Tech Tip "Cylinder prep before instalation"

[KM Burgess]

I've been reading this post with interest as it has some great ideas in it. Just a quick tech point:

1. Rings can, BUT don't usually rotate in a bore, they usually stay where they were mounted. That's why you need to stagger the rings in a car engine at the appropriate places. The rings on a 2 stroke dirt bike are pinned only as a SAFE GUARD against any rotation that MAY occur to avoid the POSSABILITY of the ring ends getting caught in the open ports.

2. Some X hatching is always good for oil retention BUT not to the detriment of premeturely wearing out the cylinder or reducing the usable life of the cylinder. (What's best? wearing out your cylinder while riding it or wearing it out on the workshop bench with a hone?) Personally for me the cost of a new cylinder or replating is too high to start wearing cylinders out premeturely.

3. By all means de-glaze if needed but don't reduce the life of your cylinders! Most people wouldn't notice any performance difference whether they did or didn't hone their cylinders and I would rather get 4 or 5 new pistons to a cylinder than only 2 or 3, unless I just won the lottery, and that ain't likely to happen.

Cheers
PS. What about a thread on premix ratios?, I bet that could fire up a few people!

 

[Pete Payne]

Slapnut,
You would have to hone for days in a row and go through many ball hones before you could remove enough qualitycylinder plating to actually open up the cylinder bore a couple of thosandths.
When you use a rigid cylinder hone Sunnen, (the leader in honing equipment) recomends that you use a cork based honing stone . This special stone will not remove nicasil , it just removes burnt on oil and deposits.
You do make a good point as to when you use a ball hone to go slow and to not use one that is way larger than the cylinder bore as you could damage the plating at the port edge. All you have to do is use some sense.
This is probably why wiseco recomends to use the brush type or plateu style hone.
hope this helps ---Pete.

 

[pelayo]

Help! I run a 1997 KX80 motor in my shifter kart. I have never had a problem with this engine until recently when the piston seized. I noticed the carb was starting to fall off, so I assumed there was too much air getting in and the motor overheated before the piston stuck. Well, I got the cylinder honed and put in new piston and rings and after heat cycling the engine a couple of times, the first time I decided to go for it, a half of a lap later, the piston seized yet again. This time my local shop sent the cylinder out for replating. What should I be thinking about next time I put this thing back together? Do you think I just didn't break it in (or simply warm up the motor) enough last time? Also, I really don't know the optimal operating temperature (using the CHT sensor) for KX80s. Any thoughts? Last question, I see guys at the track use premix ratios from 30:1 to 60:1. What really is the best ratio for a stock motor? Any and all help would be greatly appreciated.

 

[Coolector]

KDX cyl prep

Sorry to barge in guys but I had a quick question, I'm going to be tearing down my 81KDX250 next week to replace the piston and rings ect, and I was wondering if the cyl was the plating or a sleeve, I was able to stick a magnet through the exaust port which was attracted to the cyl wall. Some people also said I should get the cyl machined due to the fact it may be oval, couldn't measurements take care of this.
Thanks

 

[Bodge]

Do you guies ever notice or worry about all that gas and stuff in your dishwasher? Could i just use an old tooth brush, hot water and the dish soap in the sink? If so should I go in any particular manner w/ the tooth brush? The cylinder has a good cross hatch, there isnt any possibility of messing it up w/ the tooth brush is there? Thanks

 

[RM_guy]

Sorry to barge in guys but I had a quick question, I'm going to be tearing down my 81KDX250 next week to replace the piston and rings ect, and I was wondering if the cyl was the plating or a sleeve, I was able to stick a magnet through the exaust port which was attracted to the cyl wall. Some people also said I should get the cyl machined due to the fact it may be oval, couldn't measurements take care of this.
Thanks

You'e got a sleeved cylinder. The magnet sticking to the cylinder wall was dead give away (not to mention the age of the bike). If you have the right measurement tools you can check to see if it needs to be bored out or not. A machine shop could do it for you if you needed help. If it has a lot of hours on it then it probably does but check it first.

 

[Pete Payne]

Pelayo,
If this was an engine that I WAS GOING TO RUN . I would take the time to run in (break in) the engine properly.
I would use a premix ratio of 28:1.
Also after the engine is reassembled I would pressure check it to be sure ther were no air leaks.
Pete

 

[MikeT]

Forgive me if this has already been stated. I know that you want to use a 240 grit aluminum oxide ball hone, but what diameter for an original plated 250?

