[RotaxFlyer]

"When I built MY big bore I got the liners from Union Bay it came with with the port modified to use with a stroker crank, I cut the bottom of the jug off to get the deck to match the piston! The problem is this was 20 years ago and Union bay is not in business any more."

@mydoo,
what kind of stroker crank do you use? The watercraft crank fits plug and play or modify with parts from the stock crank? Where can I get data about the watercraft pistons (dimensions)? Interested in mod a 670 in a torque monster. 7200 rpm with lots of torque would be great for me. Sadly I am 20years late, but let this old stuff live and race
Would be great to get some information, thanks a lot
greetings from the snowless Germany

I have never heard of any Stroker Cranks being made for the 670. Stock is (78mm x 70mm) 669.2cc. Big Bores that I have seen made. 670 used 132mm Rods.

(81mm x 70mm) 721.7cc called a 720.

(82mm x 70mm) 739.6cc called a 740.

(82.5mm x 70mm) 748.6cc called a 748.

Seadoo made a 718/720 Stock was (82mm x 68mm) 718.5cc. Used 125mm Rods. Seadoo PTO's are Threaded. I think the 718/720's use a bigger 102-104mm Head (6) bolt pattern. Never got a 718/720 Cylinder to figure it out. With a little effort, I'm sure they could be adapted to fit. I have always wondered if a 650/657/718/720 68mm Stroke Crank would work in a 583/617/670 Case. Could the Threaded PTO still be turned down to either 33mm 1:7.5 or 30mm 1:10 Taper? Hot Rods in Des Moines, IA still make New Seadoo 650/720 68mm Stroke Cranks.

Seadoo also made a 650/657(Non RAVE) Stock was (78mm x 68mm) 650.1cc. Used 125mm Rods. The 650/657 use's the same (6) Bolt Head Bolt pattern as the 670 based off a 100mm Bolt Circle. On the 650/657 Cylinders, two bolt holes line up, and two are offset a little.

Skidoo made a 617 (76mm x 68mm) 617.2cc. The only Big Bore I have seen offered was (79.5mm x 68mm) 675.3cc. Used 125mm Rods.

Same 79.4mm Piston was also used to Big Bore a 580/582/583 Stock (76mm x 64mm) 580.9cc and (79.4mm x 64mm) 634.0cc. Used 125mm Rods.

There was a Big Bore Sleeve made for the 521 (72mm x 64mm) 521.3cc to make it also into a 580. Seadoo also made a 580/587.

My Yellow Seadoo 650/657 Cylinders on a 670 Case. You would also need to plug or Weld up those two little holes on Exhaust Side.

 

[RotaxFlyer]

Union Bay Billet Heads, Billet Rotary Valve Covers, and other Brands come up on eBay once in while. You could use a Seadoo 650/657 Billet Head with an Inline Thermostat, and a Bolt on Water Neck.

 

[RotaxFlyer]

Bolt-On Water Necks for using Seadoo Heads.

 

[Agriaspeedfreak]

@ Rotax Flyer,

many nice information.

Thought to get an information about the crank that mydoo use. may be possible to use the 787 crank with 74mm stroke in combination with 82mm bore.

Read in another thread about the "800" RV engine vs a 800 series III engine... That sound awesome and more torque in the bottom end.

Thanks for any statements about that

Greetings

Agriaspeedfreak

 

[mydoo]

@ Rotax Flyer,

many nice information.

Thought to get an information about the crank that mydoo use. may be possible to use the 787 crank with 74mm stroke in combination with 82mm bore.

Read in another thread about the "800" RV engine vs a 800 series III engine... That sound awesome and more torque in the bottom end.

Thanks for any statements about that

Greetings

Agriaspeedfreak

We tried the 787 sea doo Crank for the skidoo but it is larger main bearings and is not interchangeable with the skidoo Cases. The Crank that was used in our stroked and bored engine was a Custom billet made by Union bay. When that Crank destroyed it self it was replaced by a stock

670 crank. the engine was originally 82 mm bore with 74 mm stroke for 782 cc. When the original crank broke is when I modified the jugs and switched to a stock 670 crank, by cutting the bottom off the jug it lowers the port witch shortens the port timing! The amount I cut off was the distance fron the top of the piston to the top of the bore!

 

[RotaxFlyer]

@ Rotax Flyer,

many nice information.

Thought to get an information about the crank that mydoo use. may be possible to use the 787 crank with 74mm stroke in combination with 82mm bore.

Read in another thread about the "800" RV engine vs a 800 series III engine... That sound awesome and more torque in the bottom end.

Thanks for any statements about that

Greetings

Agriaspeedfreak

While I have never heard of anyone putting in a 787/800 RV Crank in a 670 Case, just about anything is possible if you have the Time & $$$ to throw at the R&D Machining needed done. Are you sure it's 74mm Stroke and not some oddball, like 74.5mm or 74.9mm? There were a few places that use Off Set Crank Pins or Welded up the Crank and Re-Machine them for a Higher Stroke. I would Think it might be easier to just swap out Engines, 670 for a 787/800. Another Option is to add a Turbo to the 670. You also have a Standard 670 1992-1999 and a 670HO Option 1998-1999. So what does a Stock 670HO actually Dyno?

