BMST question on displacement for production class

[edinlr]

Okay we are a bit slow here in Arkansas, so bear with me on the questions.  First off, do any of the links work for current BMST rules?  I keep getting the generic "use this site garbage"  or else I get some FIM stuff.  Is there just an easy set of rules like SCTA uses?

Here's my question, I run a 673 cc bike in SCTA production.  The bike runs in the 750 class because it is too large to run in 650.  As I understand the SCTA rules, I can bore or stroke the bike to 750 cc.  I believe I have read that this is not allowed in BMST for production.  Is any overbore allowed, even .5 mm?  I guess stroking it to 750 cc is out of the question too?  Sorry to ask what is probably such a dumb, basic question, but it is a 1600 mile drive each way, and I would really hate to explain to the Mrs. why I was so stupid to miss this detail.  Am I safe in assuming modified can bore or stroke to the class limit?

If the BMST rules are online, can someone post a link for this dummy to follow.  Thanks

[maj]

http://bonnevillespeedtrials.com/wp-content/uploads/2014/02/AMA-LSGC-SR-2014-v11-1.pdf

Congrats on the new bike too

[donpearsall]

Happy New Year Ed.
Under AMA production rules, you can't bore or stroke an engine, even if the displacement is still under the class limit. The rule says: "OEM displacement specifications must remain stock."

I was burned by that rule when I ran my Kawasaki 750 Turbo in the Production class. The bike has a published displacement of 738cc but the inspector said it measured over the stock bore. Since it had stock pistons the overbore measurement had to be due to mere wear at the top of the cylinder (common for high mileage turbo bikes) or micrometer error.

Don

[edinlr]

I thought that is how the rules read but wanted to be sure.  I was thinking about trying to run Speed Week and BMST next year and wanted to make sure I had a bike that would comply both places.  Hopefully run the new Kawasaki too if it hasn't scared me to death by then.

[JimL]

I would guess that NO manufacturer throws away newly machined blocks/cylinders that are slightly over listed bore size.  That is why there are select-fit pistons on the engine build line.  This has been going on, for some manufacturers, since the 70s that I know of.  The manufacturing process has several needs, including round and straight bores, bore finish smoothness, final piston fit, and REDUCTION OF WASTE!  For some, match numbers were on the blocks to tell what select piston to use during initial build.  That is probably computerized for decades, now, to eliminate that extra "marking process' step.

I have seen up to 0.10 mm over advertised bore (in one or more cylinder) on a brand new production car engine.  If you are measuring a bore on an engine that has never been apart, and it comes out some amount less than 0.25mm over, it is most likely absolutely stock from the production line.

This situation is old news.  Here is an interesting peek at how complicated this issue really is (try to ignore all the attitude stuff...its just the facts that are interesting):

http://www.pistonslap.com/

Note:   These folks writing this opinion don't understand that piston slap occurs at the ends of the stroke when the piston is basically stopped, and the crown/lands are not expanded under high thermal load, such as towing a trailer up a hill.  Sometimes, you have to make a choice for piston slap, for piston survival.....like...maybe...a bike set up for turbo conversion?  (hmmm....  How much over were those bores, Don?)  This slap issue is always a fight between the engineering side and the Customer Satisfaction side, and seldom strictly a "cost cutting" measure, regardless what some folks might claim.

This goes a lot farther than bores and bearings.  Some pushrod engines had select-fit lifters on the production line.  If you took apart a brand new engine, for whatever reason, you had to make sure to get the lifters back in the correct hole.  If you did it wrong you might seize a lifter, or (on hemi engines) give the customer a persistent valve noise.  I know a young mechanic who got bit on that one, until the Service Manager straightened him out.  It took a lot of fiddling to get that sorted out.

Complicated.

[**RP**]

Ed....It would be good to wait until the 2015 rules are published before making any engine building plans in case there are rule revisions.  Right now the 2014 AMA / BMST rules do not allow any overbore in Production.....Modified can go to the class limit.

Don....What was the bore wear amount measured that was not legal in AMA Production?   I though they allowed a little for caliper error.

[donpearsall]

There is no allowance for wear. If there is it would only be at the inspector's discretion to ignore it as it is not written into the rules. In my case the published spec for the bore is 66 mm. The inspector said it measured over but did not tell me how much. When I got it home I used my micrometers to measure and it was slightly over at the top and right on at the bottom of the cylinders. The inspectors only measure at the top of the cylinders. I know the inspectors can only go by the rules, but it sure seemed to me that they could allow a little for wear.

Don

[makr]

Don, the bores would have to be 66.5 mm to be over the class limit size. That is .5mm or .02 inch larger than stock. With a .02 over bore, stock pistons would destroy themselves in a hurry, if you could get it started in the first place. As a machinist I cringed watching my engine being measured, but thought the way the calipers were being used there was no way the engine would measure big.