What's a new top end?

[jds22]

I am just getting back into riding and I see alot of people talking about new top ends on two strokes. Can somebody explain what is involved in that?

Thanks,
Jerry

 

[MX265]

The opposite of the bottom end. The top end is cylinder, piston, head, rings, power valves. the bottom end is the crankshaft, rod and lower end bearings. When you do a top-end, you generally clean all the carbon and oil deposits and replace the piston, rings, wrist pin, wrist pin bearings, and the wrist pin circlips. When you replace the bottom end, you replace the rod, lower end rod bearing, and balance the crank and replace the main bearings. Hope this helps.

 

[jl2x]

jds22- Thanx for posting a question I was too embarassed to ask. I've been riding street bikes for years, and my DRZ is my first dirt bike. Now I'm wanting a *real* dirt bike with light weight and 2-strokes are calling my name.

Follow ups - Roughly how often does the top end get rebuilt? Once per year? How often does the bottom end get rebuilt? I've heard proper air filter maintenance can play a big role in frequency of rebuilds. Also, how much time and money go into each rebuild? Thank you very much from someone just joining the fun.

 

[High Lord Gomer]

I think Honda recommends 7 hours between top-end rebuilds. :eek: On a 250, I try to do rings every 6 months and a complete top end every other time (once a year). For 125s and below I try to do a complete top end every 6 months. I only do the bottom ends when something goes wrong down there.

I say I "try" to follow the above schedule...my 97 CR had 2 1/2 years on it when the piston broke in February. :ugg:

Parts (including gaskets) or a top end should be between $100 and $200. The first one could take days! After you've done a few, it should take only an hour or so.

 

[jl2x]

Thanks for the speedy response.

Every 7 hours a rebuild! I could see the advantage of a new CRF. How do you know when you need a rebuild? Loss of performance, harder to start, more peculiar noises than normal?

 

[MX265]

Basicly the idea is to put it on a maintenence schedule so you can replace the required parts before you NEED to. Yes some of the symptoms of a whipped top end are low power, flat sound, sometimes knocking and extra smoke. But you will know when it is an absolute requirement when you bike locks up from a seized top-end or a cracked piston. Perform the recomended scheduled maintenence and you should be good.

 

[jl2x]

Thanks.

 

[Chief]

My bike is actually running six hours a week or less. I don't race, I'm not on it hard for the most part, and the weather let's me ride about 9 months out of the year. I plan on doing a top end (piston included) once a year.

I did one so far. I took my time and had no problems. When the job was done I had one extra nut in the basin, I still haven't figured out where it goes. I like to think one of my kids threw it in and it's not from the bike. What? it could happen:) :) :)

The downside about mx 2strokes is the maintenence requirements. The upside is they are lightweight, crazy powerful, and easy to work on. I've heard a lot of guys rave about the mx thumpers. It would take a ride on one to change my mind. My guess is I would still prefer a 2 stroke.

Joe Chief

 

[Durt Cycler]

I think it all depends on the 2-stroke oil and octane level and how often you clean your air-filter. I run good oil, high octane, and clean my filters very often and my bike is still running strong with a over a year old topend in it. Don't worry tho my bikes topend is getting torn down this week for a new topend and reeds.

 

[hillclimberr]

Honda recomends putting a new piston and rings in every 15 hrs. The 450 is a high maintenance 4 stroke.

 

[spanky250]

Originally posted by hillclimberr
Honda recomends putting a new piston and rings in every 15 hrs. The 450 is a high maintenance 4 stroke.

That engine doesn't even have what I would call a piston, it is nothing more than a big coke bottle top with grooves to hold the rings on the rod.;) It pobably costs $400 for a piston, too...

As for rebuild frequency on a top-end, A good habit to get into is doing regular compression checks. I check it after beak-in, then re-check evey couple of rides. Once I have lost approx. 10% of the original reading, it's tear-down time.

One thing to consider. Most four-strokes will go longer between rebuilds than a two-stroke, but they are much more expensive to rebuild, and require much more work.

 

[mxracer724]

:eek: I better learn how to rebuild top-ends before I get my 02 YZ125 or my bike will be in the shop almost ever month! I can ride 7 hours in 1 week!!!:scream: