket Set; Engine Rebuild
Brand New on eBay $263.00. You still have Carb Rebuild kits to buy. Ski-Doo MXZ 670 Carburetor Carb Repair Rebuild Kit 1996 1997 1998 Brand New$71.06

Just $289.99 +$263.00 = $552.99 + Mikuni kit$71.06 = $624.05, Labor is usually$500-$800+ depending on WHO is ReBuilding it. But I believe Rotax Rick puts on New Bing 54 Carbs, about$300-\$350 each.

9. Jul 29, 2019

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I have the majority of the parts that I need to convert my engine already....
Bing carbs,582 head,Ducati ignition....etc.

Kevin

10. Jul 29, 2019

### Armilite

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===========================
Is your's a 93-94 or 95-99 670? One thing, there were about (4) different Rotary Valves made for the 670, I would try to find out which one Rotax Rick uses. The 582UL Head will need to be opened up for the 670 78mm Bore, usually 1mm oversize. If you want it to live a long time, do what he does, use 40:1 Amsoil Interceptor Oil, use Non-Oil Injection, and use a Ceramic Top Coated Piston with Moly Sidewall Coating. Use a Cageless Top Rod Bearing's. The important thing is what are you going to do for a Tuned Pipe? Somewhere I have a Tuned Pipe Designed by some German guys who built a 670HO for a Plane years ago. It would be best to Plug in your Standard 670 Engines Spec's into one of the Tuned Pipe programs if you make your own.

How To Rotax.
http://www.homebuilthelp.com/New_Rotaxengine.htm

ROTAX 582 REBUILD DVD
Part # 15-05904
https://www.cps-parts.com/catalog/rtxpages/15-05904.php

11. Jul 29, 2019

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93-94 Mach I ....sled engine

I'm not sure what I'm going to do about the pipe yet.
Rotax Rick has someone making pipes for him but I'm not far enough into my build to start mounting everything up to check clearances....his pipe may fit or it may not....dont know right now.

Kevin

12. Jul 31, 2019

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