617/618UL Big Bore Option

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Armilite

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I saw this AD on FB the other day where a guy put 670, 78mm Cylinders on a 617. So (78mm x 68mm) 650.1cc. 670's have been Big bored to 82.5mm. (82.5mm x 68mm) 727.3cc. Not sure yet what all he did to accomplish this. You have the (617) 76mm vs (670) 78mm Bore, the (617) 68mm vs (670) 70mm Stroke, different Top Rod Bearings, and (617) 125mm Rods vs (670) 132mm Rods. Skidoo 521's and 617's were all Provision 4 Cases. Would be a better setup if he could have put a 617 Crank in a Provision 8 Case. He has a Good Price on them. Stock 582UL (65hp), Stock 618UL (73.4hp), Stock 670 (95hp). Lots of 580/582/583/582UL Provision 8 Cases out there.

Has anybody ever done one of these conversions?
 

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Armilite

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617/618 Pistons come in 0.080" Over Bore. I have seen years ago a 79.4mm Big Bore Piston for the 580/582/582UL/583, 617/618. I don know if they used a new sleeve or just overbored the Stock sleeve. Wiseco made the Piston. A 580/617 Big Bore (79.4mm x 64mm) 634.0cc and (79.4mm x 68mm) 673.6cc. Rotax Rick's 670 669.2cc makes [email protected] Most Stock Sleeves can be Bored +2mm. You need a minimum 2mm Wall per Dave from LA Sleeve. So Measure your Cylinder Sleeve OD and minus 4mm and that will give you its Max Bore.

I know you can put 580 76mm Sleeves in 521/532UL Cylinder.

132mm Rod minus 125mm Rod = 7mm Difference. One fellow said they just Mill the bottom of the 670 Cylinder to lower it to put Piston Top at TDC. I would think you would have the Port Height to contend with also unless the Piston has different Heights. 2mm = 0.07873992".

Many other Brand Name Sled Engines used the same 18x22x22 Bearing. Arctic Cat had some 81mm Big Bore Pistions.

One option might be to rebuild the 617 Crank using 670 Rods and use the 670 Cylinders as is then you would have more Piston options.
 

WonderousMountain

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Doing the math on 78-68 Blueprint comes under 650cc if there is ever a situation you need to know. My 63-52 4cyl model comes tp +648cc w/18cc chambers. So essentially the same size. It would be better to use great sleeves so no loss in compression over a reasonable time period. Rods should not matter & Pistons can be the ones best suited to it. Trying to match the original seems folly.

Hope it works out,
CK LouPai
 

Armilite

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Doing the math on 78-68 Blueprint comes under 650cc if there is ever a situation you need to know. My 63-52 4cyl model comes tp +648cc w/18cc chambers. So essentially the same size. It would be better to use great sleeves so no loss in compression over a reasonable time period. Rods should not matter & Pistons can be the ones best suited to it. Trying to match the original seems folly.

Hope it works out,
CK LouPai
=====================

Yes, you're talking about a Twin 650.1cc vs your 648.6cc, 4 Cylinder. Rotax used 11.5cr on all of its Water Cooled Engines.

The Rods used do matter account they are 125mm vs 132mm and use different Top Bearings. So Stroke affects Port Timing, hence Piston Height to the Ports will be different. You can Machine the Cylinder Base to lower it for the Shorter 125mm Rod, but you still have the difference in Port Height. You can Machine the Piston with a Ramp where the Port is to accommodate that 2mm/0.07873992" difference if needed.

There are very few Piston options using 617/618UL Pistons & Rods, they only go 0.080" Over or +2mm.

A very long time ago there was a 583/617 Big Bore kit that used a 79.4mm Piston with a Custom made Sleeve. LA Sleeve makes Custom Sleeves and Wiseco makes Custom Pistons, but I would like to find Off the Shelf parts options if I can.

I do have some Seadoo 650/657 (78mm x 68mm) 650.1cc Non-RAVE Cylinders also, but two of the four Bolt Holes don't match up to the Sled/UL Case. Fixable, but another hurdle. Uses the same Head Bolt Pattern. 617 and 650/657 125mm Rods are different than the 617 Rod also. Once again, Fixable.

I got this 670 Case & Crank given to me as is. Guy had a Hydro Lock at high rpm. Crank appears to be ok. I was going to practice Welding Aluminum on it. Only 93-94 670 Cases don't have the Pro8 Holes altered and can accept the Dual Ignition. 95+ usually have top 4 Holes Doctored, some even have the bottom Left boss cut off, and are Single Ignition only. Once again, Fixable with Welding and Ez-Loc Inserts.

670 Case with Seadoo 650 Cylinders.
 

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n3puppy

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=====================

Yes, you're talking about a Twin 650.1cc vs your 648.6cc, 4 Cylinder.

