Topic: CB750 front end swap onto CB350 questions

[ceruzziracing]

I've searched and am only getting bits and pieces of answers. I'm wanting to swap a 74 750 front onto my 72 350K4. I know that they both use the same tapered steering bearings, albeit, there will be a portion of the top bearing protruding. Measuring the necks shows the 350's at 7 1/8" long and the 750 at 7 1/2", with the difference in length due to a shallower top bearing mount on the 350 (thus the protrusion).

My questions are: 1.) Do I need to replace the 750's stem with the 350's or can I run it safely with the longer stem? 2.) If I have to run the shorter length stem would it be easier/better to press the 350's into the 750 tree or should I shorten the 750 stem the approriate amount and reweld it?
Both bikes are completely apart (all the 750 components already powdercoated, really would hate to have to recoat the 750 tree if I have to screw with it). I decided to go with a GL front on my pseudo CR750 build while at the same time I stupidly am building a pseudo CR350 

[kghost]

The triple trees are not the same. Width wise...

So yes you'll have to use the stem from the 750 no matter what you do.

The fork tubes on a 350 front end are smaller in diameter...once again...750 lower has to be used.

bunch of other differences too...just don't have my tape measure handy

[crazypj]

You only need to fit a spacer 3~4mm thick to take up the difference. 750 forks are much longer than 350 ones, going chopper?
PJ

[ceruzziracing]

KG, like I said, I'm doing a complete swap. I'm aware of the tubes being bigger, that's the reason for the swap  .

Crazy, not a chopper, more along the lines of a CR style 350 for the track. Most of the upgrades I've seen to put the 350 on the track have to do with going to bigger fork tubes and better front brakes. She probably won't be AHRMA legal with the conversion to front disc brake, but she'll probably just see some time on trackdays. If I decide to compete, I can always figure a drum set-up to swap over. I woke up in the middle of the night and the spacer thing popped into my head, funny how the simple solutions elude people sometimes  . Thanks for the input guys....anyone else done this swap? May even put dual front discs on her...

[crazypj]

I was going to swap Yamaha XS650 forks onto 360 but they are at least 3" longer, 2" in the slider.
 I don't remember length of CB750 forks but they are longer than the CB500/550 ones by a least an inch.
 PJ

[lordmoonpie]

My questions are: 1.) Do I need to replace the 750's stem with the 350's or can I run it safely with the longer stem? 2.) If I have to run the shorter length stem would it be easier/better to press the 350's into the 750 tree or should I shorten the 750 stem the approriate amount and reweld it?
Both bikes are completely apart (all the 750 components already powdercoated, really would hate to have to recoat the 750 tree if I have to screw with it). I decided to go with a GL front on my pseudo CR750 build while at the same time I stupidly am building a pseudo CR350 

Did the same for my CB350 K4 racer - just use a spacer on the stem and maybe an open top nut RSC style! The benefit of this is that you get better clearance width wise so you can then fit a classic AP lockheed brake and large rotors - 280mm is easily possible. You need the etxra width of the 750 front end to do this or the caliper simply won't clear the rims and the rotor won't clear the fork legs. This is the modification that everyone does in England for classic racing. The only difficult bit is machining a bracket to carry the lockheed caliper - or you can just buy one from Davies Motorsport in Nottingham, England to fit straight on! Otherwise, the 750 front end just drops in...

[JLeather]

I'd suggest shortening the CB750 stem.  It's somewhere around 1/2"-3/4" too long.  My recommendation it cut out the right amount, make a slug, drill some holes to be able to weld through, and weld it back together.  I was gonna do it to my 350 but I got a great deal on billet CB350 trees and went with those instead.

[kghost]

Ya know the 350F is kinda portly compared to the twin.

Have a look at what Ohiocaferacer is doing

[ceruzziracing]

Lord, that's the stuff! Would you have a pic of your setup by chance? I'll have to post up some pics of my 750 frame soon, the powdercoating came out beautifully-blood red metallic. Going to PC the 350 frame the same color and go for a matching set of CR lookalikes! Whilst tearing the 350 down the other night, I found a crack all the way through the smaller downtube where the passenger peg bracket is( was hidden at base of bracket). Glad I decided to tear her down...

[lordmoonpie]

Lord, that's the stuff! Would you have a pic of your setup by chance?

As luck would have it - no pictures of the CB350 at all 
However, I run a 750 front end on the black bomber too. Originally it had a CB550 disc which is kind of an interim size - lots of the race guys use a disc off a Suzuki which is cross drilled but I don't know which model bike. I then changed this again to custom made magnesium fork lowers from Dick Hunt racing in the UK (meant for a Norton but fit the 35mm Honda stanchions just perfect) and that allows me to run a 280mm cast iron disc which can put me over the bars with one pull! Here's a pic. of the bomber with the 550 disc fitted before I changed the fork lowers...

[dusterdude]

if you want to trim the stem,have a machinist thread the stem lower and then cut the top off enought to shorten it.

[ceruzziracing]

Lord, she's a beauty! Nicely done.
Duster, had thought about that one, got a few days off this week, I'll assemble with the bearings and decide its fate.

[dusterdude]

i have to do the same thing when i put my gw front end on my drag frame.the frame is setup for a harley neck and the gw neck is a tad too long.

[ceruzziracing]

Well, got buggered by the flu this weekend  but bikeweek is here in Daytona and I've got most of the week of to play with these things (as well as ride  ). Got a #$%*eload of parts in as well but didn't even have the energy to open the damn boxes. Couple of nights at work and then it's playtime!

[mystic_1]

Years ago I helped swap a 750 front end onto the wife's 350.  We used the stock 350 trees, everything else was from the 750, except for perhaps the fender, don't remember which one we used and we no longer own the bike.  A machinist friend of mine bored out the trees to accept the 750 tubes, and clearanced the lower right fork bottom to clear the speedo drive.  She rode it like that for several years with no issues.

mystic_1