Crank repairs or not worth it?

WesG · Feb 18, 2010

I have a 797 that a rod broke on the pto side cylinder just below the wrist pin eye. The cases are wrecked but the crank seems ok, meaning if you hold the other piston and the broken rod out of the way the crank turns over fine with the primary(I cleaned all the shrapnel out of the case first). Can this be repaired? Is it worth it to repair it? What am I looking at for cost to repair this if possible? I have not dealt with crank repairs before so I am curious what can and cannot be done. How about pressing the shorter 600 rods on to the 797 crank, can that be done so it will fit in a stock case and turn it into a 660-685? Or are the 797 rods offset on the crank?
Thanks

JET FUEL · Feb 18, 2010

You'll have to check but my 797 the crank is pinned and welded. They did this to help keep it from twisting out of phase during the harsh environment of racing. If yours is pinned and welded you won't be able to repair it, you will have to start over. Pull it out and make sure its not welded, if it isn't than you can get it repaired.

JET FUEL · Feb 18, 2010

If you use a 600 crank you will have a 727/726 (depending on what pistons you have) It might be a cheaper faster option to find a 600 motor and use your top end, then fix your crank as time and money permits. The 797 crank shares the stroke with the 800 cfi motor.

Angermangement890 · Feb 19, 2010

I don't think it's as easy as swapping in a 600 HO crank to make it a 727, if my memory serves me right the cylinders are different between the 727 and 797 due to port location and port timing due to the difference in strokes of the engines. The next problem is if it was a snocross 797 that was converted to trail use, there is a good chance it uses the one off cranks which the race dept had specially made. Finding parts might be a real bear.

meathooker · Feb 19, 2010

Angermangement890 said:
I don't think it's as easy as swapping in a 600 HO crank to make it a 727, if my memory serves me right the cylinders are different between the 727 and 797 due to port location and port timing due to the difference in strokes of the engines. The next problem is if it was a snocross 797 that was converted to trail use, there is a good chance it uses the one off cranks which the race dept had specially made. Finding parts might be a real bear.

The Carl's Cycle performace department stated the 797 polaris race motors are built by SLP and use their 727 cylinders...They also said your could but a 600HO/VES (dependingo on cases used) crank and run it as a 727.

i've never tried first hand it but that's what they told Jet and I.

Angermangement890 · Feb 19, 2010

meathooker said:
The Carl's Cycle performace department stated the 797 polaris race motors are built by SLP and use their 727 cylinders...They also said your could but a 600HO/VES (dependingo on cases used) crank and run it as a 727.

i've never tried first hand it but that's what they told Jet and I.

I spoke with Jim Fairchild at SLP, he is the one that told me it would not be compatible due to the differences in port timing. He also says he had no idea where the cylinders for the 797 that Carls built were coming from. And when the 726/797 engines were in production, SLP was the subcontractor, Polaris put all the pieces in place as far as suppliers went, SLP made the pipes and assembled the engines. After the 600 became the mod engine, SLP licensed the right to cast the 726 cylinder from Polaris, they made slight porting changes to make it pump gas friendly and then renamed it the 727.

On a complete side note, it makes sense to me the cylinders would be different heights, the 797 crank has 6mm more throw, they use the same piston between the 727/797 so the extra distance the crank travels has to be accounted for someplace, a taller cylinder makes logical sense to me.

meathooker · Feb 19, 2010

that does make sense the cylinders would be different to account for the longer stroke...never thought of that.

i wonder if you took 6mm off of the bottom of the 797 cylinder if it would work for a 727??? it's probably not that simple

sounds like the guy at carl's was talking out of his a$$

Angermangement890 said:
I spoke with Jim Fairchild at SLP, he is the one that told me it would not be compatible due to the differences in port timing. He also says he had no idea where the cylinders for the 797 that Carls built were coming from. And when the 726/797 engines were in production, SLP was the subcontractor, Polaris put all the pieces in place as far as suppliers went, SLP made the pipes and assembled the engines. After the 600 became the mod engine, SLP licensed the right to cast the 726 cylinder from Polaris, they made slight porting changes to make it pump gas friendly and then renamed it the 727.

On a complete side note, it makes sense to me the cylinders would be different heights, the 797 crank has 6mm more throw, they use the same piston between the 727/797 so the extra distance the crank travels has to be accounted for someplace, a taller cylinder makes logical sense to me.

JET FUEL · Feb 19, 2010

Angermangement890 said:
I spoke with Jim Fairchild at SLP, he is the one that told me it would not be compatible due to the differences in port timing. He also says he had no idea where the cylinders for the 797 that Carls built were coming from. And when the 726/797 engines were in production, SLP was the subcontractor, Polaris put all the pieces in place as far as suppliers went, SLP made the pipes and assembled the engines. After the 600 became the mod engine, SLP licensed the right to cast the 726 cylinder from Polaris, they made slight porting changes to make it pump gas friendly and then renamed it the 727.

