2T Engine experts -> your opinions on top end rebuilds

DieselTech

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Jan 21, 2007
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This is a question for the "experts" (Rich, Eric, etc.) regarding top end rebuilds on 2 strokes. I've always heard so many different opinions about when to do a top end rebuild on a 2T, and I'd like to get some definitive answers!

Now, I'm a Diesel Mechanic by trade, and I'm having trouble understanding why so many people think 2T's need seemingly constant top end rebuilds - "do it every 20 hours", "every year at least", "tear down and inspect/measure every XX hours", so on and so forth. I understand that the piston is aluminum, and is lubricated only by the premix. Can someone like Rich or Eric please go further into depth on this subject, including how to determine when it's time to rebuild, or tear down for inspection, and what to inspect for specifically? I have searched these forums, and I have yet to find this topic answered in this way. Although I'm sure that some of the other topics on this subject have experienced people providing their input, I haven't found where Eric or Rich have stated their "professional" opinions (which I would trust without question). So, if either of them would be willing to tackle this subject, I'd be greatly appreciative of their doing so, and if by chance they already have and I've missed it, then by all means point me to it and disregard my request. Thank you very much, in advance. :)
 

tx246

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May 8, 2001
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Now Im not one of the qualified experts but I do have some suggestions.

When to build a top end depends on a lot of things. Ive seen trail riders that go 2k between top ends. These guys are not slow either but they dont twist em rpm wise. If you ride with the bike under a heavier loads, your replacement interval will be sooner. Heavier load meaning riding in sand or mud.

A slower mx rider isnt going to need a top end as soon as the local pro in the same number of laps ridden. Do you see a pattern here?

Top end jobs are really preventive in nature. You get to check side play on the crank, check for rod problems, check burn patterns on the piston dome (jetting)and take apart the power valve and clean it ect. On my bikes, the piston/rings are rarely out of spec. but I replace once a year as insurance which is 1000 miles mixed trail/track. Its a big bike and I dont hammer it. I used to just rering and go, but I got bit once when a piston fatigue fractured and cost me a weekend of riding and a new cylinder.

Again that is just why I top end when I do.
 

DieselTech

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Jan 21, 2007
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tx246-

Thanks for your input. Please understand that I value your opinions & advice, even if you are not "one of the experts". So, are you saying that even though you may find the piston and rings to be within specs and reusable, you still replace them regardless just as "insurance"? I can understand this logic, especially if pistons are known to fatigue and crack over time. I totally understand that piston life is relative to the engine's use - obviously, higher rpm and heavier loads lead to more stress, and therefore, less life. Again, thanks for your comments.
 

76GMC1500

Uhhh...
Oct 19, 2006
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Your diesel engine may loose about .001" of piston skirt diameter every 100k miles, a 2-stroke will loose that about every 5-10 hours. The rings wear even more quickly. You can see that if you run your piston for 30-40 hours, piston skirt wear becomes an issue. This is when piston slap starts and this is when pistons break skirts. Most of us don't have the tools to properly evalulate the wear of a piston which is why we have our every 20 hours or 30 hours rules of thumb. The same methods of evaluation you use for the diesel apply here.
 

DieselTech

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Jan 21, 2007
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So, ultimately, piston/ring wear is extremely accelerated in 2T's, and rather than measuring and comparing to specs, it's just easier to go ahead and replace them while you are in there? Considering what you said, this also makes sense to me - especially if you consider the time involved and chances for mistakes or even the possibility of catastrophic failure shortly after a "passing" inspection due to fatigue. Does this sound correct?

The "proper" thing to do then would be to disassemble, inspect, and measure the piston, rings, rod bearing/pin, and cylinder, then replace parts that are out of service due to condition/wear, correct? I can see how just replacing the piston/rings can be much faster, plus act as "cheap insurance". Thanks!

