EKINSUN LTD is a custom parts manufacturer, and the trunnion is where this site's whole worn-pivot doctrine converges: the pin is a turned part, the bushing is a bushing, and the boss is a hole that has gone oval — each one covered elsewhere on these pages, and all three arriving together the day a tipper bed starts to knock. The machines that knock loudest are the ones the repair forums know by heart: orphaned splitters, old dump trailers, loaders whose maker folded — where the forum's standard answer is "have one made." This is where that answer lands.
One Word, Three Products — Which Trunnion Do You Have?
| You searched "trunnion" and mean… | What it is | The honest route |
|---|---|---|
| A trunnion ball valve | A valve-industry product for pipelines | A different industry entirely — valve suppliers, not us |
| Concrete-mixer trunnion rollers | Wear rollers under the drum | Stocked aftermarket for common mixers — buy there; obsolete drums are a quote |
| The pivot a machine swings on — dump bed, splitter beam, cylinder mid-mount | Two coaxial pins in two bosses, wallowing loose | This page. Pin, bushing and boss work, machined from 1 piece |
The Three-Part Decision
A knocking trunnion is never one part. The pin, the bushing and the boss eye wear together, and the repair splits three ways:
| The part | What wear does to it | The remake answer |
|---|---|---|
| The pin | Grooved and stepped where the load sat | Turned oversize in 4140 or C45, induction hardened where duty is real, grease cross-drill and groove reproduced |
| The bushing | The sacrificial part — if the machine ever had one | Renewed in bearing bronze; on bare-bore machines, added for the first time |
| The boss eye | Wallowed oval; on bare-bore machines it took all the wear itself | Bored round and true, then bushed — the same bore-and-bush doctrine as every worn hole here |
The honest fix is an upgrade. Cheap equipment often ran the pin straight in the bare casting — no bushing, so the boss itself became the wear part. The remake bores the eye true, fits the bronze bushing the factory saved two dollars on, and machines the pin oversize to suit. From then on, wear costs a bushing, not a boss — the pivot is repairable forever, which is more than the original could say.
Two Bosses, One Bore
The whole point of a trunnion is that both pins share an axis — the machine swings about a line, not about two nearly-parallel guesses. So trunnion seats are treated as one bore that happens to pass through two bosses: machined in line, in one setup, and checked as a pair. On a trunnion-mounted hydraulic cylinder the penalty for skipping this is paid quickly — misaligned bosses make the cylinder fight its own mount, side-load the rod, and the seals die young; the cylinder's other end has its own page at the gland nut. It is also why the hand-drill repair that "fixed" one side never lasts: the second bore must be placed by the first, not by eye.
Materials — Stated, Not Assumed
| Material | Where it belongs |
|---|---|
| 4140 / 42CrMo4 · C45 | The pins — induction hardening on the working surface where duty is real; the full grade story is on the 4140 page |
| Bearing bronze | The bushings — sacrificial by design, and the upgrade bare-bore machines never got |
| 304 stainless | Pins for washdown and weather duty — at a strength cost we state in the quote |
| 6061 / 7075 aluminium | Only in instrument- and gimbal-class trunnions where the load is grams — never in an equipment pivot |
What to Buy, and What to Have Machined
| What you need | Stocked? | The honest route |
|---|---|---|
| Weld-on trunnion mounts, standard cylinder ODs | Yes — cylinder-parts catalogues stock them | Buy them. A stocked mount beats any one-off on price, no argument from us |
| Trunnion ball valves | Yes — the valve industry's product | Not our trade at all — a valve supplier's catalogue |
| Mixer trunnion rollers, common drums | Yes — stocked aftermarket | Buy there; an obsolete drum's rollers become a quote |
| Pin + bushing + bored boss for an orphaned machine | No — the maker is gone | Machined as a set from the worn parts or your measurements, from 1 piece |
| The bare-bore upgrade — add the bushing the factory skipped | Not a product — it is a machining decision | Bore true, bush, pin to suit; wear becomes replaceable |
| Mounts for an odd cylinder OD, pin size or spacing | No catalogue row exists | Machined to the cylinder and frame you actually have |
| Matched pin pairs with grease ways | Model-specific only | Turned as a pair, cross-drilled and grooved like the originals — or like they should have been |
"The tipper bed on our old farm trailer knocks like a door — the trunnion pins are grooved halfway through and the bosses were never bushed, just holes in the casting, now egg-shaped. Can you make the pins and bushings if I send you one pin and the measurements?"
