4 stroke death toll....

[300rman]

AMA MX season, engine death toll:

1. Chad Reed YZ450F, Daytona
2. Kyle Calderini CRF450R, Edward Jones Dome
3. Austin Stroupe KX250F, Wortham
4. Jason Lawrence YZ250F, Mt. Morris
5. Mike Brown YZ250F, Red Bud
6. Ryan Dungey RMZ250, Red Bud
7. Trey Connard CRF250 (no start) Budds Creek
8. Michael Pugrab KX250F (no start) Budds Creek
9. Martin Davalos 250SXF (exploded 1st moto) Unadilla
10. Broc Tickle YZ250F, Washougal
11. Ryan Clarke CR450F, Washougal



funny, i never remember seeing that many 2 strokes blow up on riders in races over the course of many seasons, much less the same season..........

[brapp]

yeah thats why i went from the trx450 back to a trx 250r for runnign xc and when they do blow its alott cheaper to fix

[3leggeddog]

here we go with the 4 stroke bashin.

I built my trx motor last year,3rd race of the season.i rode the piss out of it from then till the 5th race of the season this year.I'm talking many indoors,lots of practice,and a whole season,minus 3 races.Before the 5th race this year we out a new piston,and rings,and honed it.cost me 200 bux.this is a 57.2 hp motor,at the wheels,with 47 ft. lb. tourqe.it's a monster.very reliable.the pro's are obviuosly alot harder on thier machines but I still don't think reliabilty is that bad for the power you get out of these new 4 strokes.

[300rman]

here we go with the 4 stroke bashin.

not bashing......pointing out a blatant truth. the pro-circuit 2 strokes rarely grenaded.......and thats 11 4 strokes this season i am aware of.........

[fabiodriven]

2 strokes truck- 4 strokes suck!!!!!!!!!

Just kidding, guys. Although I do like two strokes.

[Dirtcrasher]

It's got some truth to it.... Those four strokes can be time bombs, but so isn't a built cam'd and stroked 350X or an 85 200X built to the hills. My 86 200X crank isn't gonna last long with the 12:1, cam, race gas, porting and a lightened flywheel.

The new 4strokes need really good maintenance and the valves need to be shimmed correctly.

Were not Ricky Carmichael so I'm thinking that once a year after some good rides that a topend can be done and they can last.

I've got a KXF250F here now, I read the book and found that even the large end of the con rod has a direct oil passage. No more splash system like the old thumpers.

[SWIGIN]

don't forget, the pro's also used to rebuild there 2 strokes between every run.

i haven't bin in a pro pit since the 2 stroke days but i highly doubt they are tearing into the 4 stroke as much except for normal ajustments.

[300rman]

don't forget, the pro's also used to rebuild there 2 strokes between every run.

yeah, rings and maybe a piston. that probably takes as long as doing the valves on a 4 stroke

[Billy Golightly]

The thing is, with most all of the newer generation 4 strokes, its just a fundamental engineering issue. They have extremely short strokes, and short connecting rods. This makes big hp, but it puts side load on the pistons and the shorter connecting rods actually change how fast the piston accelerates to TDC, and BDC. Because when the engines running, the piston actually briefly "stops" at each one of these points, and the shorter connecting rods make the time from stopped, to moving, even shorter, and thats bad for reliability because it just more or less is on the bleeding edge of flinging itself apart. This a lone is like the single biggest killer of engine reliability, but its also, obviously, a nice way to pickup power too.

Most all of the 2 strokes just are not designed or built with connecting rod ratios as short as the newer generation 450s, so they have the reliability advantage.

[300rman]

The thing is, with most all of the newer generation 4 strokes, its just a fundamental engineering issue. They have extremely short strokes, and short connecting rods. This makes big hp, but it puts side load on the pistons and the shorter connecting rods actually change how fast the piston accelerates to TDC, and BDC. Because when the engines running, the piston actually briefly "stops" at each one of these points, and the shorter connecting rods make the time from stopped, to moving, even shorter, and thats bad for reliability because it just more or less is on the bleeding edge of flinging itself apart. This a lone is like the single biggest killer of engine reliability, but its also, obviously, a nice way to pickup power too.

Most all of the 2 strokes just are not designed or built with connecting rod ratios as short as the newer generation 450s, so they have the reliability advantage.

Very well said.

but, the short stroke doesnt affect the valve issues. that comes from the engine revving to the moon. i have no doubt these motors would last longer if they were kept out of the upper RPM range most of the time.

[The Goat]

yeah, but you'd be cruising around at 35. It seems to me that if you had a piston made of titanium...and a cylinder coated with LiquidMetal's high heat application...you could seriously reduce the weight of the piston...of course this would put even more stress on the valves...as coupled with a decent rod kit, this thing would rev and rev and rev.

no way to upgrade the valves? besides springs, keepers, titanium guides, titanium valves?

[300rman]

titanium valves?

stainless valves are actually a better option for lasting reliability, at least thats the consensus among people on the web.


the REAL problem with the whole valvetrain is that a 4 stroke is inherently heavy because of all the extra parts.

well, to make them lighter to be able to compete with the 2 strokes, they cut WAY down on the sizes and thicknesses of the components, so they are smaller, lighter, and of course, less reliable because they are already on the "edge" of breaking, so to speak.
Now combine that with the short stroke of the crank/rod, and you have a grenade with the pin pulled.

[The Goat]

I was thinking chromoly as I typed that actually. But even still...the difference in weight between a 250r and a 450r engine can't be that damned much. can it?

[KASEY]

HOW DO YOU GET LONGIVITY OUT OF THIS????

[SCEADU]

Where did the rest of the piston go on these new machines? I watched a friend build one in his shop and the way those pistons are, you would think they could turn sideways in the cylinder.