Motorcycle displacement issues...

[Hans Blom]

Also for the unknowing the A65 is a 60s, early 70s bike, no vintage here. So I guess this particular type of motor would never be allowed to run in  production650 since it is over 650cc.  What is the process when measuring a bike? If they measure a 650 class bike for instance and it comes to 655, do they look up the specs on that particular bike, figure the new cyl volume with an .020 bore and make sure it is under that new larger volume?  As was mentioned earlier, a 20 over 50cc single doesn't gain near as much volume as a 4 or 6(cbx) cyl would. Percentage wise it might be a bit more but I digress.  I just want to make for damn sure when I arrive to run in 650 modified I am allowed to do so and do so with in the rules.

[1212FBGS]

+20 on a 50cc single does make a big difference. the tech guys dont care what displacement the manufacturer calls it, all they do is check bore, stroke and take overbore into account. do your math correctly and you should be able to run your bsa in the 650 class the same as all the triumph 650 guys do

[JackD]

A bike engine in a car gets whatever it measures to the inch. A 60 inch motor that measures 60.900 isn't 61 until it measures 61 or above.
A bike has the overbore allowance for the same dumb reason the cars seem to do without.
So the same motor in a bike might be OK but too big for a car.
If you follow that logic "The smell may vary but the view will always be the same".

[yamagamma]

Hans, Nortonist has it correct. If the bike was originally built as a 653cc production motorcycle it runs in 750 production. Period.

 It is important to stay within the "spirit of the rule", an oft used line from your friendly Tech Inspector! The overbore rule was introduced to allow racers one overbore and get a little more life out of their cylinders, basically a cost saving measure to help the racers budget. It was not intended to allow people to build a bogus oversize engine that stretches the limits of the rulebook.

[JackD]

I guess you did not see it happen and on who's behalf. The car guys laughed it down for their use.

[Sumner]

A bike engine in a car gets whatever it measures to the inch. A 60 inch motor that measures 60.900 isn't 61 until it measures 61 or above.
A bike has the overbore allowance for the same dumb reason the cars seem to do without.
So the same motor in a bike might be OK but too big for a car.

This concept has confused me, not hard to do, and I have wondered about it.  I'm using  a 750 cc Suzuki with a bore & Stroke of 2.834 & 1.811 in my lakester.  Using the SCTA formula: bore x bore x .7854 x stroke x number of cylinders the motor is 45.70 cu. in. and 0.7488 liters.

The rule book says J Class is 31 thru 45 cid [ Approx. Liter Equiv. of (0.51 to 0.75)].  I Class is 46 thru 61 cid.  So I'm over 45, but under 46, but it says "thru 45 cid", so I'm thinking that means up to 45.999999, so I'm OK.  Am I right :  :

One other thing when they are measuring (with tools, not air or oil), what are they measuring to, tenths, hundreds, or thousands of an inch?  If I measure the above to hundreds then the motor is 45.54 cid.  

Looks like no overbore for sure .  Of course all of this is meaningless unless you actually set a record and I have my doubts I'll be doing that.  I will have some fun though   .

c ya, Sum

[JackD]

You are right about how you read it and deserve to be confused.
The bore is measured to 1/1000 of an inch if direct measured.
You will set a record, you just have to work at it and a big part of that is don't repeat the mistakes of others.
The overbore allowance for a bike answers the same question as "How deep should a hole be?"

[1212FBGS]

Yamma
Ya got it wrong man... although the spirit is good.... the written rule is what is governed!!!!!!!! I'll say it again... the tech guys don?t care what a manufacturer calls the bike as long as your engine measures within the allowable limit it will be legal. Do the math! good example... Keenan Tatro takes a 900cc sportster and runs it in 500cc and 650cc class all the time!!!! has tons of records!!!! how did he do it? he did the math!!

[JackD]

The reason it does not apply to production is because the person that wanted it does not run production. Is that so hard to figure and does it have the same validity as the rest of the application ?
The hydraulic method developed by the bikers prior to all this overbore stupidity is really pretty accurate. If it measures you in excess if the class limit, it is up to you to take it apart and show them how unless of course they take your word for it.
Consistent fair application and the ability to do the math are presumed to be skills that are highly developed on the part of the tech leaders but you suspect I suspect otherwise.

[1212FBGS]

Scott
Your on the right track... the letter of the rule (not spirit to be left to interpolation) is measurement!!! You can take a 1290 busa and bore it to 1350 then add .020 to each of the 4 holes and still run in the 1350 class. lots and lots 1363 busas running in the 1350 class. In this case 650cc +. 020. Again... they don?t care about claimed displacement!!! The tech guys will measure the hole and then the stroke and after they play with a calculator for a little while they will tell ya if you?re legal or not. DO THE MATH you may be legal for the 650 class
kent

[yamagamma]

So why is it when I enquired about putting a set of new 2006 bandit cylinders that measure 656cc on my 750 gixxer so I could run in 650 class TECH said NO!

If you do  the math, sure a 650 engine bored 0.020 is more than 656cc, so on purely a measured basis, the 656cc engine would be legal. The problem is ....it's purpose built to curcumvent the rule which clearly reads... TO PERMIT RECONDITIONING OF A WORN BLOCK. In this case the 656 engine has NEW cylinders and hasn't been re-bored, so it's NOT LEGAL.

So..... do we have a lot of ILLEGAL BUSA'S running around? or am I running under the wrong flag?

[JackD]

You measure 656 cc ,are above 650, you are are therefore in the 750 class.
The overbore allowance was intended for a small 2 smoke that didn't want to buy or sleeve a new jug.
On a 4 stroke, make the displacement smaller and that is a lot cheaper than a lot of head work on a larger setup.

[Sumner]

So why is it when I enquired about putting a set of new 2006 bandit cylinders that measure 656cc on my 750 gixxer so I could run in 650 class TECH said NO!

I've completely lost track on this thread, but if you are running "production" I could see where they could have said "no" because you were changing the cylinders on the motor to ones that didn't come on the motor, not just "adjusting" the bore size of the stock cylinders.

Just a thought,

Sum

[bak189]

Many, many, moons ago we had a 1300 c.c. class.....I ran a big bore KZ
Kawa that I knew was 1260c.c......
the oil-pump method showed it to be over 1300c.c.....had to tear the motor down to show I was right (1260c.c.) so how accurate is the oil-pump.....is it better these days? Or do they now know what they are doing???  Lets hope so !!!

[yamagamma]

Sum, my request was for a modified bike.

My question now is: If I got my request turned down by Tech to build a 656cc bike to run in 650 based on "it doesn't fall within the spirit of the overbore rule, which was designed to allow a re-bore of old cylinders"  WTF are all these 1363 Busa's doing running in the 1350 class?  Looks like a double standard to me.

The production guys (me included) didn't want the overbore rule. It was some guy running a flathead Harley who couldn't find new cylinders.