So Stroke affects Port Timing, hence Piston Height to the Ports will be different. You can Machine the Cylinder / Base to lower it for the Shorter 125mm Rod, but you still have the difference in Port Height.
He is talking about the fact 78x68 does not seem equal 650.1 cc
As he says it is "under 650" cc

Here are some calculators that give different results than whatever math you are using.
1) 78x68=649.86cc
2) 78x68=649.9cc
3) 78x68=649.86cc
4) 78x68=649.86cc
5) 78x68=649.857cc

BTW - If the 670 cylinders are machined so the 617 pistons are at the same cylinder height as the 670 piston at TDC, the 1mm difference at the top of the 68 vs 70mm stroke will be less than 5 degrees of exhaust duration. Not a lot compared to the stock 670 that is over 195 duration to start. (Way less than 670 vs 670HO)

Make sure you find an accurate calculator to figure out exactly what the duration would be.

47BB9112-E77E-477B-978F-471B03028A5F.jpeg
 
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Armilite

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He is talking about the fact 78x68 does not seem equal 650.1 cc
As he says it is "under 650" cc

Here are some calculators that give different results than whatever math you are using.
1) 78x68=649.86cc
2) 78x68=649.9cc
3) 78x68=649.86cc
4) 78x68=649.86cc
5) 78x68=649.857cc

BTW - If the 670 cylinders are machined so the 617 pistons are at the same cylinder height as the 670 piston at TDC, the 1mm difference at the top of the 68 vs 70mm stroke will be less than 5 degrees of exhaust duration. Not a lot compared to the stock 670 that is over 195 duration to start. (Way less than 670 vs 670HO)

Make sure you find an accurate calculator to figure out exactly what the duration would be.

View attachment 115837
==============================

The Calc I have used for 10+ Years. Some round up some don't. (78mm x 68mm) shows 650.1251451cc

If the 670 Cylinder Bottom was Machined to lower it so the 617 Piston was at the same TDC, the 670 CYLINDER Ports would still be 2mm Lower 68mm vs 70mm STROKES than the Top of the piston at BDC. You could Ramp the Piston where the Ports meet the Piston. Will that 2mm difference make a big deal I don't know. I don't have any 670HO Cylinders to compare against the Standard 670 Cylinders. I don't have any 617 Pistons to measure or the Seadoo 650/657 Pistons.

I still want to see if there are any Big Bore Pistons for the Seadoo 650/657 out there. I'm not a big fan of RAVE Valves myself, so I'm thinking the Seadoo 650/657 Non RAVE Cylinders are my best option.
 

n3puppy

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==============================
The Calc I have used for 10+ Years. Some round up some don't. (78mm x 68mm) shows 650.1251451cc

If the 670 Cylinder Bottom was Machined to lower it so the 617 Piston was at the same TDC, the 670 CYLINDER Ports would still be 2mm Lower 68mm vs 70mm STROKES than the Top of the piston at BDC.
You could Ramp the Piston where the Ports meet the Piston.

I still want to see if there are any Big Bore Pistons for the Seadoo 650/657 out there. I'm not a big fan of RAVE Valves myself, so I'm thinking the Seadoo 650/657 Non RAVE Cylinders are my best option.
You should delete that calculator from your bookmarks- it has been wrong for 10+ years
Not rounding error - Somebody coded the formulas for mm to inch conversion wrong
Use your excel sheet - 78x68= 649.85728995097 like all the calculators I showed (649.86)
649.86 rounds to 649.9 - NOT 650.1

68 mm stroke does not lower ports 2mm.
More like approx 1mm because 1mm at top of stroke and 1mm at bottom.
Less than 5 degrees duration difference. Not a big deal in the real world. Especially at 6500 rpm.
Plus you can't RAMP a L-ring piston like the 617 or 650 or 670 uses.

If less than 5 degrees still bothers you - there is another option you haven't mentioned - and is commonly used by engine builders. Machine the bottom AND the top.
Machine the bottom to get the port in proper relation for 195 degrees duration at 68mm stroke
Machine the cylinder top/head to get compression back to original.

Leaving that approximately 1mm extra on top would drop the compression from 11.5 to 10ish
Much better for detonation prevention like modern engines. If lower compression bothers you
Mill the head or cylinder or both to get it back up to 11.5 CR

Wiseco makes 650 PWC pistons you are looking for. Up to 80mm but use the same 20mm pin as 670.

You haven't mentioned the 717 Sea Doo - 82x68mm 82-83.5mm pistons available 20mm pin

If you don't like Rave valves - why not remove valves and weld passage closed on a cylinder that actually works on the crankcase?

F4AF6A33-E66D-4F0D-8EEE-22C58B0A2090.jpegCBEEE7C4-D170-48B1-BBE0-A3D566829945.jpeg 01A88027-A1B1-4221-92B1-4CF1D585F470.jpeg
 
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WonderousMountain

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Not to promote Jerry Rigging, but if you have Case, Crank, Cylinders and Rods, then all you need is to jig the thing up right? If I needed to gain 6mm, my go-to would be a copper spacer. Dual ignition would be nice, although debating experts is a poor use of my tongue. Messing around for 10% more cc capacity isn't that special, and rod length is unlikely to be the problem or solution. However, if you want to go with 7075 aluminum, they can be CnC'd reasonably.

Also buy a real calculator.,
CK LouPai
 
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