“Splitting the cases” is often referred to as a daunting or undesirable task, but if you are well prepared and properly equipped then it can be a straightforward job. To alleviate any concerns you may have with the task, I want to discuss best practices and share some tips that you may find useful when dealing with crank bearings that utilize an interference fit with the crankshaft. We’ll get started by discussing preparatory items and work through to completing the job.
Today I want to share some pointers on preparing new or re-plated cylinders that will help ensure your engines run stronger and last longer. Plus, I've got an update on the two-stroke book I've been working on that I'd like to share. Let's get started!
Today I'm going to cover how to check and set cam timing, which is something you can do if you have adjustable cam gears in your four stroke dirt bike engine. This is a procedure often performed by race engine builders to ensure the valvetrain performs just as they intend, and ultimately so that they extract the desired performance out of the engine. Adjustable cam gears typically aren't a stock option but are abundantly available in the aftermarket. The following text is exerted from my book, The Four Stroke Dirt Bike Engine Building Handbook, so if you find this info valuable please take a look at the entire book.
It's time to open up a can of worms and talk about a hotly debated topic in the powersport community - four stroke cylinder head reconditioning best practices. I've perused the forums and had discussions with people about reconditioning four stroke cylinder heads and there appears to be a lot of mixed opinion and beliefs on what is right or wrong. I'm certainly not going to say my take on the subject is the only way, but I do want to share my opinion, explain the technical details, as well as touch on the machining process. The text below is out of my book, The Four Stroke Dirt Bike Engine Building Handbook, and details why cylinder heads should be reconditioned a certain way.
Whenever new valves are installed in a cylinder head, it is best practice to recut the valve seats since the valves and seats are mated parts, otherwise the new valves are very susceptible to premature wear when run in the old seats. If a major overhaul is being performed, there is a good chance that enough seat wear will have occurred during the engine’s life that the valve seats will need to be recut before new valves are installed. This may be news to you, so I want to provide an explanation of why this is necessary.
The term concentricity is used to describe the relationship between the axis of two circular objects. When two objects are perfectly concentric, their axis match up precisely with one another.
In the case of the cylinder head, the valve guide axis and the valve seat axis must be as close to perfectly concentric as possible and parallel to one another. Usually, guide to seat concentricity is kept around 0.001” (0.025mm) or even less for racing applications. This is achieved by the factory by using a manufacturing process where the valve guides are reamed first. Then the freshly reamed valve guide bore is used to center the valve seat cutter. Once centered, the valve seat is cut. This process is then repeated for all the valves and results in very good concentricity between the valve guides and valve seats.
As the engine is run, the valve guides, valve seats, and valve faces will wear. The valve guides will wear from front to back in an oval shape at the top and bottom of the guides. In a cross sectioned view the valve guide will take on an hourglass shape. The guide will become oval as a result of thrust forces stemming from the way the camshaft contacts the lifter bucket or rocker arm. These forces are transmitted to the valves and cause the valves to thrust against the sides of the guides, eventually causing the guides to become oval shaped.
Once the guides start to become oval shaped, the valve faces will no longer be as concentric to the valve seats as they originally were. When this happens the valves will start to slide against the seats, causing the seats and valve faces to wear. The valve seats will eventually become out of round and the sealing between the valve face and seat will suffer. Installing new valves into oval shaped guides and out of round seats will ensure that the new valves wear out very quickly!
To ensure the new valves being installed last as long as possible, the cylinder head’s seats and guides must be reconditioned once they are worn out. Complete cylinder head replacement is always an option, but I want to focus on freshening up the original head which is usually a more economical option, but comes with many more variables surrounding the quality of the job.
There are numerous services offered in the marketplace for valve seat cutting, however, not all valve seat cutting methods are equal in terms of quality. There are hand operated seat cutters, dedicated seat cutting machines, and a few other options to choose from. Selecting the correct seat cutting process and entrusting the work to a competent engine builder is very important. The valve seat cutting process should mimic the OEM process as closely as possible. A concentric valve seat will never be able to be cut without first servicing the valve guides. If the valve guides are out of round then they will either be reamed to a slightly larger size if they are not too oval in shape or they will be replaced. Once any issues with the valve guides are addressed and they are perfectly round from top to bottom, it will be possible to cut the valve seat. Ensuring the valve guide is perfectly round is extremely important since the valve seat cutter is centered off of the valve guide bore.
Cutting the valve seat concentrically to the guide requires a combination of skill and using modern machinery. The best valve seat cutting equipment in the world is worthless without a good experienced operator running it. There are two main factors which make cutting a seat concentric to the valve guide difficult. To start with, the valve seat cutter uses a pilot which locates in the valve guide. Since the valve stems are very small in diameter the pilots used to guide the seat cutters are also very small in diameter. A small diameter pilot shaft that centers the cutting tool can flex easily, which presents a real problem when cutting the seats. The next issue that arises when reconditioning seats is that often times the cutting tool will try to follow the path of the old valve seat which can make it hard to cut a concentric seat. Couple these factors together with slop within the machine, setup error, and operator error and you can see how quickly things can come out of alignment and you can end up with a poorly cut seat.