I wonder then about the 4T's (high-compression, multi-valve OHC engines)? Do they wear pistons as quickly as 2T's, or is it just the 2T's that have this accelerated wear because of their design & lubrication? I mean, these 4T's are revving pretty high, like 13,000 rpm, and wear is incrementally proportional to speed, something like 4x as much wear at each doubling of the speed (so 4x as much wear @ 4000 rpm as compared to 2000 rpm, and 16x as much wear @ 8000 rpm as compared to 2000 rpm). Of course, diesels spin much slower, therefore less wear, which if you apply my crude, partially correct (?) formula, makes some sense. :nod:
 

76GMC1500

Uhhh...
Oct 19, 2006
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4-stroke pistons and rings do last quite a bit longer than their 2-stroke counterparts due to reduced temperatures and better lubrication. But, agressive cams eat the valves and springs. While a complete overhaul on a diesel engine may run $12k-20k, a piston and ring set in a bike is only $75-100.
 

DieselTech

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Jan 21, 2007
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76GMC1500 said:
But, agressive cams eat the valves and springs.

True, but mainly due to high rpm and valve/seat/spring materials. I would be interested in knowing what the rule of thumb is for 4T valve wear without teardown - by that, I mean judging valve seat wear based on shim size, so that if you are checking valve clearances you can have an idea of when the valves are in need of replacement just by the size of the shim installed. My 15 year old and I just did the first valve adjustment on his '05 KXF250 - it has almost 30 hours on it, and only 1 exhaust valve was out of spec, the rest were still in spec, though right at the lower limit. We adjusted all of them to be at the upper limit (.009" exh/.006" int). Anyone care to provide some guidance on that issue? Keep in mind that he's a beginner, so he's not hitting the rev limiter all the time, hence the minimal adjustment at almost 30 hours. An expert rider would probably experience more valve wear than what we have seen on this bike.
 

Redrodent

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Jul 31, 2002
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Rule of thumb for a high strung (thumper) 4t.

Once the valve starts to move (needing smaller shims) it won't stop. Watch out, could be a matter of hours then :ohmy:
 

_JOE_

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May 10, 2007
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Generally the rule of thumb is when the 3rd shim is needed, replace the valves. 4t 250 pistons usually last an average of 50 hours, 100 for 450s. Average valve life is around 40 hours. If you are easy on it(like your son), you will get longer life. As redrodent said, once the valve tightens it indicates wear. Once it starts, it doesn't stop. If you're boy isn't an a-class racer, swap the valves/springs etc with Kibblewhites. They are good for a couple hundred hours, with a small sacrifice in performance.

You seem interested in learning more of the why and hows of this hobby. I highly suggest buying Eric's book(s). I learned a ton from them, and this forum. There are alot of people on here that know what they are doing, and as you can imagine, some who think they do but don't. Hang around a while and you'll learn who to trust.
 

DieselTech

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Jan 21, 2007
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Joe,

Yes, I understand valve wear and that it "doesn't stop", but really, as long as the valves are sealing and have the correct clearance, it's not an issue (I would think). What I'm interested in is how to judge valve face wear based on shimming - you seem to address that with your times, but where do your numbers (50 hours, 40 hours, etc.) come from? Empirical data derived from hundreds of valve jobs on these 4T's? For instance, consider this: if the bike is new, and 1)the length of the valve is known (manufacturing tolerances) and 2)the width and angle of the valve seat face is known (again, manufacturing tolerances), couldn't a formula be worked out that would allow you to compute the decrease in valve seat face width based upon diminishing valve stem tip-to-shim-to-bucket-to cam lobe clearance? In a nutshell, you baseline the valves before it's run, then at every valve clearance check/adjustment, you could accurately calculate actual valve seat face wear based on the required shimming to keep the valves at thier initial adjustment (baseline). It's an interesting topic for me, at least.....

I do know about the Kibblewhite components and longer life, and when his bike is ready for valves, that's most likely what I'll put in it. Yes, I do need to get Eric's book, and more actual hands-on experience with bikes too. That's why I try to do all the work on our bikes myself. I don't mind buying tools and books, as that's how I learn to do this stuff. But, it always helps to have your questions answered by more knowledgeable people. Thanks for your suggestions!
 

DieselTech

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Jan 21, 2007
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yamarider88 said:
this is off subject but did you know back in the day Detroit Diesel made a 2 stroke diesel engine that was turboed and supercharged and but used a 4 stroke type lubrication system. i wonder if they ever had to rebuild those things like crazy but i herd those things were beast and could pull a huge load.

