YZ125G Build

[alexbrown64]

My old race number on the G when i was 16 was 278.  I was a restricted license holder and we had to run yellow numbers on a black background. We also had to butcher our plates and make them square to conform to regulations.
This pic is in 1980 at Coastal Park and has me getting me usual crap start with Harry Banken next to me (bike 56 YZ125G) and a young Jeff Leisk on Honda bike 5.
Needless to say, Jeff wiped the field.  But, at the Prince of the Cross race at Southern Cross in the same year, Harry Banken on his G hands down beat Jeff Leisk in what i still remember as the race of the century.  They were battling all race and it was Jeff who made the mistake and went down.  Harry deservedly won and the G had its day of glory. 



Cheers,

[alexbrown64]

Geoff at GMC has given the green light on the cone pipe concept.
I put the bike back together for a photo shoot. Cleaned and repainted the oem pipe and fitted it with the oem bracket and original muffler.
I then went around and measured and marked every point on the pipe that is in close proximity to other parts, such as airbox, frame, mudguard, sidecover etc.. Ended up with about 50 measurements all over the pipe. Also had the check the pipe was sitting square in the spigot and everything was mounted correctly.

This is not the normal way GMC would build a pipe.  I believe he does normally require a bike.  But, in this case, he already builds a silencer for the G, and he builds an IT175G pipe, which has an almost identical frame and motor. He has also looked at the clearances and believes he can do it.  As part of the concept, i will be trial fitting the final product, and i can make any minor modifications with a tig and send it back so that he can finish off the final jig and give it the thumbs up.
It may take a bit of effort, but i think it will work out great.

Just one of the 50 or so pics....



Cheers,

[alexbrown64]

You are going to get setbacks as you rebuild these old girls.

This old girl must of been sitting in a damp barn for years, covered in crud.
On final inspection before the frame went to the powdercoaters, i noticed a pin head hole in the gusset which attaches to the down tube.  More prodding led to a some rot.  I think its a common problem on these older YZ's.  Moisture and crud has got in behind the gusset and sat. This rotted some of the gusset out, and started a pin hole in the main down tube, which slowly grew to be about 10-15mm in diameter.  The only thing to do was cut the gusset off, and cut away a main section of downtube back to good metal.

I will now fabricate some new panels to go in.  For this i have 3mm plate, but really would like 2mm.  I also have to hit my dad up for some tig welding, which means i am up for a bottle of argon.  Still, its a win win.  I get to spend some quality project time with dad, and the job gets done satisfactorily.



Cheers,

[GMC]

Unfortunately that's pretty normal for those model YZ's, the gussets are a moisture trap that eats out the frames from the inside, any other secondhand frame will most likely be the same.

I would try to squeeze a tube or part thereof inside the downtube and plug weld in before welding a patch back in, the longer the better. When you make your patch make it with a 2mm gap all round so your weld will penetrate through to the tube inside.
Ideally you would be able to squeeze a tube in from the bottom but there is probably mounts and things in the way that stop you doing that so your going to have to perform some magic tricks to get it in there.

[alexbrown64]

Ho man, this frame is becoming a headache....

I do have some ideas.
I should be able to slip some 38mm (1.5") exhaust tube down inside the main downtube. I can cut a section to drop in, slide it along, fit another section in, then slide it back half way, fully weld them together and then plug weld it. That should strengthen the area.
Next step would be to make a patch section to fit in, and as suggested, leave a gap so that the whole thing will be welded to the internal pipe and frame. That will take care of the downtube.
The final step is to make a new gusset. I want to keep it as original looking as possible, but replace the entire thing. For this, i hope to use a 45 degree bend 32mm (1.25") mandrel bent exhaust section. I can slice this lengthwise and it will give me the radius and bend i will need, as the original was a pressed steel molded section. I will also fabricate the sides out of sheet metal and all this will be tig welded to 47mm o.d piece of pipe which will be sliced lengthways to fit over the downtube. The whole gusset can be fabricated, welded and flap disc smoothed off the bike, and then fitted and tig welded.

If the bike was to be raced, i would probably seal weld the gusset, with perhaps a small drain hole at the bottom, but i want the frame to be as close to OEM as possible, so the option i am taking will keep it looking original, but be stronger than it was when new.

Sending the pipe to GMC this week, then the project goes on hold again for a while.... got to get my two YZ125K's going again..... it never ends......

