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Grenading dirt bike motors!

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My friend with no mechanical skills at all wanted to do mechanic things with me so I said he can help me do a top end on my pit bike cr85 two smoke. Anyway he helped me and I showed him what to do well I worked on my other bike, he put it all back together and kept kicking it. It sounded like metal on metal. He didn't put the damn piston ring in and he had the piston facing the wrong way lol I fixed it

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The only people that 'grenade' an engine are those that fail to take care of the bike. Sad really.

That's not 100% true. There are a certain percentage of manufacturing problems. There are also probably a certain percentage where the people don't do their own maintenance and the shop where they have it done screws up.

This probably doesn't happen as often in MX, but I used to race karts. In the 4 strokes class we ran Briggs motors on methanol and turned them to engine speeds that the designers never intended. I saw rods exit engine cases on a number of occasions. When I ran 2 stroke karts we would reach back and turn the mixture screw one turn richer going into a turn. After exiting the turn and making it about mid way down the straight we'd reach back and lean the mixture up a turn. All you had to do was forget one time to richen it up entering a turn and you'd go lean when you leaned it up on the next straight. Melting pistons and spark plugs was pretty common.

Edited by Doc_d
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That's not 100% true. There are a certain percentage of manufacturing problems. There are also probably a certain percentage where the people don't do their own maintenance and the shop where they have it done screws up.

This probably doesn't happen as often in MX, but I used to race karts. In the 4 strokes class we ran Briggs motors on methanol and turned them to engine speeds that the designers never intended. I saw rods exit engine cases on a number of occasions. When I ran 2 stroke karts we would reach back and turn the mixture screw one turn richer going into a turn. After exiting the turn and making it about mid way down the straight we'd reach back and lean the mixture up a turn. All you had to do was forget one time to richen it up entering a turn and you'd go lean when you leaned it up on the next straight. Melting pistons and spark plugs was pretty common.

That just backs up what I said. Running a Briggs motor way beyond design tolerance, you have to expect it to blow. If you never exceeded 3,600 rpm the engine would of lasted forever. Take it to 5,000 rpm, a 38% increase would be like taking a 12,000 rpm mx engine to 16,500.

Failing to richen the kart was an error. You could of left it slightly rich for the entire race. You could of not messed with the governor on the Briggs engine.You do these things and it is a service issue, not a design issue. Flirting wih the Devil. Not to say there have not been engines with design flaws. Rather they are very few and far between.

 

The Polaris issue Snow points out could of easily been avoided with some additional machining. The rare design flaw that once happened, you'd think that it would be a standard servicing task to increase the clearance by a few thou.

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That particular failure was due to the con rod bearing that blew apart. 

I got over 5k miles out of that motor with nothing more than a top-end rebuild every few years.  Most 900s were lucky to hit 2k before they failed.  The 05's were really bad.

Polaris is finally getting over that mistake, 10 years later...

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That particular failure was due to the con rod bearing that blew apart. 

I got over 5k miles out of that motor with nothing more than a top-end rebuild every few years.  Most 900s were lucky to hit 2k before they failed.  The 05's were really bad.

Polaris is finally getting over that mistake, 10 years later...

Ah... what was the design failure on that?

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Ah... what was the design failure on that?

Polaris tried to build a big-bore🤣

 

06 cranks were 8mm larger diameter than the 05s. 

Chassis was built for an 800... wasn't designed to take the torque the 900s put out.

 

Motor mounts were soft and would tear.  Stiffer mounts held the engine in place better, but would crack the crossmember holding them (also,  vvvviiibbrraaaatttiiioonnnnnn...).  As soon as the mounts failed, the clutches weren't aligned, and the crank was usually next in line to go.

I ended up designing/machining a solid billet crossmember, which ended up simply breaking the grade-8 bolts that secured the crossmember to the bulkhead.

 

The stock skis were terribad.  The whole steering/geometry needed some big changes to be useable.  Geared it down so the track didn't spin out and trench as soon as the clutches engaged. 

 

Had a front radiator that was supposed to improve cooling.... It didn't.  Remove and close off that front radiator, pushing the coolant through the tunnel/heat exchanger, sled didn't overheat anymore.

 

Air intake was a massive fusterlcuck.  Designed my own...

 

Handlebar mounting was this plastic adjustable mess.  Mine broke in <300mi.  Another custom part, and after installation.

