Author Topic: Motorcycle Displacement limit/overbore for Production Vintage engines.  (Read 736 times)

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Offline metermatch

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I keep reading this rule over and over and I can't figure out it's meaning for Vintage motorcycle engines regarding allowed overbore and effect on displacement class:  example:   "In all cases, the resulting displacement shall be exceeded to qualify for the next higher class."???  seems contradictory to me.

7.D   CLASSIFICATION of DISPLACEMENTS, FRAMES, ENGINES, and ENGINE TYPES:
7.D.3   Engine Displacement Classes:
Displacement shall be greater than the maximum allowable for the next lower class.  To permit minor reconditioning of worn cylinder blocks in classes other than Production, it is permitted to increase cylinder bore diameter .020 in. (.508 mm) beyond that which provides maximum displacement for the class only if the OEM bore diameter is within .020 in. (.508 mm) of the maximum class displacement. In all cases, the resulting displacement shall be exceeded to qualify for the next higher class.  The .020 in. (.508 mm) will be discounted for record certification and will be noted on the certification card and in the logbook.
Vintage engines are allowed +.050 in. overbore, see section 7.J.10.


My example:  I have a real, correct 1954 BSA 500cc Gold Star I would like to race in the vintage pushrod class.  Stock Bore/stroke is 85mm x 88mm for 499cc.  The rule above says I can overbore .050" (1.27mm).  the resulting displacement will be 86.27mm x 88mm for 514ccc.  Does the rule above mean that the .050 (1.27mm) overbore will be discounted/forgiven for record setting purposes, and bike would be treated as 499cc bike, to run in the  (500cc class)?

Another question about the same bike.  Again, it is a correct 1954 500cc Gold Star.  But the bike was also available with a 350cc engine.  Same frame, but had a different number designation.  Same engine,  just the bore (barrel) was different.  Same outside appearance.  Could I put a 350cc barrel on it and run it in the 350cc class?

Couldn't find this exact question in the search.  Maybe the answer is obvious to everyone else.


Thanks,  Jeff

Offline Stainless1

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Jeff, if you wanted to run your bike in the 650 class with stock stroke, your bore would have to exceed the allowed .050 allowance for vintage.  As is your vintage bike can be overbored .050 and stay legal in the 500 cc displacement class. 

I doubt you will be able to run in production class with the same bike in 2 displacements... at the same meet... but stranger things have happened.

Just my opinion... not necessarily that of the staff and management of the SCTA
 :cheers:
Stainless
Red Hat 228.039, 2001, 65ci, MSA Bockscar Lakester with a little N20 
MSA Bockscar Lakester #1000 my fastest mile 245 and change, 84 ci turbobusa motor... but Corey's 233 MPH H/BFL record is still 3MPH faster than mine.

Offline Stan Back

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I dunnot know ? but cars have run in differing classes (say gas to fuel) at the same meet.  I believe it takes a re-entry under another number ? usually adding a one with tape before or after the original number.
Member of the San Berdoo Roadsters -- "California's Most-Exclusive Roadster Club".
Celebrating 67th anniversary of racing on the salt.

Offline Stainless1

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Yes Stan, but he is talking about running a bike in a production class in 500cc and then running the same production bike in 350cc.  If he ran in Modified, I see no issue, but like I said, I guess we will see what the Guru's say is legal when it happens. 
I suggest bringing lots of literature showing bike.  :cheers:
Stainless
Red Hat 228.039, 2001, 65ci, MSA Bockscar Lakester with a little N20 
MSA Bockscar Lakester #1000 my fastest mile 245 and change, 84 ci turbobusa motor... but Corey's 233 MPH H/BFL record is still 3MPH faster than mine.

Offline panic

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Not a GS expert, but I doubt the 350 has the same head casting, cams, valves, carburetor. If you located those parts the flywheels would need to be re-balanced for the lighter piston.

Offline metermatch

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Thanks for the replies.

I wasn't really thinking of making the change from 500cc to 350cc at the same meet.  I am far too slow of a mechanic.  I had just noticed that for the Production (pushrod vintage) class, the 350cc records seemed a little "soft" compared to the 500cc records.  I was wondering if I had to just go and buy another original 350cc Gold Star to run in the 350cc class, or could I just put the vintage correct 350cc parts on my 500cc motor and run the 350cc class, because my bike was sold in both 350cc and 500cc versions with very few changes except for the displacement.

Jeff
« Last Edit: July 29, 2019, 09:22:38 PM by metermatch »

Offline panic

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Sounds like a plan, but as I said, your crank will be out of balance with the much lighter piston, not a 1-day job.
I'll bet there has been extensive use of the many, many 500 (and even M20/21) cams in a 350 in various combinations. I have a list of grinds somewhere.
There are also a wide range of internal ratios available for that box, I think you'll find the close 3-4 shift valuable for top speed. Shameless self-promotion: http://victorylibrary.com/GEARZ.htm
The 500 may have bigger brakes (which just makes a 350 engine swap heavier, no advantage to LSR).