 

[motox4eva]

DEA ,
Good advice . I did not bring up that point--Thanks !!!
Pete

Also, never use a spring [loaded?] hone either, that will also get hung up in the ports.

(dunno if it was worth bumping a old thread or not but...)

 

[Bodge]

I dont know if this has been covered in this post but im about to send my 97yz125 cylinder out for a replate to US chrome. Will this cylinder need anything but a washing(to make sure there isnt any metal shavings in it)? I assume they will put a good cross hatch on it, but correct me if im wrong.

 

[RM_guy]

Part of the process for plating is to strip off the old plating. That cleans it up for you. I'd get it as clean as you can before you send it but it doesn't have to be spotless. Also, remove everything that can be removed. They charge labor if they have to remove components. Check their website or call them if your not sure what has to be taken off.

 

[raydog]

Some people do not oil the cylinder, The oil is in the premix, if it got gas it got oil.

 

[Pete Payne]

One thing not to do is install the cylinder dry and then pump the kickstarter to watch the piston go up and down fast . I had a friend , fellow DRN'er, do this and it seized the piston to the bore . Now he always uses oil and puts it all the way together before he pushes the kickstarter

 

[2-strokes-4-life]

i wanted to ask what does this cross hatch look like that alot of people keep referring to.i am about to do my first solo top end job and dont know what i am supposed to be looking for.if someone could please let me know that would be great,if anybody has any pics to go along with the advice that would be much appreciated also.

 

[kx125412]

So after you have everything ready, piston in, cylinder on, you have to break the bike in again? Like you have to warm it up, shut it down let it cool, do that a couple of times then slowly ride it around, then do the warm up cool down process and then ride it hard?

 

[2strokerfun]

On the question about the cross hatch on a chromed cylinder, there typically is not a cross hatch per se as in a steel cylinder. The chrome is either really smooth or looks sort of crazed a bit. Make sure you run iron rings in a chromed cylinder.

 

[CBRob]

Very interesting thread. I've see the the same discussions on my favorite snowmobile website, snowest. The sled disscusions took a different point of view.
The consensus was... use scotch brite to remove some cylinder glaze and varnish build up. Don't use dingle balls due to the potential damage of port edges.
It was also mentioned that nicasil is so hard that the cheap hones wont really touch it anyway, and if they could scratch the nicasil, how could you make sure that your honing was at the right pitch/angle. Sounds like a machine shop is required to get a proper hone on nicasil.

 

[f4istunna]

Lots of very good tips and helpful info in this thread, i recently just did my first 2 stroke rebuild. Done a few 4 strokes but so knew the basic principals but wasnt sure about cross hatching 2 stroke. so i did it anyway, good to know for sure now.

 

[dirtracker]

Certainly the amount of ring seating needed would vary widely depending on the mismatch in profile between the rings and the cylinder. For example, I just changed out the original top end in my CR250. It had about 1 1/2 seasons on it. I'm not going to guess at the hours, because you guys will yell at me. The compression was at 190, and measuring the old and new pistons side by side I had trouble finding any wear on the old piston (using a good digital micrometer).

The ring gap on the old rings was about .002" wider than the new ones. Certainly most of this is normal wear rather than seating, but let's say for argument's sake 1/2 of this wear took place before the ring conformed to the cylinder, which puts us at .001" difference in circumference. Mathematically, since circumference = dia. x pi, this would equate to a difference in diameter of only 1/3.14 (about one third) of the difference in circumference, or about .00035". This puts us at .00017" per side. I would venture a guess that this amount of gap, or mismatch would not result in any significant loss in compression...in fact the film of oil would effectively seal a gap larger than that.

My point? I think machining and quality control practices have produced engines that require minimal ring seating. Certainly, there are engines out there on the loose end of the tolerance that would require more break in and seating, thus furthering the need for carefull break in and good cross hatch on the cylinder.




Regarding the finish on chrome fork tubes...if these imperfections or scratches carry oil, why doesn't the fork tube act like a dust magnet?I would think that some of these dust particles would then be carried back into the fork, then washed off by the oil. The oil would become dirty very quickly.

That is the reason fork oil needs to be change frequently :think: