Yamaha IT frames, chromoly or mild steel?

[motomaniac]

Reynolds chromoly tubing was seamless. There were different grades available as well. The reason it is lighter than mild steel is that it is much stronger so in a frame it can be a much thinner gauge and still have the strength required.
If we accept that YZ went cro-mo in 77 and IT in 79, what are the dates for the other Japanese manufacturers bikes?
Cheers, Grahame

77 YZ's are mild steel,78 cro -mo.

[GT43]

I have two IT490 frames, and one of them is not real good with internal rust in the rear tubes.
I took it to an engineer and he welded it with mild steel, and said he had problems getting the weld to cling to the original metal.
He put it down to rust, but said it should be strong enough and hold for years to come. He also mentioned to hurry up and squirt some fish oil through the frame, otherwise it will start to crumble soon at some period .

Do todays MX chro-mo frames have a more advanced steel/alloy mix technology compared to the old bikes?

Because I think the old chro-mo may have been crap perhaps? Or as Grahame mentioned "different grades".

Apparently the YZ K series (83) had lots of frame issues of cracks at the steering head. Some 490's have been known to look like choppers.

[Zakk]

sorrrry to dig up this old topic... 

i've been repairing my friends 1981 IT250H and IT265H frames and have found both use seamless chromoly tubing for all the smaller diameter frame tubes, only the larger front down tube and upper shock mounting tube have seams.
this looks to be the same for the YZ's of the same era, although doing some more reading i understand that the 1983 YZ250/490K's used a thinner diameter tubing and this one was prone to cracking under stress of heavy landings

[GMC]

Just because it has no seam doesn’t mean it is Cro-Mo.

Just because it has a seam doesn’t mean it isn’t Cro-mo.

But if it does have a seam then it’s not aircraft grade 4130 Cro-mo.

And I doubt that any of the Japs would have used aircraft grade 4130 Cro-mo.
They would have got their own 'mill run' made to similar specs.