Author Topic: Production motorcycle overbore?  (Read 6563 times)

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Offline TurboCat

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Production motorcycle overbore?
« on: May 19, 2013, 06:18:58 PM »
Another member has raised questions about my project and I'm looking for confirmation. Plans are to run a '07 Sportster 883 in P/PP 1000 class using an overbore and custom slugs for a displacement of 997cc. Is this still allowed in Production? I know an almost identical 883 set a record at Bonneville using XR1000 pistons in 2001.

Offline Stainless1

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Re: Production motorcycle overbore?
« Reply #1 on: May 19, 2013, 06:23:31 PM »
yep, as long as you stay under 1000cc and over 750cc you are good to go.  You can't have a clean up overbore that goes over the class limit in production. 
Making the size to class is good.
Stainless
Red Hat 228.039, 2001, 65ci, Bockscar Lakester #1000 with a little N2O

Offline TurboCat

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Re: Production motorcycle overbore?
« Reply #2 on: May 19, 2013, 07:17:02 PM »
What I thought...thanks for the verify.  :cheers:

Offline wfojohn

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Re: Production motorcycle overbore?
« Reply #3 on: May 21, 2013, 08:43:00 AM »
Hey Turbocat,

Sorry for causing you confusion. I am glad to be wrong because the way I thought the new changes were was its not allowed and that would suck. Excuse me while I hit the fridge for some milk to go with my Humble Pie   :-D

Offline rouse

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Re: Production motorcycle overbore?
« Reply #4 on: May 21, 2013, 09:06:55 AM »
yep, as long as you stay under 1000cc and over 750cc you are good to go.  You can't have a clean up overbore that goes over the class limit in production. 
Making the size to class is good.

I second that.

rouse
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Offline edinlr

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Re: Production motorcycle overbore?
« Reply #5 on: September 08, 2013, 10:26:08 PM »
As a caution, I read where one racer at BUB a few weeks ago was measured as too big with a 738cc bike in the 750 class.  He must have had a lot of wear in that cylinder.  I guess if I was ordering custom pistons I might go a little smaller as they seem to have measured his bore, not the piston size.  It may not be that big a deal, but if you are doing custom, why risk it.

While we are on this subject, how are measurements done at the ECTA?  If you have to tear down is it after a run, or at the end of the meet?  My pushrod bike isn't bad, but with a desmo it can be an all day affair.
« Last Edit: September 08, 2013, 10:28:38 PM by edinlr »
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Offline Stainless1

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Re: Production motorcycle overbore?
« Reply #6 on: September 08, 2013, 11:15:59 PM »





Size is size... if you don't know what size your motor is you should not be racing.
bore x bore x stroke x .7854 x number of holes.
Using bike motors in car classes can cause a few seconds of WTF but the 748cc motor is 45.7 inches, still within the less than 46 inches for J class.










Stainless
Red Hat 228.039, 2001, 65ci, Bockscar Lakester #1000 with a little N2O

Offline RansomT

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Re: Production motorcycle overbore?
« Reply #7 on: September 09, 2013, 06:43:52 AM »

While we are on this subject, how are measurements done at the ECTA?  If you have to tear down is it after a run, or at the end of the meet?  My pushrod bike isn't bad, but with a desmo it can be an all day affair.

No tear down at ECTA.  It was considered many years ago, but not much time in a 2 day event (it's actually a 1 3/4 day event).

Offline runt13

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Re: Production motorcycle overbore?
« Reply #8 on: September 09, 2013, 09:36:38 AM »
however you can be protested, then the tear down will take place. and a complete inspection of the vehicle.

who would have guessed it would take so long to go so fast for such a short time?

2012 Wilmington mile
April meet,
p/pp-1350/4 record, 132.601mph

September meet,
p/pp-1350/4 record 139.915mph [bump]
p/pp-1650/4 record 142.364 mph
p/pp-2000/4 record 140.009 mph
p/pp-3000/4 record 144.511 mph

2013 Wilmington mile
July meet,
m/pg-1650/4 record 137.278
m/pg-2000/4 record 136.695

Sept meet,
p/pp-1350/4 record 141.1986 [bump]

2014  Wilmington mile
May meet
p/pp-1340/4 record 142.5855 [bump]

Offline ol38y

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Re: Production motorcycle overbore?
« Reply #9 on: September 09, 2013, 10:46:13 AM »
As a caution, I read where one racer at BUB a few weeks ago was measured as too big with a 738cc bike in the 750 class.  He must have had a lot of wear in that cylinder.  I guess if I was ordering custom pistons I might go a little smaller as they seem to have measured his bore, not the piston size.  It may not be that big a deal, but if you are doing custom, why risk it.

While we are on this subject, how are measurements done at the ECTA?  If you have to tear down is it after a run, or at the end of the meet?  My pushrod bike isn't bad, but with a desmo it can be an all day affair.

I don't know if I would read too much into that. When my engine was certified at Bub they measured it 4 times and never did get the correct measurement. Cept with mine they measured it smaller than it actually was.

I'll also add the only way I know is to measure the bore.  Engine size is determined by bore X stroke.   
Larry Cason
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2010 BUB 1350 M-PG record
2012 Speedweek  1350 A-PG record 169.975
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Offline jacksoni

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Re: Production motorcycle overbore?
« Reply #10 on: September 09, 2013, 03:57:30 PM »
I'm going to take a swing from a slightly different perspective: Nobody cares what size, type, origin etc your PISTONS are (except your engine builder -might be you) so that they fit in the perfectly honed BORE you have prepared, the size of which is used, as Stainless and others have pointed out, to determine the displacement (along with stroke and some math). Difficulty measuring same is another issue.  :roll:
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Offline edinlr

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Re: Production motorcycle overbore?
« Reply #11 on: September 09, 2013, 11:41:13 PM »
I agree with Jacksoni's observation above regarding the bore size versus the piston size.  I am just suggesting that someone planning to build a motor visit with their engine builder to make sure their calculations are spot on.  You would hate to spend all that money and have a warm motor that measures a cc too large. 
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