Top End Time

[boondocker97]

About 2700 miles on these on a Bikeman 900 kit. Has run 32:1 premix 91 octane and Cat APV oil its entire life. No cleaning on any of the parts except a wipe with a shop towel. Overall pretty clean inside. Little to no deposits in the exhaust ports of the cylinders.

Kind of wish I had a leak down tester before I tore it down, but doesn't much matter since it was getting a fresh set of pistons anyway. Think I got my use out of these? Lol. Note the ring plating starting to go.

 

[boondocker97]

Cylinder pictures for those that are interested. Some spots look bad from the pictures depending on which way you are looking at them, but nothing I can really feel a finger nail catch on. Especially if I close my eyes Hit a few spots with muriatic acid where I thought there might be a small aluminum deposit, but no change before and after. I've seen much worse cylinders put together and run again. Maybe these will get a replate on the next set.

 

[Sleds R Fun]

Thanks for sharing. Impressive!

 

[boondocker97]

Little more coating on the new ones.

 

[Pinchecharlie]

I gotta dumb question for you. Does wear on the piston skirt =detonation or slap on two stroke? Sorry never built a two stroke. I'd say those cylinders look really good...can still see the cross hatch. How do two stroke wear as far as out of round from top to bottom? Curious too how the rings weaer. Do they get flat ? Are they super sharp and get ridges? What's the spec for piston to wall . Just curious to see how these engine like to run since theres no oiling lol. Tolerances must be pretty loose huh? Those pistons are pretty tech looking! Can you do the reassembly with lower end in the chassis or do you pull the engine? Newbie questions sorry

 

[boondocker97]

If you get too much piston wear it can start getting some slap and eventually break the skirts off of the pistons. Usually two stroke cylinders will wear out the thin areas between the ports first. The holes in the piston skirts are to allow fuel/oil to come through from the bottom side of the piston and help provide extra lubrication to those thinner spots. I've heard on high mileage ones they can get a ring wear ring at the top. I think the rings get more wear on the edges. As they pass the ports they are constantly trying to expand outward into them and then hit the edge and are forced back in. Get more of a rounded shape, or the plating starts wearing/chipping off like mine were starting to do.

According to the service manual the stock piston to cylinder clearance is 0.0031"-0.0041" for this engine. I'm not sure what Bikeman sets up their cylinders and pistons to run. Might make a difference on piston material and expansion factors. Wiseco forged pistons will expand more than stock cast pistons. The Bikeman pistons are made by Wossner.

I left the bottom end in the sled, but had to take the chassis spar loose on one end that's above the clutches to get the PTO side cylinder on and off. The locating dowels in the rear of my cylinders were all rusty and stuck so I had a fight getting them off. Probably would have been equal time to remove the whole thing in this case. Most two strokes you can leave the bottom end in the chassis if only pulling the top end off. The G4 skidoos are designed to completely remove the engine more quickly for any service work.

 

[sno*jet]

I see you let that front rivet do the lightweight clutch cover mod for ya HaHa, nice sled man

 

[boondocker97]

Heck yeah! Removes material evenly so you don't need to rebalance it! It actually barely touched this clutch. The old clutch got it hot and melted it in flush with the plastic after I pulled and tucked the panels or else I would have drilled it out when the new clutch went on.

It's a constant work in progress... It'll finally be done when a new one takes its place hahaha.

 

[Pinchecharlie]

Oh shi@ is that a thing that happens to them all lol!

 

[boondocker97]

Yeah, if they are tipped over on that side with the motor running in harder snow. Can remove that rivet out of the belly pan if you're worried about it.

 

[FIBLIFE]

If you get too much piston wear it can start getting some slap and eventually break the skirts off of the pistons. Usually two stroke cylinders will wear out the thin areas between the ports first. The holes in the piston skirts are to allow fuel/oil to come through from the bottom side of the piston and help provide extra lubrication to those thinner spots. I've heard on high mileage ones they can get a ring wear ring at the top. I think the rings get more wear on the edges. As they pass the ports they are constantly trying to expand outward into them and then hit the edge and are forced back in. Get more of a rounded shape, or the plating starts wearing/chipping off like mine were starting to do.

According to the service manual the stock piston to cylinder clearance is 0.0031"-0.0041" for this engine. I'm not sure what Bikeman sets up their cylinders and pistons to run. Might make a difference on piston material and expansion factors. Wiseco forged pistons will expand more than stock cast pistons. The Bikeman pistons are made by Wossner.

I left the bottom end in the sled, but had to take the chassis spar loose on one end that's above the clutches to get the PTO side cylinder on and off. The locating dowels in the rear of my cylinders were all rusty and stuck so I had a fight getting them off. Probably would have been equal time to remove the whole thing in this case. Most two strokes you can leave the bottom end in the chassis if only pulling the top end off. The G4 skidoos are designed to completely remove the engine more quickly for any service work.View attachment 366124

Appreciate you sharing all this info, thanks!

