How to tell if she's bored out ?


12 replies to this topic
  • slothman

Posted December 17, 2012 - 08:37 AM

#1

Just curious guys....how can I tell if my yz450 has a big bore kit or not ? The previous owner didn't have any paper work, so I can't really trust anything. Will the head be stamped "Athena" ....or maybe have another type of stamp ? Does the stock head have any particular stamps? Thanks !

  • vic2340

Posted December 17, 2012 - 09:30 AM

#2

If the bike has a big bore kit on it then yes it will definitely have a new cylinder jug with the aftermarket brands name on it. The kit in that sentence is the key word. Now if the guy before you simply fried the top end and needed to bore out the scratches on the jug and then he fitted it with the next size piston (ex 455, or something) that is not considered a big bore kit but simply bored over.

  • grayracer513

Posted December 17, 2012 - 09:44 AM

#3

... if the guy before you simply fried the top end and needed to bore out the scratches on the jug and then he fitted it with the next size piston (ex 455, or something) that is not considered a big bore kit but simply bored over.


There are no such pistons available, so that will not be the case. Because the original bore is plated aluminum, and cannot be bored for wear without replating it, the factory does not offer wear oversizes as is usually done when using boreable iron or steel cylinders. Only the standard and 3mm oversize pistons exist.

If a 3mm big bore kit has been used from a company such as Luke's Racing that uses the OEM cylinder as a starting point, there is no way to tell without measuring the bore.

  • slothman

Posted December 17, 2012 - 10:19 AM

#4

There are no such pistons available, so that will not be the case. Because the original bore is plated aluminum, and cannot be bored for wear without replating it, the factory does not offer wear oversizes as is usually done when using boreable iron or steel cylinders. Only the standard and 3mm oversize pistons exist.

If a 3mm big bore kit has been used from a company such as Luke's Racing that uses the OEM cylinder as a starting point, there is no way to tell without measuring the bore.


Well, I was told that the bike has a 480 big bore kit. The head does look new, however I didn't see any stamps such as "Athena" or anything else.

  • grayracer513

Posted December 17, 2012 - 10:31 AM

#5

The head is not what gets bored out, the cylinder is, and there are all kinds of 480 kits done by boring the original cylinder. So, as I said, the only way to know for certain is to pull the head and see if the bore is 95mm or 98mm.

  • slothman

Posted December 17, 2012 - 10:52 AM

#6

The head is not what gets bored out, the cylinder is, and there are all kinds of 480 kits done by boring the original cylinder. So, as I said, the only way to know for certain is to pull the head and see if the bore is 95mm or 98mm.


Sorry, I meant the cylinder. I guess I will never know lol.....until it's time for another rebuild :)

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  • 72degrees

Posted December 17, 2012 - 11:00 AM

#7

I suppose, with the engine out, you could use a burette in the same way that tuners used to calculate compression ratio. With a bit of maths and knowing the original swept volume and ratio very acurately you would only have to measure the combustion chamber volume, assuming a 'stroke' conversion hadn't also been done. Far too much hassle to bother doing unless the engine had to come out anyway.

  • grayracer513

Posted December 17, 2012 - 02:08 PM

#8

Displacement is unrelated to combustion chamber volume or compression ratio, and is dependent only on the bore and stroke. If one were to disconnect the cams so the valves would be closed at BDC, and run the piston down to true BDC, you could simply measure the air displaced through the spark plug hole between there and TDC directly without having any idea what the CC volume or compression ratio is.

The equipment used to do this is not very commonly available, however.

  • 72degrees

Posted December 17, 2012 - 03:07 PM

#9

Yes, I see the error in my logic. Assuming the head is not altered and the big bore piston maintains the same combustion chamber volume the compression ratio will have changed as a result of the big bore. So the calculation Swept Volume = (Compression Ratio * Combustion Volume) - Combustion Volume, won't work.

  • grayracer513

Posted December 17, 2012 - 03:51 PM

#10

No, it won't, but again, nobody is trying to determine the compression ratio, just the swept volume, and that could be measured if one had access to the equipment.

  • vic2340

Posted January 14, 2013 - 08:40 PM

#11

There are no such pistons available, so that will not be the case. Because the original bore is plated aluminum, and cannot be bored for wear without replating it, the factory does not offer wear oversizes as is usually done when using boreable iron or steel cylinders. Only the standard and 3mm oversize pistons exist.

If a 3mm big bore kit has been used from a company such as Luke's Racing that uses the OEM cylinder as a starting point, there is no way to tell without measuring the bore.

I wasnt being exact with the 455 I was just making up a number that was bigger. Thanks for the correction though

  • vic2340

Posted January 14, 2013 - 08:43 PM

#12

Oh and about that answer you gave me, On the 450's and newer four strokes when you over bore the bikes to the next size (according to you its 480mm) do you need a new sleeve or just a bore and replating job?

  • grayracer513

Posted January 15, 2013 - 07:48 AM

#13

480 cc, not 480mm, and that depends on the the original size of the bore relative to the stroke, and on the amount of oversizing done. The point is that whereas in the past, pistons were produced in standard sizes and then typically .25, .50, and occasionally 1.0mm oversizes for wear so that the cylinder could be bored whatever minimal amount was needed to clean it up and restore a good surface, this practice ended with the advent of hard-plated aluminum bores. On those engines, the official factory method of repairing an unserviceable bore is to replace the cylinder with a new one.

A plated cylinder can only tolerate a maximum of about .002" total wear (0.05mm) before the plating is worn through. Regardless of whether the cylinder is to be bored oversize or restored to standard size, the bore has to be bored or honed to a true round and straight condition, then replated and honed to size. In the case of the most minimally worn cylinders, it is possible to replate without any repair work on the bore. If the bore is damaged by deep scratches, etc., then the damaged area is usually welded over and then rebored, plated, and resized back to standard.

In big bore kits, the limiting factor is the original thickness of the cylinder walls and the skirt extension below the cylinder base. Some bikes will remain acceptably strong with 3mm removed, some won't, so the final size of the overbored engine depends on how much it can safely be enlarged. In either case, only the "standard" sizes are offered, and not "next size" oversizes for wear adjustment.

Sleeving cylinders is an old school repair method in which an iron or steel sleeve is fit to and pressed/shrink fitted into an overbored cylinder to return it to the stock dimensions when boring for wear won't clean up the damage. The process has several disadvantages, particularly when used on modern plated bore engines. For one thing, there is a natural thermal barrier between the sleeve and the cylinder because of there being two different pieces of metal involved, and heat trying to move from the cylinder wall to the cooling media is hampered by having to cross that barrier. Secondly, the piston fit has to be adjusted looser than normal to accommodate the different thermal expansion rate of the sleeve vs. the original aluminum bore. Overall, sleeving for repairs is not as good a choice as repair and replate is.





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