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Allowable P1 CB350 Frame Modifications

Bike preparation, class eligibility, technical questions.

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Allowable P1 CB350 Frame Modifications

Postby pacomotorstuff » Sun Feb 17, 2019 8:15 am

Popped an email last week to Tech regarding the subject line, "allowable P1 CB350 frame modifications", but sometimes my emails don't get through so... here are my questions:
1) Are additional frame tubes for bracing an acceptable modification to Period 1 CB350’s? Any limits?
2) Can the original stamped, C-section top frame tubes be replaced with tubular steel? I know most guys just box them but...? I have one frame which is particularly gruesome in that area, so looking for some direction from you guys for my repairs.
3) If a CB350 motor was put in another frame that was Period 1 correct (a copy or the real thing), how would the Tech Committee view such a bike as eligible for P1 - or should this be done on a case by case basis?
The reason I'm asking is, it seems I'm getting dragged into yet another CB350 project and don't want to waste my time and $$$ on "inappropriate" modifications.
Also, if any of the other P1 CB350 guys want to share what they've done to their frames, I'd appreciate it. Maybe we could start a thread or something on the topic, as it would be a great help to the newbies.
It's always better to ask permission now than get turned away at Tech Inspection in June, right?
Let me know so I / we can figure out our next steps.
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Re: Allowable P1 CB350 Frame Modifications

Postby The Flying Fossil » Tue Feb 19, 2019 4:27 pm

I welded all the seams on the pressed steel backbone, deluged the frame and drilled a sixteenth inch hole in the lowest point on the lower frame tubes when on the stand to stop water accumulation and hence tube rotting - that's it.

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Re: Allowable P1 CB350 Frame Modifications

Postby Michael Vinten » Mon Feb 25, 2019 4:19 pm

The Tech Committee has discussed P1 frame modifications. A clarification was made in early 2018 to allow extra bracing typical of the period in P1 as is permitted in Pre 65. Tubing used must be round.

Replacing the pressed steel section with round tubing would be acceptable.

As long as the CB350 engine is stock it can be mounted in any P1 legal frame. This is already in the rules.
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Re: Allowable P1 CB350 Frame Modifications

Postby pacomotorstuff » Mon Feb 25, 2019 5:46 pm

Mike (and of course all of the Tech Committee Members).
A BIG THANK YOU for clarification on my questions - I really appreciate it.
I "sort of knew" about the P1 frame rule, back when I was shoehorning a '67 Yamaha R1 350cc motor into a T20 Suzuki frame, but sometimes I need a reminder.
Keep up the great work!
See you at the TT if not before.
Best regards,
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Re: Allowable P1 CB350 Frame Modifications

Postby racerchris46 » Mon Mar 18, 2019 8:59 pm

Hi Pat

my 350 has 1 inch cro-mo side braces from head stock to swingarm area with right side removable for engine swaps. I did this in 1999 or 2000 when I lived in USA and had P2 motor with big (almost double stock) horsepower. then it made a difference. Now with a P1 motor while I think it looks cool I dont think its needed. I would weld the spine and the outer swingarm pivot plates into place and maybe an angled brace just above swingarm pivots and call it good. The hp available in a stock engine doesnt push the frame hard enough to twist it. I cant prove it as I dont have another frame to run!

I do also use an SL350 swingarm which is one inch longer and cb750 triples which have 25mm more offset. longer wheelbase and more weight on front wheel as measured by gmd computrack 20 years ago when that was still a thing.

If you need pics drop me email.

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