# Rotax 670

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#### Phenomdriver

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

#### PW_Plack

##### Well-Known Member
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.

#### Armilite

##### Well-Known Member
Anyone here have experience with the Rotax 670, such as reliability?
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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!

#### PW_Plack

##### Well-Known Member
Pitting in the Combustion Chambers is a Visual Blemish, it doesn't affect Airworthiness! Cracks do!
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.

#### Phenomdriver

##### Member
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.

#### Armilite

##### Well-Known Member
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.
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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.

##### Well-Known Member
Log Member
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

#### Armilite

##### Well-Known Member
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
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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. #### MadProfessor8138 ##### Well-Known Member Log Member I have the majority of the parts that I need to convert my engine already.... Bing carbs,582 head,Ducati ignition....etc. Kevin #### Armilite ##### Well-Known Member I have the majority of the parts that I need to convert my engine already.... Bing carbs,582 head,Ducati ignition....etc. Kevin =========================== 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 #### MadProfessor8138 ##### Well-Known Member Log Member 93-94 Mach I ....sled engine Rotax 582...head,ignition,carbs...etc. 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 #### Armilite ##### Well-Known Member ROTAX RICK 670 DYNO TEST with his Tuned Pipe, 92hp@6350rpm. Notice NO Major Dead Spots. #### MadProfessor8138 ##### Well-Known Member Log Member I know this thread is a little old but after reading it again I have a question.... PW_Plack made the comment to make sure the 670 is a "Rotax Original". What was meant by that statement? Is he saying to make sure the engine is a true 670 that has been pulled from a sled instead of being a 582 or 618 that has been bored to make it a 670 ? Even a "true 670" that is pulled from a sled will need to be "Modified/Frankensteined" for aircraft use. Or is he saying to use the 670 sled engine straight from the sled and bolt it on the airframe without "Modifications/Frankensteining" it ? I'm confused as to what was meant by "Rotax Original" 670..... Kevin #### PW_Plack ##### Well-Known Member My intent was to suggest I'd want mine to be an original Rotax 670 as identified on the manufacturer's data plate. #### Rotax Rick ##### Member PW Plack. Before you comment you should have a idea about what your talking about. You cannot make a 670 out of a 618 or 582 . The 670 has a longer stroke & the crank & a 670 crank would never fit into a 618 case or vice versa. The Gyro guy is Jyro Jake who builds erector set gyrocopters with no engineer background. I know of a sever accident in one of his gyros that got hurt bad. He also never had me rebuild the engine he got from a guy for free. That guy never made to to 25 hours in any of the 6 Rotax 2 stroke engine he owned before my 670 ( O BOY ) He ran it at a fly-in at 100% power to beat everybody & blew it up at 12 hours. I warrantied it & even paid the shipping. I had to use a complete new 670 to rebuild as hen blew a hole in the case. At 25 hours he called me to brag that he finally made it to 25 hours but the oil was getting expensive, & asked if he could use a cheaper brand or use less oil. At 52 hours he blew it up again. I installed Cageless lower rod bearings to see if I could build a engine that would hold up to this guys abuse. In the 4 years from installing them until Jake got the engine, we tested Cageless lower rod bearings & did not like the result. I removed them for Jake for$200 to cover the parts in a engine that was way out of warranty. He brought it up to me a second time to remove the Ducati Dual ignition & install the 670's single ignition. ( Why ? Who does that??? ) He claims on another blog that I rebuilt it 4 times & he never even flew it yet. Thats a lie .The 2 things I stated above is all the work I did since Jake got it. As far as sealant the calm I use sealant on the crankcase halfs & the main bearings to prevent spinning in these high hp engines & I use it on the water pump cover along with the gasket. That''s the only place I use any sealant. The 670 OEM fag Pin bearings are no longer made by the "FAG" bearing company. The pin bearings that are out there are cheap China bearings. I will only use the OEM Fag German bearings like Still uses. I use the very best base gasket made & the head uses the Viton OEM o-rings. The parts used off the 582 are : the cylinder head, the Bing 54 carbs , the RV shaft, the Ducati Dual ignition & the little RV oil bottle. That's it. The rest is a factory built 670 SkiDoo short block that gets all my upgrades. I machined the case for the gearbox & ceramic water pump seal. I also have a custom jig to drill the bolt holes for the gearbox. The C or E gearbox flywheel or clutch is machined at a machine shop for me. When a lower rod bearings fails or detonation occurs the head gets damaged. Most of the time they can be cleaned up to look about like new . If