New seals for old as David Silvester, boss of Sussex specialists Southern Hydraulic Rams, steps Andrew Pearce through a reseal

Rams, like just about every other mechanical thing, have a habit of giving up the ghost only when you're using them. Which means an immediate lack of pressure where required, and a new, unwelcome, pressure to get the thing fixed.

At that point the choices are simple: Buy a new ram, have the old one sorted out or repair it yourself.

If you like the DIY option, be prepared for potential grief for time (or the sheer bloody-mindedness of machinery) renders some rams reluctant to come apart. A word on that from specialist David Silvester: "Rams are held together in all sorts of ways, some less obvious than others. So it pays to look very carefully before trying to take anything apart, including the internal piston.

"Then be sure to find the cause of failure. For example, 95% of the rams we repair have suffered piston rod damage rather than age-related seal deterioration. If you just fit new seals and reuse the damaged rod, sooner or later the ram is sure to expire again."

Take your time over stripdown, he urges. "Sketch or take a picture of the location, order and the orientation of every seal and O-ring you come across. Then you can get the new parts back in the right way round and in the right sequence."

So, if you're ready for a little likely angst, the following picture sequence shows how to go about the job. The principles apply to all varieties of ram, though, naturally, the details --the ways of fixing caps and pistons, the seal design and number - will vary.

Strip

[1] This Claas combine ram leaked badly. First step to rejuvenation was a thorough external clean. Then comes stripdown. Here, the end cap (more properly called the head bush) is held by a wire clip, though others are commonly screwed on. Rotate the bush to find the end of the clip. Prise the end up, then continue turning to feed the clip from its groove. While all caps can be awkward, this design is often a right bugger to get apart, says David Silvester, if the wire is seized or snapped.

[2A and 2B] De-grease all parts, ideally by steam-cleaning. Here's the cause of death: Pits in the piston rod's chrome finish (arrows, A), which scored the head bush seal (B). A new rod, machined on site, was the only long-term solution. Also check the piston bore for scoring or other damage -- there may be more than one cause of failure.

[3] The rod must be clamped to take off and replace the piston. If the rod is to go back in service, it's essential to grip it in soft jaws like these. Scoring, flakey chrome or rust pits will quickly kill new seals.

[4] Jot down the position and orientation of the piston seals plus any backing rings, spacers or bushings. Then prise the old ones off BEFORE trying to undo the piston, as a locking pin or grubscrew sometimes lives below the seals.

[5] With any mechanical retainer(s) removed, undo the piston. If it won't budge it's probably held by locking fluid heat the piston gently to beyond 100C to soften the compound, then try again. Stilsons and/or a hammer and punch should be the tools of last, not first, resort.

[6] The rust damage shown in pic 2 meant a replacement rod had to be made. Over in the machine shop, threads were screw-cut in the lathe.

Rebuild

[7] Rebuild starts with the head bush outer seal. First on in this case goes a backing ring (1), with its concave face ready to take the O-ring seal (2). Details like backing ring orientation are important to spot during stripdown.

[8] Then fit the head bush internal seal. The flexible U-section seal squeezes in, with the open side of the seal facing pressurised oil. Lubricate the new seals and the internal bush with a generous dollop of oil-soluble grease.

[9] The piston had no locking grubscrew or pin, so the first new seal could be fitted with it clamped in an aluminium-jawed vice. Using a little force on a flexible seal is acceptable as long as you don't nick or over-stretch it.

[10] The piston must be locked to the rod, or oil pressure will loosen it. De-grease the seat area with brake cleaner or similar, then refit any lock pin or grubscrew. Here there was no mechanical retainer, so locking fluid was used on the piston's seat and threads. Tighten the piston (or its retaining nut) down hard.

[11] With the rod back in the soft-jawed vice, on goes the second seal.

[12] Check the piston tube entry has no burrs or sharp edges. Grease the new seals and the tube, then push and wriggle the piston home. Don't nick a seal or the game will be over.

[13] Locking clips usually have one hooked end. Find the matching recess in the head bush, hook the ring in it, then wind the ring back home. Continue turning until the ring's end is hidden by the tube.

[14] Southern Hydraulic Rams pressure-tests each rebuild in a purpose-made rig. When leak-checking on the farm, pass a piece of card around all seals and connections to pick up any fine oil mist. DO NOT use your fingers to find leaks -- high-pressure oil very quickly penetrates skin, possibly leading to amputation. If you're not convinced, see this non-commercial safety website. The card trick is useful to pinpoint tiny hose leaks, too.

Need seals or a rebuild?

Southern Hydraulic Rams has the facilities and expertise to rebuild old or damaged rams, or to fabricate new ones from scratch. SHR can also supply most replacement seals by post, given old ones as a sample. Where originals can't be matched, the piston may be remodelled or remade to take available seals.

The Claas unloading auger ram featured here needed a new piston rod. To make one and rebuild the ram with new seals cost £236 + VAT a genuine replacement is over £1000.

Southern Hydraulic Rams, 01403 784000 and www.southernhydraulicrams.co.uk