Ok, this has me worried.

[pidjones]

A PO of this '79 CB750F swapped forks to leading axle forks. What from I'm not certain, but suspect an '82 CB750C or a CB900C. Either way, they are 37mm fork tubes. Yeah, in 35mm trees. Did they bore the trees? No, they spread them. Seem to have pretty good contact all around the tubes, but it worries me still. Do I need to look for other forks, trees, or just go with what I have? Don't care about pretty as it is a rat rod project. Just don't want to kill myself or some future rider. The tubes don't look bad in the areas that see the oil seals. I already have one apart and seals on order. Kind of rusty at the tops around the clamps, but not deep and I don't care how they look.

Anyone with experience in spreading the trees?

 

[J-Rod10]

I'd swap em. Trees at least.

 

[irk miller]

I'd swap the trees. You can shim them when the trees are too big, but I'd reckon they're at the edge of their tensile strength spreading out.

 

[pidjones]

Shazbot. I just measured them again, and they are actually 36mm tubes. Which means they aren't even Honda. Maybe Yamaha? At least it means the trees are only stretch 1mm instead of 2. Also means I can't get a Honda triple to match them. They look a lot like Kawasaki LTD 454 forks, except they don't say Kawasaki at the top of the warning label and there is a label on each leg. And the air fitting at the top looks like Yamaha. Found a set on eBay for a Kawa KZ-750H that looks identical (I think). Same air fitting, same layout of lugs on the lowers. Online fork size chart says they ought to be 36mm. At least I know what they are now. So, is 1mm stretch safe?

I'm so confused.

 

[irk miller]

I ran a combo KZ650/750 front end on a 77 CB750. It was actually a pretty easy swap.

 

[teazer]

What you need my fine man is a pair of 37mm Gold Wing forks with 4 bolt end caps.. I happen to know someone that has a set of fork legs but no triples to match. Mix and match with any set of 37mm triple clamps that look like they should fit and Robert's your mother's brother.

 

[pidjones]

What you need my fine man is a pair of 37mm Gold Wing forks with 4 bolt end caps.. I happen to know someone that has a set of fork legs but no triples to match. Mix and match with any set of 37mm triple clamps that look like they should fit and Robert's your mother's brother.

Yeah, I do too. Left over from the Hunley build. Already have the mild pitting polished out, and have a new set of seals for them. But, would require new triples and probably a new axle. I think brakes are the same (the 750 came with rusty steel adapters to mount the original calipers to the Kaw lowers.)

 

[jpmobius]

First, let me say that spreading the bores to accept larger tubes - even for 1 mm more - is something I would absolutely advise against. Now that that's out of the way, just how dangerous is it, now that it has been done? I'll go out on that limb and promote that probably not very. The lower tree is steel, and likely has suffered hardly at all from such a small deformation, and the clamping force is, in reality probably not noticeably compromised either despite the differing radii. The aluminum upper tree is another story. Many aluminum castings are brittle and crack or break easily - and in fact it seems a lot of old trees (especially Honda, especially 350's?) are know for breaking from over tightening the pinch bolts. So if they didn't break already, likely they are ok. A close examination is in order for sure, but parts like this usually break straight away rather than just crack. In fact, if the deformation is within the "not enough to break it" range, there is a good chance the strength is actually increased, though this is not as beneficial as it sounds. The good news is that the lower tree takes the vast majority of the load, so the upper serves more to keep the whole assembly aligned. I recall seeing quite a few old bikes with broken upper trees that evidently rode around just fine seemingly forever. Anyway, if everything looks and feels solid, and it is as you say a rat bike cosmetically, I wouldn't worry so much. Give it a close look on a regular basis to be sure though, and if it keeps you up at night, swapping out the front end is always an option.