Why do you have to rebuild 2-strokes ALL the time???

Discussion in '2-Stroke Machines' started by SpeedyE, Oct 3, 2019.

  1. SpeedyE

    SpeedyE Experimental prototype, never meant for production

    I didnt own a bike in 1990, but was was lucky to be let to sprint race an 86 RS250 Honda 1x, and sprint race a 1990 TZ250 1x.
    I have been doing research this week, and apparently those type of bikes (2-stroke race bikes) need the top-end rebuilt every 200+/- miles and crank rebuild every 600-mi +/-. And even crank replacements/rewelds quite frequently.
    This seems like a lot of money in hard to find parts, if true.
    The 2-stroke streetbikes of that era (RZ350/Gamma/NSR/etc) not need this type of investment/maintenance to ride.
    Why cant you TD or race a season on the motors, why u have to rebuild constantly???? I dont understand.
    Thank you.
  2. erock768

    erock768 Well-Known Member

    I will share my opinion based on the following 1) 12+ years racing a TZ250 - reasonably quickly, and 2) my back ground as an engineer.

    First, your service intervals are off. Per the Yamaha TZ and Honda RS manuals the top end interval is 500 km and the crank replacement interval is 1500 km. I loosely go on 300mi/1000mi for the top end / crank respectively.

    What you must keep in mind regarding these bikes is that that are not detuned or restricted in anyway and have very high rev ceilings and no airfilters. That is why your RZ350 can go almost forever, whereas a TZ250 cannot. FWIW the TZ250 makes more power than an RZ350 with 100cc less displacement, so its not a stretch to see that increased parts wear results from higher out put.

    You certainly can, and people have run full seasons or more on a TZ top end and crank. However, I think if you ride the machine the way its intended you will find that eventually things will wear out. For me, I inspect the top end after every race weekend. I don't take the cylinders off. I just check the piston and cylinder for scoring or any obvious wear. I check to make sure the ring is not stuck (check through the exhaust port) Check the reeds for cracking. In the southwest where I live now, there is quite alot of fine sand that gets sucked into the engine. I have not observed increased wear on the top end parts, but it has debited the life of my cranks. At my last rebuild at 1000mi the bottom end bearings were pretty rough.

    If you stay on top of the machine, inspect it frequently and replace items when needed you will be rewarded with a great bike that is as reliable as the guy maintaining it. If you just ride it hard, put it away wet and never look at it, it will break and bite you.

    Plus most of us that race the GP bikes like to tinker anyway. If that's not you, that's ok, thats why they make 4 strokes with electric start.
    dave3593, Killswitch and SpeedyE like this.
  3. SpeedyE

    SpeedyE Experimental prototype, never meant for production

    Thank you Eric.
    I currently cannot ride a bike, but am making slow progress and hope to someday/eventually TD once in a while.
    I was an inventor/custom-fabricator my entire adult life, until recently. And have a full machine-shop in my garage....so I am/was tinkering madness :)
    I think maintaining/rebuilding would be part of the fun of owning an old late-80's 2-stroker :)
    I have financial problems, so the cost of pistons/rings/cranks etc is scaring me away from my current dream/hope of owning one of these bikes in the future. Tires are bad enuff $, but having to a buy a rare crankshaft every year..... :eek::eek::eek:

    Thank you for your insight/info :flag:
    Newyork likes this.
  4. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    Find yourself a TZ125 - cheaper to maintain, cranks are NOT unobtanium, and they're just as much fun and damn near as fast as a 250.
    Trent and SpeedyE like this.
  5. SpeedyE

    SpeedyE Experimental prototype, never meant for production

    Thanks Don. That might be a better way to go....I am far from able to ride a bike, I am just hoping/thinking ahead, to stay motivated/smile/dream. May never happen. Time will tell.

    In regard to the cranks....
    I don't understand why the cranks need to be rebuilt/replaced so regularly. Cant you either put tighter bearings in it, or, just replace the bearings at recommended intervals.
    I have rebuilt many oem gsxr bottom ends, and a SV SB, replaced many crank/rod bearings in them, never had to replace/rebuild crankshaft.
    Loose rod bearings ='d maintenance motor, tight bearings ='d no worries/maintenance.....i don't understand why the crankshaft on the two strokes need replacing/rebuilding. I am not understanding something mechanically about these bikes.
    Thank you.
  6. pscook

    pscook Well-Known Member

    A two stroke crank is completely different than a four stroke crank, at least for the inline four engines. A two stroke crank uses needle bearings at the big end instead of plain bearings, and the lubrication is applied and rinsed as the fuel and oil mixture circulates through the engine and coats the needle bearings as they ride on the crank pin (no forced lubrication on a two stroke). It's the wearing of the crankpin that requires rebuilding, which requires separating the crank and replacing all of the rotating parts inside. Here's a link explaining two stroke engines, comments regarding the crank bearings about 3/4 of the way down:
    https://www.cycleworld.com/2015/04/...ston continues down,port in the cylinder wall.
    SpeedyE likes this.
  7. pscook

    pscook Well-Known Member

    How Two-stroke Engines Work
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  8. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    On a 4-stroke engine you don't actually rebuild the crank itself; whatever refacing and truing is needed, the pins and webs are not separated. With a 2-stroke, the crank is literally dismantled into its component parts.

