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Moto Mind
Moto Mind is a technical blog written by Paul Olesen who is a powertrain engineer working in the motorcycle industry. The blog covers a wide variety of topics relating to two and four stroke engine performance, design, and optimization.


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When to Rebuild Your Engine

Posted by Paul Olesen , December 15, 2014 · 4,943 views

When to Rebuild Your Engine This week I’d like to start a long series of posts on the proper way to rebuild a four-stroke engine. I will share with you a top to bottom rebuild where I go through the disassembly, inspection of parts, and reassembly of the engine. We’ll cover the top end, bottom end, and everything in-between. I’ll pass on the tips and tricks I’ve learned over the years from the people I’ve worked with in the motorcycle industry. Hopefully these tips will benefit you on your next engine build, save you money, and ensure you do things properly.

Before I get into the specifics I want to discuss the importance of preparing for the rebuild. This week let’s talk when to tear into the engine.

When to replace: Engine wear is directly related to RPM and mechanical stress so riders engaging in riding where the engine is on the rev limiter frequently, the engine is operating at a high RPM without any load on it, the gearbox is loaded or unloaded abruptly, or gears are selected hastily will require the rider or mechanic to service the engine frequently. Most of you will relate the scenario I have illustrated to motocross racing. At the top levels of racing, mechanics are constantly checking or rebuilding the engines to make sure they are operating at maximum power.

Fortunately for most of us we are not riding or racing at the top level, so our bikes and engines last quite awhile longer. Unfortunately everyone’s scenario is different- depending on the type of riding you do, the environment you ride in, how often the oil is changed, etc. which makes it difficult to standardize or pinpoint any sort of service interval.

As an engineer, mechanic, and rider my philosophy has always been to replace components preventatively before they fail. My reasoning here is that the costs attributed with a failed component are much higher than a replaced component. Let us consider a scenario where a piston fails. This could have been avoided had I replaced the piston and would have cost around $130. Instead it’s quite likely that the entire engine will need to be opened up and serviced, making the price of the repair extremely expensive.

From an opportunity point of view, if any part fails on the bike while I’m out riding or racing I’ve lost out on a significant amount of time, a significant amount of points if I’m racing, and a wad of cash when it comes to paying to get to and from the venue. So apart from saving a small amount of money by not replacing a serviceable part, there is no upside to trying to prolong the life of a component. The ramifications of engine neglect are nothing to scoff at.

The best way to determine when components need to be serviced is to keep careful track of the health of your engine. This means from the time of purchase to the day you sell you keep track of all the engine hours, maintenance, and repairs you do to the bike. By keeping track of engine time you’ll start to develop patterns and be able to establish your own service intervals. I wrote a nice article about maintenance logging which you can read HERE.

Along with keeping a log from day one, I also like to do a compression test any time I get a new bike so I can establish a baseline for the health of the engine. As I put hours on the bike, if I ever become suspicious that the engine is down on power I can perform another compression test. Then I can quickly refer back to my first test to determine if I have lost any compression and might need to consider servicing the top end.

The next thing you must do is pay attention to your engine. In most cases your engine will give you signs that it is time to service one component or another. Some common signs that may indicate your engine is due for servicing soon are:
  • Hard to Start - This could be due to a fueling issue, ignition issue, decompression system out of adjustment, worn rings, worn valves and seats, a stuck valve, leaking gaskets, or cam timing that is off.
  • Engine Power has Diminished - This could be due to restricted fuel flow in the carburetor or throttle body, a clogged air cleaner, the clutch slipping, worn valves and seats, worn rings, a stuck valve, leaking gaskets, or ignition issues.
  • The Top End is Noisy - A noisy top end could be caused by a loose cam chain, out of spec valve clearances, a worn cam chain guide, or worn cam bearings.
  • The Bottom End is Noisy - A worn clutch basket which has started to rattle, damaged or stuck bearings, a worn bushing and needle bearing between the clutch basket and primary shaft, or gears which are improperly lubricated may all contribute to bottom end noise.
  • Blue Smoke - Blue smoke occurs when the engine is burning oil. Either the valve seals are allowing oil to leak past them or the piston rings are no longer sealing properly. Once the engine is warm very little blue smoke should ever be seen.
  • White Smoke - White smoke is emitted when the engine is burning coolant. This typically occurs when a head gasket starts leaking.
  • The Engine Consumes Oil - Oil is getting into the combustion chamber any time the engine consumes oil. Oil can either enter into the combustion chamber from worn valve seals or worn piston rings.
  • The Engine Oil is Creamy - Whenever the engine oil is creamy in color moisture is getting into the engine oil. While some moisture getting into the oil is normal excessive amounts are a cause for alarm and may indicate that a water pump seal is leaking.
  • The Engine Oil has Large Pieces of Metal in It - Metallic particles are common in engine oil but if larger metal pieces are found in the oil this is a cause for concern and should be associated with damaged components. An example of this could be finding fragments of chipped gear teeth in oil.
  • [color=rgb(0,0,0)]The Engine Vibrates Excessively - Excessive engine vibration may be caused by an out of true crankshaft, worn crank bearings, worn counterbalance bearings, a mistimed counterbalancer, or a loose clutch.[/color]
One last tool I want to mention that is helpful in determining the health of an engine is a leak down tester. With the piston at TDC and the valves closed (compression stroke) a leak down test pressurizes the cylinder to a specified pressure. A comparison is made between how much air is supplied to the cylinder and how much leaks out. The amount of air leaking out of the cylinder is used to determine the health of the engine. For example if 70% of the air is leaking out the cylinder there are serious problems! By carefully listening for the air leak(s) it is possible to determine the cause of the problem.