Hmmm, that's funny, I just mentioned this the other day in another thread . :whoa:

Yes, they were 2-strokes, but with a 4-stroke style lubrication system. Different engine design than the typical 2-stroke, and they actually had exhaust valves as well - kinda hybrid in a way. Very unique sound also. As for rebuilding, no, they didn't require frequent rebuilds.
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
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Detroits used an external roots blower for supercharging while your 2-stroke gas uses the underside of the piston and the crankcase to supercharge. The charge air doesn't go through the crankcase on the detroit. That is how the detroit gets away with pressurized oiling system and a sump.
 

_JOE_

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May 10, 2007
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The timelines I used are straight from Eric's book. I bet he has seen enough to set a basic recommendation for replacement. He says the valves stems stretch and cause the tightening(as well as seat/face wear). If allowed to stretch too much, the valve heads can actually break off :yikes: . I have done an enormous amount of research on 4t's as my crf is supposed to be hard on valve trains. I have about 25-30 hours on it and the valves have yet to move. Some have issues from a very young age. I tend to think its more of a maintenance issue than a part quality issue. I, being paranoid about my engine, change the oil every ride, filter every other oil change, and trans oil every other oil change. I have two air filters and always swap for a fresh one every ride. Too much? Maybe. But better than not enough. I plan to change the valve springs this fall, and depending on if there is any movement of the valves, I will go Kibbelwhite at that time. If thats the case the piston will be changed as well, as I don't want to have to tear it apart again to do that. If so, in goes a high comp Wiseco. I have been eyeing up the top end packages from Eric, they look like a great deal. Box up the head, send it to him, wait a couple weeks and put it back together.

So anyway, go get the book and read up on here. It won't be long before you have a good grasp on what's going on in that little thumper motor.
 

NM_KDX200

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Dec 29, 2002
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As noted, 2S top end jobs are mostly preventative. Even on my 125's, I've yet to see one that was close to the service limit. For a 125, in "play racing mode" this is about 6 mos (20-25 h) for me. For a 250 MX'er, every year. On a KDX200, every 2 years was fine.

On all of these bikes, I'm cleaning the powervalve at this time and it usually needs more attention than the piston/rings. Since a piston/ring/wrist pin set is around $100 and it takes me about 3 hr to do a top end, it's fairly reasonable insurance to just pop a new piston/ring in there. Sure, you could do just the ring (and I do, sometimes), but a single ring is still $25 or so, and a double 250 ring is $35, so I'd just soon go ahead and do the piston, too.

I have 32 h on my YZ450F and it feels like it's just now loosening up. Valves haven't budged. When the valves start moving, I'll replace them. Again, cheap insurance compared to a new 4S top-end.
 

sx2504life

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Sep 30, 2009
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im not a real expert on this but i find on my 08 kx250f the vavles need to be adjusted about every 15 hrs untill i got my new vavles and they are up to 28 so far and havent moved yet but my last vavles i could tell when they needed to be done when the bike was had to start when warm
 

adam728

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Aug 16, 2004
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Another reason 2 strokes eat up rings faster is the ports. For a good portion of their total up/down travel some sections of the rings are over a port, and not wearing against a cylinder wall. This causes the rings to wear unevenly, allowing blow-by in less time than if the entire ring were in constant contact with a cylinder wall. I've taken rings with way too many hours on them and set them into the bore (as to check end gap) and seen light around different areas between the ring and cylinder wall due to this uneven wear.

Also as mentioned, usage determines how long between top ends. If you ride hard mx and are using an average of 35 hp on your 250 you won't see nearly the life span of the trail rider that uses an average of 10 hp of his 250. More load/heat wears things out faster.
 

DieselTech

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Jan 21, 2007
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While that may have some truth to it, it would also contradict the fact that a Detroit Diesel (2-stroke) does not have that issue (even though it also has rings that ride in the port area and not against the cylinder wall). As a matter of fact, the part of the ring that rides against the cylinder wall would be the part that wears faster than the part of the ring that rides in the port area. So, maybe the real reason for the top-end replacement interval being so much more than a 4-T is related to the possibility of a broken ring due to the unworn part of the ring (that rides in the port area) catching the port edge and breaking, or causing the ring land of the piston to break. Another reason may be related to such things as 1) improper warm-up, 2) variables in pre-mix ratios, 3) or improper jetting (too lean) and overheating of the piston. Just my thoughts..........

