I need some help with this repair

[Pete O]

I'm tackling a repair that is outside my experience and knowledge, hoping you fellas might chime in with some advice.
I posted a photo the other day of the hydraulic lift cylinder out of the 3-point linkage on my old Ford 4100 tractor being rebored in the lathe. I dropped it in to an engine rebuilder today who is going to hone it to finished size and surface.
The cylinder bore seems originally to have been 3.30". I have bored it out to 85mm being the next 'standard' size up from that; this got all the scoring out and I should end up with a good-as-new 85mm cylinder, with the plan to use a modern single-acting hydraulic piston seal in that size.
That brings me to the piston. My problem is whether I need to make a new piston, put a sleeve on the existing one, or simply cut some lands in it and run wear rings.
Here's the piston. I'm pretty sure the material is cast iron:
   
It has just the one narrow land in it that originally ran a flat rubber seal with an O-ring backup.
The more complicated part of the piston is this side
   
   
where it has a concave seat that accepts the convex end of the pushrod:
   
   

I don't think that I can replicate that concave seat. The radius is larger than my radius gauges, perhaps around 0.6" and it appears to be hardened.
I've been scratching my head over how to approach this; someone suggested sleeving the piston and cutting a land in the sleeve. another thought has been to turn the existing piston down to a small slug with the seat in it and making a new piston with a pocket to accept that slug.
I'm now wondering if there would be a type of seal that can cope with a .020" clearance between the piston and the bore? The piston is close to .040" smaller in diameter than the new bore size.
I have very little knowledge of hydraulic components so am keen to hear whatever you guys can offer.

[TomG]

I would use option #2; make a new piston with a cup shaped recess on the back, then turn down the old piston to fit. It's also possible that the hardened spherical insert could be removed and pressed into a new piston by itself.

Tom

[Dr Stan]

Is there a plating company nearby?  If so have them plate the outside of the piston.  Before you go with that option put it in an oven at 180C to bake out the oil.

[f350ca]

I agree with Tom, build a new piston and press the rod seat into it.

[SteveG]

I've little practical experience with hydraulics other than component change (aviation industry) and automotive clutch/brake systems, but....

The practical side of me says as long as you have a seal to cope, the only other issue with re-using your current piston would be the clearance and potential "cocking" etc
Something as simple as a teflon ring near the end of the piston would prevent the cocking, and would also prevent the piston itself contacting the bore (ie it would be a improvement over the original). The teflon rings are commonly used to prevent o-ring extrusion so shouldn't be hard to get hold of, but wouldn't be hard to make one and just cut it to allow fitment.

Regarding the seal, I recently had a single direction cup seal for my power hacksaw ram custom made by a small company near my work.
They make them on a small CNC machining center out of various materials depending on application and have various standard profiles. I've no relationship with them other than being a happy customer. Its pretty much a one man operation - Steve's details are on the website contact page.
Took them the old seal, said what it was for and the dimension of the bore it fitted into and picked it up the next morning - $20 if I wanted just the one, so I got a spare for $30 total.
Not particularly cheap for a rubber cup seal, but the convenience was well worth the money.

Hytech Seals

Steve

[Pete O]

ok I have come to a definite decision...maybe...kind of...

I had a look through several websites last night with details of hydraulic seals and recommended land geometry, clearances etc. Seems that .019 clearance is the largest I could find as allowable on a particular style of seal, that is total clearance on the diameter so the .040 that I will have with the old piston in the new bore is not an option.
I contemplated plating the old piston but I decided it would complicate the geometry of the seal grooves, perhaps too much for my limited abilities to deal with.

So I will go with Tom and Greg's recommendation and make a new piston that will take a standard, off-the-shelf hydraulic seal and wear rings.

My next question is, what material would you guys recommend? I'll have to make a special trip to a steel supplier for a chunk of stock of sufficient diameter, so the slate is blank.

[TomG]

Pretty much any steel you choose is going to be adequate for strength, so pick something that machines well. I would go with a free machining leaded steel. "Sissy steel" is I believe how Greg refers to it.

Tom

[f350ca]

Is the cylinder steel or part of the tractor housing? If its steel I'd go cast iron for the piston, steel on steel is likely to gall unless one is really hard, not a sissy steel.

[Pete O]

The cylinder is a separate casting that is bolted to the underside of the tractor top cover but it's cast iron. I'm planning on putting phenolic wear rings on the piston on either side of the seal, so there shouldn't be any metal-to-metal contact in future. I'll see if the steel supplier has some leaded steel, if not I'll probably go with my fallback 4140, I seem to be able to get a reasonable finish on that stuff and the seal groove is going to have to be just so.

[Pete O]

The steel supplier I went to today was not as helpful as I had hoped, would effectively only sell me offcuts out of their bin if I didn't want a full length. The tribulations of being a hobbyist in an industrial field. I bought a 700mm length of 102mm round 4140, only piece they had with sufficient diameter to get my 85x 85mm piston out of. Cost an arm and a leg. At least I'll have some stock on hand, nothing else bigger than 50mm round in the shed at present and not much of that.
I've sorted out the appropriate seal and wear rings, hopefully will tackle the piston tomorrow.

I spent a bit of time this afternoon facing the flywheel out of the tractor, I decided to attempt milling it on the rotary table to avoid having to make a mounting flange to chuck in the lathe. Didn't go as well as I had hoped. The thing is HARD. I doubt it started life that way but it has seen some slip over the years and has a crazed, oil-impregnated surface. Think I might have to make a flange and do it in the lathe after all as I'm not sure I'm not milling runout into it with the mill setup.