That is a complete enquiry, and the answer is the whole doctrine in one job: pins turned oversize as a pair, the egg-shaped eyes bored round on a shared axis, bronze bushings in — and the trailer swings on a replaceable part for the first time in its life. The wider family of this work: clevis pins for the linkage end, bushings and sleeves of every size, tractor spindles when the pivot that wallowed is the steering, and obsolete parts when the whole casting is past saving.
How to Order
- Best input: the worn pin and one bushing (if it exists) in a box — they come back with the new set
- Or photos with a caliper: pin diameter at an unworn end, boss bore, boss width, spacing between bosses
- Say what swings on it — bed, beam or cylinder — and roughly what it weighs; it decides hardening and bushing wall
- Grease: yes or no — cross-drilled pins with grooves are reproduced or added
- Quantity: pivots come in pairs; sets for the sister machine join the same price ladder — one pin alone is still a normal order
A dimensioned approval drawing comes back before machining. Quote within 12 hours; production typically 5 working days after approval.
Frequently Asked Questions
A trunnion is a pair of coaxial pivot pins or bosses that something heavy swings on — a dump bed, a log splitter beam, a trunnion-mounted hydraulic cylinder. That machine-pivot kind is what this page machines: pins, bosses, mounts and bushings. Two namesakes are not this page: trunnion ball valves are a valve-industry product, and concrete-mixer trunnion rollers for common drums are stocked aftermarket parts — we say so up front rather than waste your enquiry.
Measure all three, because they never wear alone. The pin is cheap to make oversize. The bushing — if there is one — is the sacrificial part and simply gets renewed. The trap is the boss eye: on cheap equipment the factory often ran the pin directly in the bare bore, so the bore itself wallowed. The honest fix is an upgrade: bore the eye true, fit the bronze bushing the factory skipped, and machine the pin to suit. From then on wear costs a bushing, not a boss.
Yes — post the worn parts or send photos with a caliper across the pin, the bore and the boss spacing. Worn surfaces are taken back to nominal, recovered from unworn sections and the mating parts, never copied as found. The grease way — cross-drill and groove — is reproduced as the original had it, or added where the original should have had it. A dimensioned approval drawing comes back before machining.
For standard cylinder outside diameters, buy them: weld-on trunnion mounts are catalogue parts and cost less than any one-off. The machined lane starts where the catalogue stops — an odd cylinder OD, a non-standard pin diameter or spacing, a mount that must match a wallowed frame that has already been bored oversize, or the orphaned machine whose maker never sold spares at all.
Treat two bosses as one bore. Trunnion seats are machined in line, in one setup, because the whole point of a trunnion is that both pins share an axis — misaligned bosses make a cylinder fight its own mount, side-load the rod and kill the seals early. That is also why a repair that hand-drills one side never lasts: the second bore has to be placed by the first, not by eye.
Pins in 4140 (42CrMo4) or C45, induction hardening on the working surface where the duty is real; bushings in bearing bronze, sacrificial by design; 304 stainless pins where washdown or weather demands it, at a strength cost we state. Aluminium — 6061 or 7075 — belongs only in instrument- and gimbal-class trunnions where the load is grams, never in equipment pivots.
EKINSUN — a custom parts manufacturer machining pins, bosses, mounts and bushings from the worn originals or your measurements, MOQ 1, pin-bushing-boss work quoted as a set, quote in 12 hours. Honest boundaries: weld-on mounts for standard cylinder ODs are catalogue buys; trunnion ball valves are the valve industry's product; mixer rollers for common drums are stocked aftermarket parts.
Get a Quote
Post the worn pin, or send four caliper readings. Reply in 12 hours.
// In short
- →Ball valve or mixer roller? Not this page — honestly
- →Machine pivot wallowed? Pin + bushing + boss, as a set
- →Bare bore? We add the missing bushing
- →Two bosses, one line-bored axis
- →Quote in 12 hours
or email: [email protected]