In addition to seat concentricity, the depth the seat is cut to will influence valve spring pressure, shim sizes, and the compression ratio of the engine. All three of these variables will be reduced the deeper the seat is cut, which is not a good thing. The surface finish of the seat itself will influence how well the valve seals. A seat with chatter marks or other machining blemishes will not seal as effectively as a smooth seat. The valve seat width and the contact point between the seat and the valve face are also very important.
Due to the complexities involved with cutting valve seats on modern four-stroke dirt bike engines, the job should not be left up to just anybody. There are numerous businesses which specialize in valve seat cutting which have both the right equipment and expertise to do the job correctly. I highly recommend spending some time researching and finding a reputable cylinder head machining company when it comes time to recondition your head. If the cylinder head must be shipped off in order to do business with a reputable company, the additional wait will be worthwhile.
If you found this information helpful and would like more technical info on maintaining your four stroke engine, check out my book, The Four Stroke Dirt Bike Engine Building Handbook.
Thanks for reading and happy wrenching!
As always if you have comments or want to share your thoughts please leave a note below.
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I hope you’re all enjoying the holiday season so far. For those of you in northern states, I hope that you got in some last minute riding or are gearing up for riding on the ice this winter. This month I want to touch on bolted joints and the importance of adhering to tightening techniques outlined in your model’s service manual.
So with all the new reasons to get out and shop post-Thanksgiving (now you've got Black Friday, Small Business Saturday, and Cyber Monday) here at DIY Moto Fix, we figured we would make it simple and just have one big four day sale.
Whether you're the one getting up at 5am to get the best Black Friday deals, or you're the one driving 90 minutes north to go ride in the dirt all day (that would be us), from today through Monday the 28th, everything on the DIY Moto Fix site will be available at 30% off. We normally don't go this steep with our discounts, so you definitely want to jump on board for this one before Monday the 28th is over.
Below we have a list of our top selling products with links so you can learn more. Scroll through and start making your wishlist.
If you are taking off for a weekend of riding, then be sure to share this post with your significant other so they know what to put in your stocking this year!
The Four Stroke Dirt Bike Engine Building Handbook
SHOWA & KYB SHOCK BUILDING INSTRUCTIONALS
The Honda CRF 450 Full Engine Rebuild Video Manuals
Do you want more suggestions? Check out our entire store at this link. Happy Holidays!
This week I wanted to share an excerpt from the Race and Performance Engine Building chapter in my book, The Four Stroke Dirt Bike Engine Building Handbook. If you've been wondering how high compression pistons work and if they are right for your application, read on!
Piston upgrades are normally considered when changing the compression ratio is desired or larger valves are installed. In both instances the shape of the piston is altered either to reduce the volume in the combustion chamber or to allocate additional room for larger valve pockets.
The compression ratio defines how much the original air/fuel mixture which was sucked into the engine is compressed. The following equation shows how an engine’s compression ratio can be calculated.
Alright guys, this week I just want to share a short and simple tip with you on how to stay more organized during an engine build.
When it comes to major engine maintenance or repairs, usually the engine covers have to come off or the crankcases must be split. The covers and cases are almost always retained using different length bolts. The repercussions of installing the bolts in the wrong order upon reassembly can be very damaging. This is especially true if you install a bolt that is too short for its location and only a couple of threads engage, ultimately stripping the threads when you tighten the bolt.
So what’s an easy way to keep track of cover or case bolts that are arranged in a pattern of different lengths?
My favorite way to organize these bolts is to take a thin piece of cardboard (think cereal box thickness) and then slit the approximate bolt pattern into the cardboard so that the bolts cannot get mixed up. A picture is worth a thousand words so check out the one below. You need not be an artist to apply this tip, simply slit the pattern, add a couple reference points and you’re done!
Do you have any organizational tips you’d like to share? Leave a comment below because I'd love to hear about them!
In today's post I'm going to shift focus back to the engine and talk a little about valve technology. Valve technology and manufacturing techniques have changed substantially from the earlier days of engine development and I want to share with you some information about the current valve technology being implemented in your engines. I also want to discuss one way you can get a feel for how much life is left in your valves. Let’s get started.
The following excerpt is copied directly from my book, The Four Stroke Dirt Bike Engine Building Handbook. If you want to learn more helpful tips, which will bring your maintenance knowledge and engine building skills to the next level, I’d like to invite you to pick up a copy of my book by clicking here. Be sure to use the offer code rebuild2016 to get 15% off when ordering!
Alright, on to valves shim sizes.