 

Throttle body boots were prone to cracking, leaning out the mixture, burning down the motor.

 

Oil pump had a short (like 6-8") linkage cable between pump and throttle body.  With the tight angle in that cable, it usually frayed, broke, and stuck... Usually broke holding the pump wide open, dumping a ton of oil into the engine... sometimes it'd break closed and starve the engine of oil.  Custom throttle cable assembly of mine fixed that issue... also enabled me to take the throttle body off without removing the entire engine from the chassis to get to that short linkage.

 

They were not good sleds.  It's why Polaris never bothered with big-bores before, nor have they made one since.  They never should have left the factory in that condition.

Needless to say, the Pro i'm riding now has been a huge breath of fresh air after owning a 900.  I pull it into the garage, add fuel/oil and ... go ride the next weekend.  Don't have to constantly wrench on it.

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When I was a younger dude, I had a really nice 76 yz-100, alloy tank good 70s style yz strobe graphics, the cool mono-x shock with a funky ball shaped resivior , all kited out! Well I had a love interest who was upset with me, so me being a dummy did not know she was out to get even, i let her ride the yellow screamer, she left it in 1st gear and held it wide open on a fire road and grenaded the motor! ouch, My gals dad offered to split the cases and fix the molten mess, he could not finish it, so i was left with a pile of parts, back then i had no real mechanical skills, so i worked and mickey ds and bought a nice 83 yz 100, but the genade was pretty bad!.....live and learn10.jpg

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Polaris tried to build a big-bore🤣

06 cranks were 8mm larger diameter than the 05s.

Chassis was built for an 800... wasn't designed to take the torque the 900s put out.

Motor mounts were soft and would tear. Stiffer mounts held the engine in place better, but would crack the crossmember holding them (also, vvvviiibbrraaaatttiiioonnnnnn...). As soon as the mounts failed, the clutches weren't aligned, and the crank was usually next in line to go.

I ended up designing/machining a solid billet crossmember, which ended up simply breaking the grade-8 bolts that secured the crossmember to the bulkhead.

The stock skis were terribad. The whole steering/geometry needed some big changes to be useable. Geared it down so the track didn't spin out and trench as soon as the clutches engaged.

Had a front radiator that was supposed to improve cooling.... It didn't. Remove and close off that front radiator, pushing the coolant through the tunnel/heat exchanger, sled didn't overheat anymore.

Air intake was a massive fusterlcuck. Designed my own...

Handlebar mounting was this plastic adjustable mess. Mine broke in <300mi. Another custom part, and after installation.

Throttle body boots were prone to cracking, leaning out the mixture, burning down the motor.

Oil pump had a short (like 6-8") linkage cable between pump and throttle body. With the tight angle in that cable, it usually frayed, broke, and stuck... Usually broke holding the pump wide open, dumping a ton of oil into the engine... sometimes it'd break closed and starve the engine of oil. Custom throttle cable assembly of mine fixed that issue... also enabled me to take the throttle body off without removing the entire engine from the chassis to get to that short linkage.

They were not good sleds. It's why Polaris never bothered with big-bores before, nor have they made one since. They never should have left the factory in that condition.

Needless to say, the Pro i'm riding now has been a huge breath of fresh air after owning a 900. I pull it into the garage, add fuel/oil and ... go ride the next weekend. Don't have to constantly wrench on it.

wow,that's a poor design.

You have basically built a new ski.

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https://www.youtube.com/watch?v=6_45YYQ0MG4

That's what you call a grenade!

That's terminal!  :(

 

When I was racing dirt karts I ran an 85 cr500. Before the start of the season we had a "play-day" at the track at prairie city. My 500 had fresh top-end in it. Well, my buddy, and he also owned the kart, decided that it would be a good idea,even over my protests, to let one of the park rangers take a few laps. I used an Odyssey steering wheel with the throttle and clutch on it. Left foot was brakes and right foot was a shifter that had a "pocket" that your foot would fit into. Well, mr. park ranger gets in with his clod-hopper boots on and starts going around the track under green and I could hear the motor winding out crazy. His foot, from all the bouncing around, had shifted it into 1st gear. I think "racing gear" was 3rd. Well, I tried to wave him off but to no avail. After a few laps the kart comes skidding to a stop on the track. I knew then that it was over. The big end of the rod was "welded" to the crank. That was end of that motor. Ended up getting and 86 cr500 after that, but never did like it as well as the 85.

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