(78mm x 70mm) 669.2cc, Stock 115.7@7750rpm, for 669cc making 115.7hp = 94% Volumetric Efficiency!

(82.5mm x 70mm) 748.6cc. So 748cc using 94% Volumetric Efficiency at 7750rpm = 128hp!

--------------------------------------------------------------------------------------------------------------

(82.5mm x 74mm) 791.4cc. So 791cc using 94% Volumetric Efficiency at 7750rpm = 136hp!

(82.5mm x 74.5mm) 796.8cc. So 796cc using 94% Volumetric Efficiency at 7750rpm = 137hp!

(82.5mm x 74.9mm) 801.1cc. So 801cc using 94% Volumetric Efficiency at 7750rpm = 137hp!

https://www.mk5cortinaestate.co.uk/calculator3.php

Stock 1993 670 Dynoed & and a 670 with a Turbo using 7 lbs & 11 lbs.

 

[RotaxFlyer]

We tried the 787 sea doo Crank for the skidoo but it is larger main bearings and is not interchangeable with the skidoo Cases. The Crank that was used in our stroked and bored engine was a Custom billet made by Union bay. When that Crank destroyed it self it was replaced by a stock

670 crank. the engine was originally 82 mm bore with 74 mm stroke for 782 cc. When the original crank broke is when I modified the jugs and switched to a stock 670 crank, by cutting the bottom off the jug it lowers the port witch shortens the port timing! The amount I cut off was the distance fron the top of the piston to the top of the bore!

Ok, a Seadoo 787/800 Crank would then require the Case to be Align Bored for the Bigger Bearings. I thought he was talking about a newer Skidoo 787/800. Couldn't the 787/800 Seadoo Crank just be rebuilt with 670 Smaller OD Bearings? Do you know if the Seadoo PTO can be Machined to the Standard Skidoo 30mm 1:10 or 670 33mm 1:7.5?

With that much HP, for plane use, I would want to use an RK400 Clutch with the Rotax C Gear Drive. I bet you could Drill & Thread the RK400 since it was Designed for the Smaller 30mm Taper.

Putting a New Hot Rods Seadoo 650/657, 68mm Stroke Crank in a Provision 8 Skidoo Case, 580, 582, 583, 582UL, 670 for Plane use would be nice. All Skidoo 617's were only Provision 4 Case. The 618UL(RAVE) was 73.4hp@6750rpm with Rotax's Muffler. A 618UL with an R&D Tuned Pipe was 90.6hp@6750rpm. It would be Awesome if you could Big Bore some Seadoo 650/657 78mm Cylinders to 82mm and put on a 670 Case.

 

[Agriaspeedfreak]

"The amount I cut off was the distance fron the top of the piston to the top of the bore!"

That is what I want to know. What kind d of piston do you use? Was still UB stuff? So would be possible to optimize the squish with cutting the bottom of the jug to lower the ports a bit to gain bottom end torque? This in best case with big sleeves to take the 82mm pistons. Where are the differences of the deck height of Pro X for 717 and 787? I just wanted to ask what kind of existing parts for example from the watercrafts or bigger engines could be used. Thanks for your data

Agriaspeedfreak

 

[mydoo]

"The amount I cut off was the distance fron the top of the piston to the top of the bore!"

That is what I want to know. What kind d of piston do you use? Was still UB stuff? So would be possible to optimize the squish with cutting the bottom of the jug to lower the ports a bit to gain bottom end torque? This in best case with big sleeves to take the 82mm pistons. Where are the differences of the deck height of Pro X for 717 and 787? I just wanted to ask what kind of existing parts for example from the watercrafts or bigger engines could be used. Thanks for your data

Agriaspeedfreak

The pistons I used in my water craft were Pro X 785 Seadoo pistons because I was using the jugs from the stroker big bore , if you are using a standard big bore liner for 670 you need the 717 Seadoo pistons because of the deck height differance!

 

[turbopete]

https://www.kijiji.ca/v-snowmobile/cambridge/skidoo-asphalt-drag-sled-670-791-cc-big-bore-stroker/1425426675?utm_campaign=socialbuttons&utm_content=app_ios&utm_medium=social&utm_source=email

 

[mydoo]

https://www.kijiji.ca/v-snowmobile/cambridge/skidoo-asphalt-drag-sled-670-791-cc-big-bore-stroker/1425426675?utm_campaign=socialbuttons&utm_content=app_ios&utm_medium=social&utm_source=email

That big bore stroker built by Split Second would be a hot setup! My Sons Drag raced against guys that used there engines and they always worked real well! Split second made real good pipes that made big power! I wonder were that Sled went to!