Specifics of classic production class limitations

[gschuld]

So in classic production, it seems clear that you need to have factory:

Frame
Engine including original displacement
Tank
Side covers
Wheels
Seat
Fenders
Exhaust
Swingarm
Drive type
Etc.

That make sense.  But how about less obvious examples such as:

1-Must the original exhaust retain 100 original unmodified(gutted or partially) innards in the muffler section(s)?

2-is the addition of an oil cooler OK?

3-is any form of minor non speed or aero benefitting chassis bracing(minor gussets,etc) OK?

4-the ride height must remain 100% stock?

5-vintage (but higher quality) aftermarket rear shocks of the same length OK?

6-engine may have non OEM (yet factory bore) pistons, cam, cylinder head work correct?

7-would the engine specifically need to be from the same year of the bike of the same model?  As in for the sohc CB750 which the model ran from 1969 through 1978(with ALL years being rules compliant), could someone install a 1978 sohc CB750 engine in a 1970 sohc CB750 chassis with ALL the rest on the bike being 1970 vintage?  Same basic engine architecture, same bore/stroke, identical in outward appearance, same original power rating.

Thanks in advance.

George

[Stainless1]

George
Tech@bonnevillespeedtrials.com
If you read the rules online for BMST and have specific questions you can address them to Drew Gatewood.  He is a member here and hopefully will chime in

Be aware some rules change so read the new rules to be sure you are OK before going racing.
With any luck someone familiar with the BMST rules can help direct you.... I have not been to a BMST race for about 5 years and not up on alll the rule changes in all the classes these days....
Guys... Jump in here and help George out 

[Old Scrambler]

1-Must the original exhaust retain 100 original unmodified(gutted or partially) innards in the muffler section(s)?....Yes, some have had minor alterations that may have slipped past tech.

2-is the addition of an oil cooler OK?...........No

3-is any form of minor non speed or aero benefitting chassis bracing(minor gussets,etc) OK? ............No

4-the ride height must remain 100% stock?.............yes

5-vintage (but higher quality) aftermarket rear shocks of the same length OK? ...............No

6-engine may have non OEM (yet factory bore) pistons, cam, cylinder head work correct?..........only if not detected..........

7-would the engine specifically need to be from the same year of the bike of the same model?..........Probably.......... 

As in for the sohc CB750 which the model ran from 1969 through 1978(with ALL years being rules compliant), could someone install a 1978 sohc CB750 engine in a 1970 sohc CB750 chassis with ALL the rest on the bike being 1970 vintage?............You could try but the sprockets don;t align

[gschuld]

Thanks Dennis. 👍

So what is the final word on displacement overbore.  The CB750 is 736cc from the factory.  There is an overbore allowance, but the full overbore allowance puts the 736cc to a little over 750cc.

So any replacement stock type pistons would have to be small enough of an overbore to remain under 750cc limit no matter what?

This is the same for Production, Modified, and Altered classes?

George

[donpearsall]

The way the rules read is that you cannot go over 750cc as that would put you out of the displacement class as well as out of the production class. And you cannot go over .050" if you bore the engine even if that still puts you under 750cc. That is the same rule for all the classic engine classes.

Don

[Koncretekid]

I read the BMST rule about Production Engine Class, " OEM displacement specifications must remain stock," as meaning no overbore allowed in Production Engine class, but maybe someone else knows otherwise.  

But the Classic Production rule reads as Don Pearsall has said, so up to .050" but not over 750cc.

Tom

[gschuld]

I have admit, those are not the answers I was expecting.

Classic Production:

“Allowable overbore in this class is .050-inches over standard bore to remain in displacement class”.

This is the same wording as in all the other classic and vintage classes.

That reads to me as if I have a 736cc OEM displacement (CB750) I am allowed to freshen up the 40-49 year old engine with a reasonable limit of .050- inches without being bumped out of the class it fit into when stock.  It’s crazy to think a motorcycle engine from the disco era would still be running on the original factory pistons and bore AND still be healthy enough to run WOT for 30+ seconds at altitude in the heat and salt.  And 1mm over pistons(less that the .050-inches allowed) is the standard next size up.  It puts the displacement at 760cc.  So unless you take a 40-49yr old CB750 that has never been apart out to the salt flats, or a freshen it up with just a back alley re-ring job on the old worn OEM pistons and cylinders(who would do that?😏), it would not be eligible for Classic Production 750?

That does not make sense to me.🤔.

Anyway, I have sent a detailed set of questions to Drew Gatewood, and I’d be happy to share what I find out as the official word on the subject when he gets back to me.

George

[Koncretekid]

Consider yourself lucky - - My BSA B50 as well as my TR25/B25 are both at the limit as OEM, so no overbore allowed, no matter what class I want to run.

If you find out otherwise, please let us know.

[gschuld]

I was hoping I could use stronger than stock replacement pistons.  30 seconds of WOT is asking a lot from a 45 year old cast piston.  They all start at 62mm bore(760cc)

Strange since many of the bikes were intentionally made to be just under the 500, 750, etc displacement for various reasons. 🤔

George

[tortoise]

Consider yourself lucky - - My BSA B50 as well as my TR25/B25 are both at the limit as OEM, so no overbore allowed, no matter what class I want to run.

If you find out otherwise, please let us know.

Is there a rule against destroking?

[Doc B.]

Older Honda overbores are in 0.25mm (~ 0.010") increments, so there are three other choices before you get to 1.0mm overbore. I run a 0.5mm (second) overbore on my CB77.

https://www.dimecitycycles.com/piston-kit-honda-cb750-1969-76-0-25mm-size.html

For Honda piston part numbers look here and scroll down to 61 Millimeter

https://4-stroke.net/library/honda-piston-size-chart.html

[Chris V]

as gschuld said I agree as in BMST rule 12,n and 12,o .050 overbore of stock displacement, so I read it to mean you can be a little over in the classic class if your stock displacement is close to or at class limit size   

[Koncretekid]

Consider yourself lucky - - My BSA B50 as well as my TR25/B25 are both at the limit as OEM, so no overbore allowed, no matter what class I want to run.

If you find out otherwise, please let us know.

Is there a rule against destroking?

No, except probably in Production and maybe Classic Production. I ran my de-stroked and bored B50 in APG and APS-PG classes, which set the records which were measured and certified by the tech inspection crew.  As long as it remains within the class limit.

But I have two other B50's which might set records in M-CG which are stock stroke but bored .020", so 507 cc.

Tom

[mtkawboy]

Couldnt you sleeve it back to std bore then use std bore new pistons ? Just asking, no dog in the race

[gschuld]

Couldnt you sleeve it back to std bore then use std bore new pistons ? Just asking, no dog in the race

Yes, for that matter so can go .5mm over OEM type replacement pistons and end up with 749.xx cc.  The only problem is only OEM type cast pistons are available, and likely lower quality than genuine Honda pistons.  I’d much prefer a higher quality piston set that is better equipped to deal with the abuse of 30 seconds of WOT on the salt.  All decent aftermarket pistons are to my knowledge 62mm or larger(61 is stock) and would put me at 760cc

George