Cheers

[GMC]

Sounds like your onto it okay, these jobs are always fiddly and I have done this same sort of repair before but all brands have some sort of uniqueness just to keep you on your toes.
Patience is the key
Also when you finish it off only use a grinding wheel to take the bulk of the head of the weld off, leave taking the last bit of weld with a sanding disc.
Not a fan of flap discs myself and much prefer sanding discs, I like the 75mm quicklock style discs, you can get the fitting that holds them to fit onto a 4" grinder and when your done sanding slip on a medium scotchbrite disc and no one will know its been done.

[rocketfrog]

The joys of the YZ G/H Frame rot repair.

Performance metals in Sydney sell a wide range of 4130 Tube with a variety of wall thicknesses. If you buy a heavy wall tube with that has enough meat to turn (or have centerless ground) that will telescope up into the downtube and then plug weld to stitch it in place, you will have a good permanent repair.

The front engine mounts have a anti crush spacer welded thru the frame at the base which stops the tube from passing. The wall thickness on the spacer is generous enough to bore out to so it will accept a specially made sleeve to re install once you over size the internal bore and cut out the spacer tubes from the bottom. Leaving the welded tubes in place ensures correct alignment after the repair is complete with out needing to Jig, Geoff will dig what I am talking about but perhaps some diagrams or photos may help if you are not following.

The seam weld on the original Down tube then becomes a problem to get a nice tight fit. A light groove machined or carefully ground axially along the tube will allow the inner repair splice to key slot into the outer tube. Freeze the inner and warm the outer to assist fitment.

The other option is tunnel boring the frame, which is not practical. I have set it up in my lathe but decided not to go that way. The tube is thin and alignment must be perfect, plus most of these old frames are not straight enough anymore. Thorough internal cleaning and light polishing is recommended to assist fitment of the repair - which must be a good solid fit to effectively transfer the bending loads.

When the inner is fitted up (bridges the damaged section) past the cracked section and extends out the bottom of the down tube, plug weld (through previously drilled holes) to hold in place. The small amount of excess tube extending from the base of the frame is trimmed after the welding is carried out.

Drill off (transfer the holes) from the engine mount anti crush tubes to the inner repair and slide the fabricated sleeves through. A 45° weld prep angle machined onto the ends of the sleeve and a 45° counter bore in the oversized frame sleeve allows for a nice weldment that can be carefully filed back to match the original dimension. Some tacks on the upper tube from the inside are done before the lower tube is installed, then the lower is internally tacked.

GMC is all over it on the Gusset repair, once again Performace metals can supply a tube mandrel bent to take the place of the rusted gusset that you have cut from the frame. The original piece is a stamped section, the repair piece will be a full tube gusset that is plated in on both sides to disguise the fact that there was a repair there at all. Carefull notching to make a nice joint to weld is key making sure the welding goes smoothly and the amount of weld bead material is minimal. The plating would be optional for a racer as the tube gusset will be plenty strong enough to omit the plates, this allows for effective cleaning of the area. Plates installed with a lightening hole is also an option to facilitate access to clean out the gunk and get some paint into the closed area to protect the metal. Either way the repair will look Sano.

It is a lot of work and it will be expensive once you have bought the materials, paid for labour that you may not be equipped to execute yourself, and post heat treated the frame (postage, couriers etc). If you mig weld in a crappy gusset, you might just as well bog it up and paint it nicely for display only in my opinion. There is a lot of load on that section of the frame and it will be dangerous to allow a sub standard repair.

This problem plagues 1980 - 1981 YZ (+82 YZ490) and 1980 - 1982 IT models with a single downtube frame, in other words 125, 175,250 and 465/490 from 1980 -82.

I get this come up a lot, as they are popular models and they all suffer the same deterioration. I feel like I need to offer a repair kit? Perhaps Geoff would consider this as a stock supplied kit + specialist fitment as this is most likely going to be necessary for most owners.

[OverTheHill]

without reading it all yet [later] i was given an IT250K frame which i kept the s/arm off but had rotted through the front downtube from the inside near just above the engine mount area by memory. Was years ago but fair shocked me at the time. Still have a 250k with a yz465h motor [worried now bugger it, better check before the wheel base gets stretched landing from a jump lol].

[jerry]

GMC can do fix anything. Ask him for photos of my Pomeroy pipe!! J

[Lozza]

I made a former to make the curved inner section of gussets from a plumbers pipe bender and a machined roller. You can form enough thin sheet(0.6 or 0.8mm) in one go to make the whole gusset

[alexbrown64]

Thanks for all the input fella's.

