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Engine Blue Printing

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Posted by: Faded

Eric and/or Rich (I bet that gets old!)

I've been reading Two Stroke Tuning by A. Graham Bell and in the begining of his book he makes references to blue printing the motor and the gains that can be had. I'm sure times have changed since the publication of this book but I was curious if the potential is still there on later model MX machines.

Another question that has been nagging me more than anything, where does a person get the specs on his/her machine? I doubt Honda would send me a set of "blue prints" for a 2002 CR250, even if I enclosed bribe money with my request.

Not really planning on doing it to this bike (the work has been done!), but just curious about the details.

Thanks in advance,
Andrew


Posted by: Eric Gorr

The term "blue-printing" refers to basic machining and metal finishing tasks to square-up components, lapp, deburr, polish, check for straightness and wear. Basically its the act of restoring the engine to the original intention of the designer. Blue-print specs can be extrapolated from a factory service manual where service limit dimensions are published.


Posted by: Faded

Eric,

Thank you for the reply.

Quote:
Originally Posted by Eric Gorr
Blue-print specs can be extrapolated from a factory service manual where service limit dimensions are published.

So is a person shooting for the tight spec or the loose spec? A general question I know...and I'm sure it depends; or is manufacturing bad enough that parts don't even fall into the specifications given?


Posted by: mtk

The parts will fall in; blueprinting is about putting them on the "good" side of tolerance.

Things like stacking up the tolerances for maximum compression ratio, desired clearances, maximum valve lift, etc.


Posted by: Faded

Quote:
Originally Posted by mtk
...blueprinting is about putting them on the "good" side of tolerance.

Things like stacking up the tolerances for ... desired clearances...

So, what's the "good" side of the tolerance and/or desired clearances?

I guess this is my question from the begining, how does one learn or teach themselves how to identify the most benificial spec out of the range of tolerances given? I understand that it's a pretty general question and it will depend on a particular application, but there must be some type of guideline(s) that a person would consider during the process.


Posted by: mtk

It depends on what you're trying to accomplish.

AMA Supersport roadracing bikes are set up with bearing clearances on the very outside of the spec. This results in minimum drag, and therefore increased horsepower output, at the cost of reduced engine life.

Along the same lines, you'd set up other clearances (deck height, wrist pin-to-piston crown dimensions, etc.) to maximize compression ratio. A lot of these are done by the factory because they have a whole production line of parts to check. As part of QC on the production line, they check the dimensions of new parts. It's a simple matter to pick out the pistons with the tallest wrist pin bore to piston crown height and set them aside as race pistons. These will give slightly higher compression ratios than pistons on the other end of the tolerance spec. You can do the same by picking camshafts with lift values on the high side of tolerance, rods on the long side of tolerance, etc. They don't mean much individually, but when you add them all up together it results in a stronger engine. That's why the factory bikes are always faster than the privateers; they have access to a LOT more parts to choose from and also have no problem with an engine that only lasts one weekend because the bearing clearances are loose.

I've also heard of folks surface grinding valve shims so they can set valve clearance specifications to the absolute minimum, which also gives you maximum lift and duration with that camshaft.


Posted by: usgpru27

Maybe one simple example is the difference I see between a dirtbike crank and a roadrace 2 stroke crank. From reading the posts on this forum I see that most just buy a crank and put it in. They are fairly good from the factory. In our roadrace bikes the crank is rebuilt and what we call balanced. In doing so it is set so that run out is set to .001 run out. Just about perfect. This costs about $200 over and above the $250 for the crank. It is an issue with application. The road race bike is simular to the supersport bike in that you are trying to bet the most out of the motor. A perfect crank may result in .2 of a HP over an out of the box unit. It is also presses together with an exact width measured in the cases. Again making it a perfect fit. Not sure if this helps. I plan on blueprinting my dirtbike in certain areas like the crank. To me it makes for a smoother running engine and yields maximum part life.


Posted by: Studboy

It doesn't seem right that they would run too much clearance in regards to the bearings. I thought that too loose would cause damage and too tight would just seize the bearing.


Posted by: splatt

Quote:
Originally Posted by Studboy
It doesn't seem right that they would run too much clearance in regards to the bearings. I thought that too loose would cause damage and too tight would just seize the bearing.


They aren't to tight or to loose if they are within a narrow spec range.Anything outside that range is unacceptable.

Steve


Posted by: jmics19067

IT goes along the lines if you're valve adjustment (on a four stroke) is between .004" and .006" .004" would offer more performance because it follows the cam closer than the .006". AT .006" there is less likelyness to burn a valve because of extra .002" Not that there is going to be a discernable amount of power difference or engine longevity between the two but if you do that with everything the addition of all pluses can be noticable. There is a set of allowances what you want to do is narrow down the allowable to the performance side. A good mechanic will double check everything to make sure that all falls withing spec. A good tuner/engine builder will make sure that it is not only in spec, but optimized for a particluar application.


Posted by: mtk

Quote:
Originally Posted by Studboy
It doesn't seem right that they would run too much clearance in regards to the bearings. I thought that too loose would cause damage and too tight would just seize the bearing.


Yes, it WILL cause damage, but the engine will last a race weekend just fine. They trade engine life for slightly less frictional drag. The fact that the engine needs rebuilt after every weekend is of no consequence to them, only winning matters.


Posted by: oldguy

Brought back to life at posters request



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