    A 2-stroke crank overhaul generally replaces the crank bearings (roller or ball), each con rod plus big end pin and bearings (1x needle bearing + 2x thrust washers), and (on a twin / multi) the labyrinth seal / bearing assembly between cylinders. It is generally this seal / bearing riggin's that is made of unicorn farts; once the supply from the factory is gone, it's gone. This shows an understated advantage of the single - no center assembly. The wrist pin and wrist pin needle bearing is considered part of a top-end job, and are replaced with the piston and ring(s).

    This crank assembly is pressed together under tons of force, and aligned to within a few thousandths of an inch. This is -not- a piece of solid steel with oil galleries drilled up the middle.

    Not only does the 2T crank wear rapidly (big end and wrist pins have very limited lives, and are replaced *before* they fail spectacularly) due to limited lubrication, it also tends to vibrate itself apart. Even with tack-welded crank pins and big-end pins, regular rebuilding is a must.
    pscook and SpeedyE like this.
  9. DonTZ125

    DonTZ125 Purveyor of Neat Toys


    A pic shamelessly stolen from elsewhere here on the Beeb ... Note the thickish cylinders in the middle of the crank; these are the bearing / seal assemblies that are the bane of all 2-stroke multies.
    tony 340 and SpeedyE like this.
  10. K51000

    K51000 Banned

    Did someone mention, among other things, because a 4 stroke rod is constantly in 4 stroke oil/ Whereas the 2 smoker gets all it's rod, cylinder oil from the fuel/oil mix?

    Just saying
    SpeedyE likes this.
  11. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    "big end and wrist pins have very limited lives ... due to limited lubrication"

    The diesel also has its big-end and crankpin lube forced into the bearings under pressure, vs the swirling mist of the smoker.
  12. pscook

    pscook Well-Known Member

    Yes, at least twice in this thread. And a third time implied.
    SpeedyE likes this.
  13. SpeedyE

    SpeedyE Experimental prototype, never meant for production

    It all makes sense now....I had no idea......Thank you guys for the education. Very Cool :)
  14. SpeedyE

    SpeedyE Experimental prototype, never meant for production

    Reduction in Friction (needle bearings, no oil pump, etc) at all costs.....Very Cool, very no-hold-bars Race!!!!!
  15. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    Well, I hope you're holding onto the bars - otherwise, it's gonna be a short ride! =)

    If you meant "no-holds barred", I remember a fellow wandering through the paddock stopped and stared at my bike. He shook his head and muttered, "There is nothing on that bike that doesn't involve going fast!"
    mattology and SpeedyE like this.
  16. SpeedyE

    SpeedyE Experimental prototype, never meant for production

    i had been racing a honda hurricane (and various other borrowed 4-strokes) for 2+ years. Was sitting on an a borrowed 86 RS250 I was getting ready to grid up for a sprint race, had never ridden it yet. I remember leaning it over almost to its side in the pits, and lifting it back up-right and saying "This thing is light!"
    I remember coming into T1 on the second lap hitting the front brake lever w the same pressure as I did on my hurricane, and and almost endoing it into the pavement...WHOA!
    I was used to the 1/4" thick windscreen on the hurricane, i remember noticing the RS paper-thin windscreen flexing/moving under wind-pressure.
    I remember that bike pulling as Hard in 4th as it did in 1st.
    Lotta Good memories from that 8-lap sprint, many decades ago :)
  17. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    Good times, good times ...
    SpeedyE likes this.
  18. mattology

    mattology Well-Known Member

    a few less RPM makes the engines last a whole lot longer. if you don't absolutely need to wring their neck every time you shift, you can sometimes double your service intervals. i religiously measure my piston wear and ring gap, as well as crankshaft play and runout. when i only rev to 12, things last much longer than when i rev to 13.

    good oil and lots of it goes a very long way also. i run 25:1 , yet my bike only smokes when cold in the morning. when the jetting is right, more oil won't hurt the jetting, but surely helps the bottom end out a lot.
    SpeedyE likes this.
  19. britx303

    britx303 Plagued with FZRs...........

    You guys are a bad influence.........you’re makin’ me ready to get back to workin’ on my rd350 race project............:p
    SpeedyE likes this.
  20. SpeedyE

    SpeedyE Experimental prototype, never meant for production

    86 RS250, 2 or 3 owners/racers. Bike was retired in 1990. I haven't seen it since 1990. I am guessing It's sitting under 3" of dust at an old friends house. I would guess there are zero spares/misc left, for it. All I remember about it it had marshianchi wheels and the frame/handling was straight. They were always messing with the jetting, i remember.
    What would be a fair price for it?

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