Any experts care to elaborate?
 

adam728

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Aug 16, 2004
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DieselTech said:
While that may have some truth to it, it would also contradict the fact that a Detroit Diesel (2-stroke) does not have that issue (even though it also has rings that ride in the port area and not against the cylinder wall). As a matter of fact, the part of the ring that rides against the cylinder wall would be the part that wears faster than the part of the ring that rides in the port area. So, maybe the real reason for the top-end replacement interval being so much more than a 4-T is related to the possibility of a broken ring due to the unworn part of the ring (that rides in the port area) catching the port edge and breaking, or causing the ring land of the piston to break. Another reason may be related to such things as 1) improper warm-up, 2) variables in pre-mix ratios, 3) or improper jetting (too lean) and overheating of the piston. Just my thoughts..........

Any experts care to elaborate?

I understand the area against the cylinder wears faster, I thought that could be implied from my post.

The comparisons with a Detroit diesel are far from apples to apples. Specific power output is massively different, not to mention the architecture of the engine (Detroit's ring design is such that it gives up sealing pressure for longer life, bikes are a no-compromise performance design). One might make 280 hp/L at 10,500 rpm, the other might make 40 hp/L at 2000 rpm. It's like saying a 40 hp 250F spinning 12K can't wear out valves and cams because my 210 hp / 4500 rpm TBI 350 in my truck doesn't. Parts are under far greater heat and stress, and see far more cycles per hour.

2 strokes put out much more power per cc than a 4 stroke thanks to the fact that they fire every revolution. Unfortunately this also means they see double the combustion cycles of a 4 stroke at the same rpm, making piston cooling more difficult. I also think that due to the dynamics of a 2 stroke there will be improvements made in life-span with DI, as fueling and lubrication can be much more precisely controlled.


EDIT:
My opinion here. I think one reasons bikes seem so "short lived" is because they are rebuild as preventative maintenance, where we rebuild pretty much anything else as-needed. I do top ends on my 250 every 50 hours, even though it's usually within speck and showing great compression still. I might get 100 hours before things get outside the limits, and maybe 200 hours before the engine just plain out dies, but we don't tend to do that with our bikes. With daily driver cars/trucks we don't typically go "wow, there's 120,000 miles on it, I better rebuild." No, we run em until something fails, then rebuild (which could be 300,000 miles).

Going to the specific power output thing again, it's not what an engine can make for hp, but what it does make in use. Someone riding single track at a leisurely pace on a 250F might average single digit hp output, and get WAY more life from the engine than say, Justin Barcia. Same as a gently driven Mustang might go 300K, but one that is used soley for 1/4 mile drag runs will be worn out long before seeing that number on the odometer.
 
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whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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Wait till catastrophic failure sure seems the preferred method, for most. Have an expert check the bore for being true. The manual maintenance schedules are for each bike, to maintain it to the highest level. It is not for Villipotto, his motors last minutes, not hours. Parts look great, nice and shiny? Where is this method of judgement in the book? Measure and inspect. Must be a grey area? The parts are still with in spec, they are supposed to be! Just like darn near all else, every part on a modern MX race bike has a limited life. That 5% power loss or so, most do not miss from running out of round bores, is some sort of superior feeling that they are smarter/better, than the idiot that wrote the book. I no longer need that 5% or so, but, I know where it is at! Most, really need a trail bike. Then, they try and mod it to the MX race bike? Why bother? Some still want to play with fire, and still does not like getting burn't? Why indeed does my 100,000 mile diesel rebuild not work with my MX race bike? AYE CARUMBA! Try running a top fuel dragster sometime? Then rag cause you can not drive it across country? And this is just the motor! The suspension needs just as much TLC as the motor! Wait till it blows oil out everywhere? No, dig deep for 6 to 800 dollars to fix what has been neglected. WHY I HATE USED BIKES. Vintage Bob
 

Uchytil

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Jun 29, 2003
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whenfoxforks-ruled said:
WHY I HATE USED BIKES. Vintage Bob

Bob, then it's really a love/hate relationship what with all the old bikes we have? Right now I hate my Maico but if I apply some love maybe it'll be ready for the track this year. Now I do know where there is a complete, unused, not ridden, only started, 1979 MC440, in the crate. I'll give you a hint, Ritchie...
 
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