I would really like to help you guys out and give you quantifiable numbers so that you know precisely when the right time is to rebuild your engine however I feel that by doing this I would be doing a disservice to a lot of you. I would either be giving you information that tells you to rebuild your engine too early or too late in its life which wouldn’t be good for anyone. As I mentioned before there are so many variables ranging from riding style, engine displacement, manufacturer, riding environment, and maintenance intervals that I can’t quantify all these things into one number for everyone or even several numbers for specific groups. Your best bet is to pay close attention to your engine, keep track of the hours on your engine, and learn as much as you can about your particular make and model so that you can begin to formulate a service interval schedule tailored to you.

Questions, comments, or additional tips leave a comment below!

Moto Mind - Empowering and Educating Riders From Garage to Trail

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Hey, thanks for writing this up - looking forward to your other entries.

 

What is light blue smoke at start-up indicative of? My bike seems to do this and then it dissipates as the engine warms up? Could it be the Piston/rings?

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Paul Olesen
Dec 15, 2014 02:48 PM

Hey, thanks for writing this up - looking forward to your other entries.

 

What is light blue smoke at start-up indicative of? My bike seems to do this and then it dissipates as the engine warms up? Could it be the Piston/rings?

 

Light blue smoke on start up is indicative of a little oil burning in the combustion chamber. Most commonly a little oil leaks past the valve stem seals into the combustion chamber once the bike is shut off or a little oil is left on the cylinder wall. Once restarted the oil is burned and the blue smoke is exhausted out the pipe. This isn't anything to worry about if the smoke goes away once the engine is warm and the bike isn't consuming a noticeable amount of oil.
 

Paul thanks for setting up this series, it will be a great reference tool!

i think logging hours is a great benefit !i was changing my oil every ride sometimes only 2hrs !The oil was still clear !So i upped it to 2 rides 4 hours still not really contaminated to much ! so now i am at three rides or 6 hrs and the oil is much darker so i know now 5 - 6hrs is my max ,i dont over rev it or stay in high revs for long periods so everyone should do their own checking as it will be different for everyone ..... Is it possible to get some pics or info on leak down tests and compression tests ,i.e tools required etc ..great article

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Paul Olesen
Dec 16, 2014 07:57 AM

Paul thanks for setting up this series, it will be a great reference tool!

Your welcome, in addition to the blogs I'm working on an e-book and some videos that will be beneficial to you as well. Keep your eyes peeled and thanks for reading and commenting on my blog!

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Paul Olesen
Dec 16, 2014 08:06 AM

i think logging hours is a great benefit !i was changing my oil every ride sometimes only 2hrs !The oil was still clear !So i upped it to 2 rides 4 hours still not really contaminated to much ! so now i am at three rides or 6 hrs and the oil is much darker so i know now 5 - 6hrs is my max ,i dont over rev it or stay in high revs for long periods so everyone should do their own checking as it will be different for everyone ..... Is it possible to get some pics or info on leak down tests and compression tests ,i.e tools required etc ..great article

Thanks for sharing your tip twigster, this really is a good example of how everyone's maintenance needs are different. I am planning on putting some info together on leak down testing and compression tests. There are a ton of topics to cover and I look forward to getting through them all. 

Thanks for commenting. It's comments like yours that help me determine what people want to know more about. Without a continuous dialog between myself and all of you I'm just shooting from the hip when it comes to creating content and discussing topics. The more all of you are able to fill me in on what you want to learn about or would like help with the less time I will spend writing and making videos on subjects you don't want.

Great info. Thanks for helping all of us "garage mechanics" out. I look forward to you explanation of leak down test. I went out and bought one and seams to work great. I have an 08 rmz250, won't run, zero compression. Did leak down and it held at 100%. Turns out one of the values came lose and the pressure held the valve closed... Ps that was not me that figured that out.
Thanks for the write up I look forward to the rest. I often forget that TT is more than just forum. The Facebook page recently came up as a suggestion so when I liked it and began checking it out I remembered it's a retailer and a blog spot. I usually go straight to the forums. I too would be interested in correct tools.