Rocketfrog, i can pretty much see the repairs you are taking about and went in my shed to nut it out. I can drill out the existing engine mounts at the bottom of the downtube. I have found replacement tube (12.5mm o.d) 1.5mm wall thickness tube on Ebay for only $10. I can use an extension on a drill to clean up the downtube internally. I am pretty sure the i.d of the downtube is 38mm and 1.5" exhaust tube may be perfect.  I will take the frame to my local exhaust dude tomorrow. Then it would be easy to drill holes, fit the internal pipe, plug weld, fab the new engine mount crush tubes, tig weld etc.. all that is relatively inexpensive, but time consuming.  But i will look into it and post pics up.

Fabricating the gusset is also interesting as i want it oem, and it has roughly a 45 bend while going from narrow to wide and having a plate.  I also picture what you are talking about with welding plate to a mandrel bend, which is what i originally had in mind.

Lozza, do you have a pic of your former.  I can't quite see how you fabbed your gussets, but i am interested.

GMC, i have a variety of 5" grinding and knifing discs and several grades of flap discs.  Yep, have to be careful with them, or can take away a lot of metal.  I also got myself a 80mm cordless disc sander with about 100 velcro backed discs of various grades, so should be able to get a reasonable finish i hope.

Read up on welding chromemoly to mild steel and there doesnt seem to be much of a problem.  Not sure if some pre and post heat would help eliminate weld cracks.

You guys have set me up for a bit more work than i was hoping, but i am up for the Pepsi challenge....

Cheers,

[GMC]

GMC can do fix anything. Ask him for photos of my Pomeroy pipe!! J

Yes I fixed it, but it is now suffering a severe case of 'grandfathers axe' syndrome

[GMC]

In a nut shell, Cro-Mo is basically mild steel with the addition of Chromium and Molybdenum (and some other elements) so yes it can be welded fine to mild steel with either Mig, Tig or Oxy.
Tig will give you the most control over the amount of weld filler and penetration although some others may argue the same for Oxy.

It can be welded with filler rod grades ER70, ER80 or ER90
ER70 is your normal Mig wire and you can use mig wire as a tig filler when you need a small diameter filler
ER 80, also known as Cro-Mo 2
ER 90 also known as Cro -Mo 1 and this is what I mostly use for my frames.

The up side of Cro-Mo is it is much stronger than ordinary mild steel
The down side of Cro-Mo is it becomes brittle in the heat affected zone of the weld which is not the weld itself but the part right next to the weld and it requires heat treating to bring it back to its normalized condition where it is at its strongest.
You will here also sorts of ways to do this heat treatment but I prefer to send it off to the experts that can do it in a controlled oven.

However I don't consider any of the manufacturers frames have used aircraft grade 4130 Cro-Mo
If you want to go down the 4130 path I have an assortment of offcuts in 2mm plate and various tube diameters and thickness, let me know what you need.

I think you should be fine using mild steel for this repair as you will be adding more material to carry the load. As Cro-Mo is stronger less thickness of tubes can be used which is where the weight saving comes from so by the same token you can use more mild steel to create more strength.

Give your exhaust place a miss and check out your industrial suburbs for steel suppliers, you should find some one that will sell you a few metres and they can also get in tubes that have a 2mm wall which would be better for your inner reinforcement, I have some here but can't recall what diameter.
Your industrial suburbs will be a better place to find industrial supplies rather than the likes of Bunnings etc.

The reinforcement tube you slide up inside you wont get a perfect fit as any welds on the original tube will suck in the wall so it won't be perfectly round, the important thing though is to get some plug welds either side of the join in the downtube to reduce the load on the join.
As the join in the down tube is covered by the gusset so you should be able to leave the head of the weld on for strength as it won't be seen.

12.5 tube with 1.5 wall sounds too thin for engine mounts

A 8 or 10 mill rod that will fit into your drill should help to clean out the down tube, cut a slot in the end that you can slip some emery cloth (around 100-150 long) into that will wrap around the rod

[rocketfrog]

Welding thin CrMo is not a problem. Preheat only becomes necessary for thick sections where the heat soaking will be slow (high amps required and concentrated local heating = Heat Affected Zone HAZ cracking).

Heat will soak through thin tube quite rapidly and so the entire joint will heat through adequately throughout the welding process. Allow the weld to cool naturally, the slower the better. Make sure all the fit up is good and the parts are not under tension when fitted in place, in other words, if the part must be secured with vice grips or clamps because it tends to "spring" away from the mating section - rework it until it stays in place naturally.