The cylinder head assembly of most engines will wear out before it resorts to telling you it has had enough by catastrophically failing. Diagnosing these wear signs and knowing when it is time to replace components is the key to keeping the cylinder head assembly from failing. Due to the aggressive camshaft profiles, high compression ratios, and high RPMs required to make a lot of power, the valves and seats typically are the first parts to wear out within the cylinder head. Worn valves and seats will cause the engine to become difficult to start, have low compression, and have reduced power.
I hope 2016 is treating you well so far! This last weekend I participated in the Steel Shoe Fund 3 Hour Endurance Ice Race out on Kettle Moraine Lake. For those of you not living in the northern midwest, last weekend averaged about 0 degrees Fahrenheit and the wind chill made it feel like 10 below. Regardless, there were over 65 teams that raced and the day was a blast. Unfortunately we had a serious injury out on the track and they had to call it before anyone officially completed the race. Last I heard the rider is doing fine now, thank goodness! Hopefully I get a few more ice races in before the winter season is over.
Today I want to talk about six characteristics that are necessary to have when one sets out to build an engine. I’ve detailed how to tackle many different jobs, but honestly that is only half the battle. If you’re in a rush or lack the desire to understand the reasons behind what you’re doing, you will make mistakes and miss out on important things. Listed below are the traits that I believe can help you take your build to the next level.
1. Being Detail Oriented
What’s worse than getting started on a build only to realize you didn’t buy an important replacement part? Focussing on the details of a project can feel tedious at times but can pay off in the grand scheme of things. Before I get started on a project I spend a hefty amount of time researching what parts I’m going to replace and where the best prices are. Also, I will have a solid idea of the sequences I’ll use for disassembly and assembly. Another good habit for the detail oriented is to take notes throughout the build, which you can use at a later date should the need arise. When you have an appreciation for all the small details that go into a build, it will make for a much smoother project.
2. Having Patience
Have you ever been in a rush to do something and after you’re done you realize if you had spent just a bit more time the project could have turned out much better? I was this way with so many of the things I did when I was younger, but have learned to slow down and be patient as I work. Engines don’t go together instantaneously and being patient throughout the process, especially when things aren’t going as planned, is very important. There is nothing worse than making a huge mistake because you’re in a rush. Imagine finishing a build and realizing you left an important part on the table, depending on where the part came from, you just bought yourself another few hours of work. Trying to skimp on time more often than not costs you more time in the long run. Have patience and enjoy the process.
3. Being Observant
Just about every mechanical thing is gleaming with a story, and that story only reveals itself if you know what to look for. An engine is no different. From the parting lines on a component left by the casting tooling used to create it to wear patterns on a piston, there are hundreds of observations that can be made while working on an engine. As you work, keep an eye out for subtle anomalies that may tell you why something failed or broke. For example, things like snail tracks across a gasket, raised edges on gasket surfaces, or covers that don’t sit flat on a table - these are all good indicators of why a particular part was leaking.
4. Being Curious
Perhaps more appropriately titled, “a desire to understand mechanical workings”. It is incredible how much can be learned about the engine just by studying how specific parts interact within it. An engine is composed of many different subsystems and they must all work in order for the engine to function. By looking at the various interactions of the parts within an engine, the condition of the parts and reasons for any failures can be more easily understood. The next time you build an engine, challenge yourself to learn how all the different subsystems of the engine work. Once you learn this, diagnosing problems and identifying all the faulty parts becomes much easier.
5. Being Meticulous
The necessity to be thorough and meticulous throughout a build cannot be overstated. Whether it be taking extra steps to inspect components, measuring new parts, or taking extra time to ensure the condition of surrounding subsystems are okay, having meticulous tendencies can pay off. As an example, on more than one occasion I’ve purchased new parts that have been mispackaged or out of spec. Had I not made the choice to carefully measure the problematic new parts, I could have ended up with an engine that was destined to fail. While it may take more time to be meticulous throughout a build, there is a lot at stake, both in terms of time and money, making it all the more important to ensure everything is done correctly.
6. Having Ambition
Building an engine can be hard, things can go south unexpectedly, and projects can easily stall. Being ambitious and having a can-do attitude is important to ensure the engine doesn’t sit half torn apart in the garage never to be completed. Until you tear into the engine, you never know what you might find. I’ve disassembled engines many times in the past only to find I need to replace a lot more parts than I had planned (this seems to be my luck when I shop for bikes on Craigslist as of late). This can be a huge downer, but keeping the end goal of getting back out and riding in mind and having the desire to push through any and all obstacles is a must.
Do you have any engine building characteristics you want to share? Leave a comment below and tell everyone what you think it takes to build a great engine!
For those of you that believe you possess the characteristics of a good engine builder, be sure to check out my book, The Four Stroke Dirt Bike Engine Building Handbook, to learn more about the how and why behind engine building. Whether you want to be taught about the relationships between all the various parts within an engine, you are in need of pointers on picking the right performance parts, or you would like to see examples of wear patterns found on engine components, my book is here to guide and help you throughout your build.
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