A-VG 500 displacement question

[dandoc]

Hello, I am currently building a bike to bring to the bub speed trials in 2011. I have a question about the displacement of my motor I am building. I am planing on running in the A-VG 500 class. My motor and head are from 1951 and the stock crank is is pretty weak in these motors in stock form. The bore stroke combo is 66x72.6 for a displacement of 496.8 cc stock. With the max overbore permited at .050 that would make it 519.7 max. I was wondering if i could change cranks for a stronger version at 74mm stroke and keep the STOCK bore of 66mm for a displacement of 506.4. would this be within the rules for the vintage class?
Thanks

[panic]

Is this a late BSA A7?
The "+.050" limit was (AFAIK) intended to allow re-use of nearly unobtainable cylinders, rather than be a cushion against the displacement limit.
However, the 1946-50 (*-ZA7 11192) cylinder is 62mm. If this were bored to 65.5mm with the A50 crank you'd have 499cc.
That's a huge bore (+.138") for that casting, and I'm not sure it's safe even with a sleeve.
Would they allow the 1951-* cylinder to be sleeved down to 65.5 (losing the .050" advantage)? Have to ask - in writing. Since all the "legal" parts are pre-1956, it might get approved.
The rods are the same length, I'm not sure where the 4.7mm deck height change is between early and late.

[dandoc]

Wow. So much for candor! At any rate yes the parts would be all pre 56. Would like to use the a50/65 crank  since its large journal and beefy rods can be fitted. The bore would be stock for the late a7 just with an additional 1.4 mm of stroke. The displacement would be well under the .050 limit in stock form.
I think I've read the rule concerning this about one hundred times and it's a bit grey. I also sent an email over to the tech dept at bub a few weeks ago and haven't got a reply yet

[bak189]

Regarding the BUB's Tech.......contact Drew.....he is on the forum.....he would certainly know if your mods. would be legal

[panic]

IMHO it's not likely to be approved, since the qualifying language is explicit. The .050" isn't added to the displacement, except as subtracted from the actual bore size.
I made an informal inquiry about sleeving an existing motor down to make a lower vintage class (no .050 question at all, all parts from the same pre-56 engine) - which was denied without explanation. The rules have since changed, making the engine too big for class anyway.

Of course, the big budget build is an A10 barrel with a unit crank de-stroked to 64.5mm. Raises the safe RPM a bit, slows up piston acceleration, reduces the rod angle and opens up the bore outside the valve curtain, but it's a lot of work.

[Nortonist 592]

The "+.050" limit was (AFAIK) intended to allow re-use of nearly unobtainable cylinders, rather than be a cushion against the displacement limit.

I'm not certain of BUB rules but I think Panic summed it up exactly with that sentence.

[1gsxrclosertodivorce]

I also have a technical question I would like to get answered for a bike I am building for BUB 2011. In researching where to ask I came across this old forum post.


"I forgot to include this info that Drew had passed along:
Please note, if you or Toots have any further technical questions, the 2009 contact list is as follows.
1. ANTHONY GRILLI: AMAsports National Technical Director - agrilli@ama-cycle.org
2. KEN SAILLANT: AMAsports Flattrack Director - ksaillant@ama-cycle.org
3. DELVENE MANNING: BUB Speed Trials - delvenem@bubracing.com"

I haven't received a reply yet from Delvene, so I am wondering who is dealing with such enquiries for 2011?

If someone can point me in the right direction it would be much appreciated.

Thanks for your time and Happy New Year
 

[1gsxrclosertodivorce]

Checking with the AMA. Thanks.

[goingfaster5]

Hey, that's my wifes record.  Break a leg, I mean good luck...

Having thought about this a bit this and reading the book a few to many times.  The .050 limit is definelty there to allow overbore of hard to find cylinders.  Following the rules, I don't particulary see any problem with increasing bore and de-stroking.  Using pre-'56 parts of course.  The rules say nothing about what I can do to the crank, so why not?  I have envisioned running '55 R60 barrels on the R50 and destroking.  Everything would be "OEM" and pre '56? I suppose that isn't really in the spririt of the rules eh?

We built to both SCTA and BUB rules so we are sleeved back to dead stock.

[Stan Back]

Don't know from bikes . . .
but is destroking OEM?

Stan

[Uncle Jimbo]

I hope this helps clarify the question Dan. The new 2011 rule changes are now posted, but as you will see engine displacement rules do not take effect till 2012 - so you can refer to last years rule book for your detail. At least I would think so. Best bet would maybe check with the AMA tech Steward for BUB or try Delvene or AMA Ken.

12. A. PRODUCTION (P)
Use the same engines (gasoline only)  originally installed in the
specific motorcycle frame at the time of production and  ensure it
meets the definition set in the „P‟ frame class (See  Chapter 4).
Original equipment  (OEM)  shall include cylinders, cases
(crankcases), heads, and carburation or throttle body (stock venture
size), kick-starter or electric starter. GASOLINE ONLY. Fuel not
permitted in this class. See Chapter 2, paragraph 2.F.
P engine class shall not run in A, APS or S frame classes

Effective 2012: Displacement determines the class. OEM
displacement specifications must remain stock.

12. C. PRODUCTION, VINTAGE (PV)*
Same as 12.A but with a production date prior to 1956. Allowable
overbore in this class is + .050- inches over OEM standard bore to
remain in displacement class.