Because of the amount of work going into a major structural area of the frame, Post heat treatment is definitely recommended to normalise the material. The recrystalisation of the metal surrounding the weldment is what creates weaknesses at the grian boundaries and leads to cracking. When you weld metal, you are essentially creating a small casting type crystal arrangement by remelting the metal and allowing the alloying elements to migrate however they please which is not the ideal dispersment for type of strength stated for any given spec.

Rolling or forging a Gusset would be great. This gusset is a compound curved section which would require some accurate dies and heat and a good degree of fettling to produce but could be done.

I will definitely drop in on this build as it developes Alex, you do a right proper job on your bikes.

[alexbrown64]

I dropped the kids off at school this morning, as i have some time off work.... also took my frame with me in the back of the car.
Went to a "Metal Land" type place... didnt have much in the way of anything i needed... water pipe, wrong o,d's etc.. went to a few scrap yards... nothing.  Ok, time to hit my local exhaust dude.  Showed him the frame and he got some 38 o.d pipe about 600 long.  It was a tight squeeze but would fit the down tube when i did the mods.  Next up was a scrap piece of 41 o.d to replace the bit of down tube i cut out.  Then 47 o.d to go over the top of the downtube for the gusset.

I walked out with a handful of tube for $10. Next stop Bunnings to get some grinding stones for the drill and some bubble wrap for the pipe i am sending to GMC.

Ok, first pic below is when i got all set up and ready to rock.
I used Rocketfrogs advice and used a 12.5mm drill bit to drill out the crush tubes at the bottom of the main down tube.  These tubes are for the engine mounts. They are much thicker on the outside than on the inside.  So, drilling them with a 12.5mm bit removes the internal tube, but leaves a nice spacer mount for when you want to tig some new crush tubes back in. 
Next step was to use an extension i had and following GMC's and Rocketfrogs advice i bored, ground and smoothed the internal tube.  This took a bit of work but i got it done with some grinding stones and some emery placed in a slot on a length of threaded bar i cut to size. I also drilled some holes for the plug welds.
Finally i could tap the 38 o.d exhaust tube into the existing down tube. I got it approx 570mm along the tube right up to the stem.  This will be as strong as an ox. The tube is also lightly galvanized so i guess a bonus will be a bit of corrosion protection.
Once the internal tube was in place, i trimmed the end off ready for a tig weld, and redrilled the engine mount holes at 12.5mm.  I will insert some new tubing through the frame and tig them into position. 
I have some 12.5mm tube coming which is 1.6 wall thickness.  Thats all i could get and not much different from what i chopped out. With the thicker downtube and the crush tubes only being 45mm long, i think i will be OK.





As you can see below, the new inner down tube has gone right through the frame and now acts as a strong back. In fact it goes right up to the steering neck.



Next pic below, i used the 41 o.d tube to fabricate a panel to replace the rusted section of downtube.  The new section has a 2mm all around. This bit of advice from GMC is so that the new panel, new insert tube and existing frame are all welded together to create a super strong repair.



All the plugs, welds and patches will be done later with the tig, when i organize my dad and some argon.

The next pic is when i decided to get stuck into the gusset. The original gusset was just a bit of relatively thin sheet metal pressed into a U bend shape with a nice 45 degree angle incorporated.
Thats a bit hard to fab up..... or is it? I looked around my shed, and bingo!!, a set of old steel bent handlebars laying around just waiting to get put back into service....as a gusset.

This gusset will be stronger than the original.  I used a half round section of 47 o.d pipe as the base.  This hugs the downtube and mates up nicely with what i had cut out.
I then used the bend in the bars to come up with this.....



Finally, i used my 3mm plate to make insertion panels and complete the gusset.  I now have a super strong gusset that fits perfectly and mates up to the old surfaces and looks almost OEM.  Of course, all the tubes, insert panels and gusset has to be fully tigged up, but the fab work is mostly done.



Now of course, there are several ways this repair could be accomplished, with varying materials, thicknesses, techniques and designs.  10 people will come up with 10 techniques.
I used some of the advice from  you guys, peppered with the materials i had available, coupled with the tools and skills i possess and within a budget and time frame.
So far it has cost me $15 and 6 hours of my time.  I still have to get the old fart around and get some argon, but that will be fun.. i'll shout him some Hungry Jacks for lunch.

Next stop is to fit the crush tubes and get everything zipped up, then another sand, smooth, check over and finally off to the powdercoaters.. which is a month away.  This frame will be going up with my XR75 frame and a bunch of knickers knackers to all get done in glorious Gloss Black.

Cheers,