not we have to buy a $800 cylinder head from Rotax & I leave that up to the customer. Every time I saw detonation in a Rotax 2 stroke engine & I mean 503, 582, 618 & the 670 the person was using "NON" Ethanol gas. Rotax has a warning out not to use "NON" Ethanol gas. The heads shown by Armlite I feel are to bad to be repaired. The engine needs to be a 1993/94. The 93 has a black head & all 8 holes are cast int the case, They need to be drilled to final size & tapped. The 94 has a purple head Like the rest of the 670, & only has the 4 bottom holes. The 95 & 96 engine has Key hole top 4 holes & is very hard to fix plus the Ducati Dual Ignition will not work with them. The 97, 98 & 99 cannot be used . They cut the 2 lower holes off the case to prevent us from using them. Ok so the concern on me retiring I wish I could but I will be doing engines for years to come plus all the parts are off the 582 so any 582 shop could do all that work & the short block can be overhauled by any SkiDoo shop. I want to warn people do not build a 93 through 96 670 unless the crank is overhauled. These engines use the very early 582 lower rod bearing ( 1990 thru 94) & they failed on the 582 so they will never last on the 670. I have custom double slotted rods that were made for the 97 through 99 engines . The rod bearings are far larger. I don't want the 670 getting a bad name from people building them that have no Idea what there doing. The case has to be cut perfectly for the gearbox. . I more thing, I could get the Mikuni carbs for free but I use the Bing 54 carbs, there easy to tune & I can ship the engine anywhere & have it jetted it right for you. The 670 install is easy. Same 582 mounting holes, carburetors & Ducati Dual ignition & it fits right in. I have sold 432 670's to date & like the 912 we had our early teething problems but the 670 has been perfected for years now. Running at 600 rpm's below the 582 their TBO will be higher. Like The Godfather Mike Stratman from Calf Power System says , the way to reliability in a 2 stroke engine is to get a BIG engine & run to easy. Thanks for your time. Rotax Rick #### MadProfessor8138 ##### Well-Known Member Log Member Rotax Rick........2 points : 1. I made the comment about building a 670 from a 582/618 so I could clarify what PW_Plack was referring to when he said "original"670. I'm guilty of that comment,not him. 2. I'm also guilty of commenting about you possibly retiring before I can have my engine built by you...not PW_Plack. I've realistically got another 1 1/2 - 2 years into my project before I can afford to send you my engine & parts to have a 670 built. I meant no disrespect by the comment but I raised a valid concern on my part. As for the rest of the conversation..... I dont have a dog in that fight so you guys can carry on....... Kevin #### Rotax Rick ##### Member Hi Kevin. I felt you asked the questions people should ask. I'm always here to help you even if you build your own 670. Feel free to call me anytime . (239) 572-0021 Rotax Rick #### Rotax Rick ##### Member PW Plack My intent was to suggest I'd want mine to be an original Rotax 670 as identified on the manufacturer's data plate. I never built a 670 that did not have the original 670 data plate with S.N. on it. So why would you say I did build 670's without this data plate ?? You are attacking me & giving out false information about me. #### Armilite ##### Well-Known Member Don't confuse a Standard 670 Head and a 670 HO Head. The Standard 670 Head gives you 11.5cr and has a different Combustion Chamber for a Domed Piston and the 670 HO Head is 12.5cr and using an almost Flat Top like Piston. This 670HO 12.5cr Head has pitting from Detonation and can be Machined for either same 12.5cr to make it look Pretty again, or to Lower the CR. Almost All Skidoo/Rotax Water Cooled Engines used 11.5cr, except the HO. I have seen a Document of a 532UL using 12.5cr in Europe, which the 532UL here in the USA was made from the Skidoo 521 Type 532 Engine that used 11.5cr. I have never found a 521 HO anywhere. I personally think all of these High 10.8 to 11.5cr Rotaxs would last longer with a lower 9.5 CR as most Hirths (1000hr TBO) and Simonini (600hr TBO) uses on their Engines today, or just use 100LL. Your not going to lose that much hp with a lower 9.5cr, probably 2-4hp. So, for Example if 92hp@6350rpm, the 618UL was turned 6750rpm, could you maybe lower the 670 11.5cr to 9.5cr, and then maybe turn it 6500rpm, or even 6750rpm to gain back any lost HP, probably. A lot has to do with where the Tuned Pipe peaks out! The 95-99 670's usually has the Top (4) Provision 8 Holes Doctored, or the bottom Left Boss Cut Off 670 Case. Many other Skidoo/Rotaxs 440 LC, 500 LC, 580/582/583 LC Cases were also doctored, but not all. They All can be Welded Up and Machined, and use Hardened Steel EZ-Loc Inserts is Best to Fix. You can also have the Head Machined for Single Plug or Dual Plug Inserts. The Bottom Head is the Standard 670 Head. Last edited: #### Armilite ##### Well-Known Member #1 670 with Bottom Left Boss cut off. #2 670 Head with Crankshop Single Plug Inserts. #3 An Arctic Cat Dual Plug Insert. #4 A Union Bay Racing 670 Insert. I like the Bottom of the UBR Insert with the Top of the Arctic Cat Insert, they can be Drilled for either Straight Up for Single Plug use, or Dual Angle for Racing or Airplane use. The Arctic Cat inserts were$90ea 3-4 years ago.

#5 These 670 15.2cr Crank Shop Inserts a Copy of UBR Insert were 116ea a few years ago.