Highly recommend carefully mic and gage'ing that crank.. My PTO rod big/small bearing was barely out of spec and it cost me a brand new piston, bottom casing and possibly a cylinder if I cant hone it out.. ?

 

[boondocker97]

Injured my back so not all the way together with this yet. My big end rod bearings don't have any up and down movement that I can feel. No plans of pulling it out and rebuilding the crank at this time. I ended up with what looks like dislodged casting sand in the bottom of a head bolt hole and it won't seat. So now I need to pull that cylinder and clean it out. Might measure the small end while it's apart again, but the surfaces and old bearing rollers still have a smooth finish so I don't suspect I'll find any significant wear.

 

[Pinchecharlie]

Oh man that sucks about your back!! I feel your pain i broke my back in 2009 and though iam happy I walk its the worst pain. Wish you a speedy recovery!!! Seems like a lower end would be fussy like any crank shaft. Wouldn't you need a fixture to measure all the bearing surfaces and run out and trueing and all that jazz? Do these little guys get balanced too? Been so long since I did an engine with cases lol. Seems like I remember always needing new parts for rod big end and having to re bush everything and fuss over tolerances. It does suck that a Itty bitty number out can cause such grief. I lost a wrist pin serclip one time that totalled a beautiful crack shaft lol. And then one time I throw a rod and it came through the block and missed everything and I drove that truck a hundred miles home on seven and killed the crap out of it lol. Dunno how these little engines stay alive spinning so fast so long! hope your back gets better!!

 

[boondocker97]

Thanks. I just slipped it out of place very badly. Finally getting back to normal after 2-1/2 weeks and three trips to the chiropractor. Since having a desk job I've had more back and neck issues than I ever did with a manual labor job.

These cranks are multiple pieces pressed together and the rods are one-piece. So to measure the rod big end and replace bearings requires a crank shop to press them apart and press back together. At that point I'd be having it fully rebuilt with new bearings, seals, rods, and having it trued. I don't have the tools for all of that, or the time these days. I'll tear apart and rebuild almost anything, but crankshafts are the one thing I leave to the professionals!

 

[Pinchecharlie]

Is there such a thing as a "short block" in the snowmobile world? Sometimes its only a few hundred more than parts ? I did notice that these parts are more expensive and the aftermarket is way smaller. That stands to reason though as they are not everywhere like a Chevy. Huh...pressed together? Thats crazy! Makes more since than Harleys they have a threaded crank pins. I used to have a trueing stand for those fly wheels but learned from a old guy to do it in a lathe. Anyway. Wonder do the cases have to be line honed too? Short block would be the ticket!

 

[boondocker97]

I don't think Cat sells short blocks for these particular machines. You can buy a complete 800 Suzuki long block for about $2400. Other brands like the Ski-Doo 850 have the lower end sold separately, but they are more expensive than the complete Cat engine.

First ride out on the new engine and had coolant bleeding issues. Have never had one be this difficult getting all the air out. Finally took the antifreeze jug with me and left it by a trail sign after it got itself bled out. Had the temp line flash on once after a session being on it pretty hard in deep snow. Leads me to believe the thermostat stuck itself partway closed while the coolant was drained for a few weeks. If it does it again i'll be ordering a new one. Otherwise the sled ran really strong. Better than it was at the end of last season. Nothing like 4' of new snow to get those new rings seated!

 

[Pinchecharlie]

Sweet!! Saved yourself a lot of money so now you can buy more mod parts!!

 

[boondocker97]

Update. Installed new thermostat after having repeated temp light flashing on. No change. Problem escalated and had more frequent poor running that I would experience before temp light would flash. Caught the fuel pressure dropping off down to 30psi when opening up the throttle and causing pre-detonation enough it would trip the knock sensor. Figured it was either the fuel pump or stator getting weak not supplying enough power to the pump.

Dug into it last night and the pump would supply a steady 43psi on my gauge hot wired to a cordless drill battery. Cat spec is 42.8-47.3psi. Before the last trip it was regularly running at 40psi. Plugged pump back in and started sled and cycled through the additional info the gage will supply (hold the top button and the one below it for 10 seconds right after startup). Voltage was reading 14.4V at idle and would drop below 12V when I whacked the throttle open. Started testing resistances in the stator plugs. Resistance for the fuel pump supply circuit was only 1 ohm. Spec is 1.52-2.28 ohms. Also states that voltage supply to the pump should be 14.5-17V. Looks like I found my problem. I've basically been fighting a bad stator this entire season. Should have a known good stator to put in this weekend. Will monitor fuel pressure after that again.

After experiencing this it makes me wonder how many people that have a sled that gets hot no matter what they do have a similar problem? The higher compression and extra fuel requirements for my engine probably exaggerated the fuel delivery issue. If it was a stock sled it might run "fine," but maybe down a little on power compared to another one. Heard more than one story about Cat sleds running better with just a new stator swapped in.