On a complete side note, it makes sense to me the cylinders would be different heights, the 797 crank has 6mm more throw, they use the same piston between the 727/797 so the extra distance the crank travels has to be accounted for someplace, a taller cylinder makes logical sense to me.

Nope, the 726 and 797 cylinders are the same even the heads. thats funny jim would say that because carls gets the cylinders from them. I have a 797 from the polaris race dept. I burned it down and carls helped me build it into a pump gas motor. How'd I do it? Carls re-nic'ed my cylinders and added nickasel so I could run SLP 727 pistons and a slp 727 head. nothing was done to the porting. My new displacement is actually 795cc same as the dragon motor. The extra throw is compensated with the rod length, 797 has a shorter rod.

The difference between the 727 and 726 is a millimeter ish in the bore and the exhaust valves has been changed, slp did that to match the valves on the motors they are trying to sell the kits for. The 777 kit is basicly the same as the 727 but uses the 700 stroke and they added the bosses for the injectors along with matching the powervalves so you can use the ves bellows and housings from the motor your putting the kit on.

So you can use the 797 cylinders on a 600 bottom end and have a 727/726 depending on if you re nic and use the new pistons. The new case will have to be machined.

Angermangement890 · Feb 19, 2010

JET FUEL said:
Nope, the 726 and 797 cylinders are the same even the heads. thats funny jim would say that because carls gets the cylinders from them. I have a 797 from the polaris race dept. I burned it down and carls helped me build it into a pump gas motor. How'd I do it? Carls re-nic'ed my cylinders and added nickasel so I could run SLP 727 pistons and a slp 727 head. nothing was done to the porting. My new displacement is actually 795cc same as the dragon motor. The extra throw is compensated with the rod length, 797 has a shorter rod.

The difference between the 727 and 726 is a millimeter ish in the bore and the exhaust valves has been changed, slp did that to match the valves on the motors they are trying to sell the kits for. The 777 kit is basicly the same as the 727 but uses the 700 stroke and they added the bosses for the injectors along with matching the powervalves so you can use the ves bellows and housings from the motor your putting the kit on.

So you can use the 797 cylinders on a 600 bottom end and have a 727/726 depending on if you re nic and use the new pistons. The new case will have to be machined.

I spoke with Jim on tuesday 2-19-10 and he was quite adamant they were different. He said the port timing would be way too high trying to put the 727 cylinder on the 70mm stroke and would not work. Interesting topic though, I wonder why he would say that its not doable in one conversation and then say it will work?

There is one way to settle the question though. I have access to a 726, lets measure the physical height of the 726 and 797 cylinders. Then we will know if they are different.

JET FUEL · Feb 19, 2010

That is curious, Let me know the height. I'll measure mine up.

Roadwarrior102 · Feb 19, 2010

I have both 797's and a 726, the cyl are defenitly different, the 797 is a few mm taller. I have not speced them out in a while, but they are diff. The bore on both engines are the same if they are old race engines, The SLP 727 does use a slightly different bore. The polaris race dept used to have slight over size pistons for cyl that needed to be honed hard, I think they were .0005" over. The cranks on the 797 were custom made, I may be able to rebuild one or two with my spare parts. PM when you know if yours is welded. Eric

waterskipper · Feb 22, 2010

Roadwarrior102 said:
I have both 797's and a 726, the cyl are defenitly different, the 797 is a few mm taller. I have not speced them out in a while, but they are diff. The bore on both engines are the same if they are old race engines, The SLP 727 does use a slightly different bore. The polaris race dept used to have slight over size pistons for cyl that needed to be honed hard, I think they were .0005" over. The cranks on the 797 were custom made, I may be able to rebuild one or two with my spare parts. PM when you know if yours is welded. Eric

Correct i have both motors also there is a slight differnce in cylinder height, ill take a pic of the 2 cylinders side by side and post it up, northern crankshaft can rebuild the 797 cranks(so they told me, theyve done a few in the past), it would take them a lil more time and money but you dont have many other options, unless you get lucky and come across some, and the polaris 797 race cyclinders are different from the new slp 797 cyls as the exhaust valve as smaller on the race and on the new slp wider. and i got lucky and was able to pick up a spare crank for my 797(thanks roadwarrior) and have been able to find other spare parts. and recently i found somebody else with a few bins of 797s parts which im gunna try and snatch up.

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Crank repairs or not worth it?

WesG

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Angermangement890

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meathooker

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