Last year I bought a leak down tester from harbor freight that did not fit my 250sxf. So I bought a compression testers from sears to use one of its adaptors with my leak tester. I was at tdc and all the air I put in quickly came out. I knew that couldn't be the case. There was nothing wrong with my bike. I was just curious to do the test. Then after doing some research I find out about the decompression mechanism on the exhaust cam..... So if you can cover that at some point in the write up I would be greatful. I you have ready done a write up on that I may find it since I will be browsing the blog section of TT more often. Thanks again!

Hi Paul,

 

I'm collecting parts to replace the top end on my YZ250F. I was thinking about replacing the piston and its related components as well while the engine is apart. Right now, the engine is still together and running without any observable (visual & audible) issues. Should I wait until the engine is apart before I order a piston kit or do you think it's a safe bet that I can order a standard bore kit? The oil has been changed frequently with full synthetic and the bike has never been raced.

 

Thank you!

Jordan

Paul,

 

One of the things I am interested in is exactly what is "Hard to Start".  I guess this should probably start with "How to Start a 4-Stroke".  And that probably depends on the specific bike.  I have an '06 YZ450, and it ranges from hard to start when I "Really Need to get it Started" to starting with one easy kick when I'm focused on starting it. 

 

In my case, this starting thing reminds me a bit of golf.  Sometimes it seems so easy and fluid, and others, I just cannot do it. 

 

Any help with the proper method of starting these bikes would be great.  I will practice that until I do it no other way.  Then I'll be able to tell when it is getting harder.

 

Thanks.

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Paul Olesen
Dec 17, 2014 03:49 PM

Great info. Thanks for helping all of us "garage mechanics" out. I look forward to you explanation of leak down test. I went out and bought one and seams to work great. I have an 08 rmz250, won't run, zero compression. Did leak down and it held at 100%. Turns out one of the values came lose and the pressure held the valve closed... Ps that was not me that figured that out.

Your welcome. I will work on putting some things together to cover compression and leak down testing. When you say the valve came loose what exactly happened? Did the two cotters come out or did a spring break? Interesting.

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Paul Olesen
Dec 17, 2014 03:50 PM

Thanks for the write up I look forward to the rest. I often forget that TT is more than just forum. The Facebook page recently came up as a suggestion so when I liked it and began checking it out I remembered it's a retailer and a blog spot. I usually go straight to the forums. I too would be interested in correct tools.

Last year I bought a leak down tester from harbor freight that did not fit my 250sxf. So I bought a compression testers from sears to use one of its adaptors with my leak tester. I was at tdc and all the air I put in quickly came out. I knew that couldn't be the case. There was nothing wrong with my bike. I was just curious to do the test. Then after doing some research I find out about the decompression mechanism on the exhaust cam..... So if you can cover that at some point in the write up I would be greatful. I you have ready done a write up on that I may find it since I will be browsing the blog section of TT more often. Thanks again!

I will work on it Sean!

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Paul Olesen
Dec 17, 2014 06:36 PM

Hi Paul,

 

I'm collecting parts to replace the top end on my YZ250F. I was thinking about replacing the piston and its related components as well while the engine is apart. Right now, the engine is still together and running without any observable (visual & audible) issues. Should I wait until the engine is apart before I order a piston kit or do you think it's a safe bet that I can order a standard bore kit? The oil has been changed frequently with full synthetic and the bike has never been raced.

 

Thank you!

Jordan

Hi Jordan,

I'm not super familiar with your bike however I suspect you have a Nikasil lined cylinder. Nikasil is a very thin plating that goes directly onto the aluminum cylinder so there probably aren't going to be any options for different size pistons. Before you put your new piston in you will want to lightly hone the cylinder to bring the cross hatch back so the rings seat properly. Then after honing you should measure your cylinder bore to make sure it is within your service manual specs for bore diameter, taper, and out of roundness. If you don't have a hone or the right measuring equipment you should be able to take the cylinder to any reputable shop and have them hone and inspect it for you. 

Awesome idea.  I am following this one for sure. Can never know too much

 

Thanks Paul :cheers: 

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SlickitySloan
Dec 24, 2014 11:09 AM

Thanks for the guidance.

 

I have an 06 CRF450R with 110 hrs on it. A high comps piston added at 40hrs. I bent the header so was running with restricted flow for a while before replacing the header. No oil loss, no smoke. Was getting hard to start and comp felt less when kicking it. Valves have never been out of spec. Leak down test showed 25% loss exhaust valves, no loss from crank or intake.

 

Can I just pull the head and have the valves serviced or should I replace the piston while I am in there?

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Paul Olesen
Dec 29, 2014 09:15 AM

Thanks for the guidance.

 

I have an 06 CRF450R with 110 hrs on it. A high comps piston added at 40hrs. I bent the header so was running with restricted flow for a while before replacing the header. No oil loss, no smoke. Was getting hard to start and comp felt less when kicking it. Valves have never been out of spec. Leak down test showed 25% loss exhaust valves, no loss from crank or intake.

 

Can I just pull the head and have the valves serviced or should I replace the piston while I am in there?

So the piston has 70 hours on it? What sort of riding are you doing?

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