If you've ever opened a toolbox and found a small tube of thick, oily blue paste, you've most likely got Prussian Blue fitting compound — also known as engineers blue or gear marking compound. It looks unremarkable, but it does one job extremely well: showing you exactly where two machined surfaces are actually making contact.
What It Actually Is
Despite the name, Permatex Prussian Blue Fitting Compound (PX80038, 22ml) isn't a dye or a coating — it's a non-drying paste built on petrolatum and mineral oil, with genuine Prussian blue pigment suspended in it purely for colour contrast. Because it's non-drying, it never hardens or sets on the part. It stays workable indefinitely in the tube and wipes straight off with solvent once you're done, leaving no residue and no mark on the finished component.
How It Works
The method hasn't changed much in over a century of trade use:
- Apply a thin, even film to one of the two mating surfaces.
- Assemble the parts in their normal working position and turn, load or rotate them as they'd operate in service.
- Carefully separate the parts again.
- Read the transfer pattern — wherever genuine contact occurred, the blue has streaked or transferred onto the opposing surface, showing exactly where the fit is tight, loose, or off-centre.
- Correct as needed, wipe the compound off, and repeat until the contact pattern is right.
No measuring equipment required — the compound does the measuring for you, visually.
Where It Earns Its Keep in an Australian Workshop
- Bearing fits — checking a shaft or housing for proper contact before pressing a bearing home, so you're not guessing at an interference fit.
- Gear tooth contact patterns — this is literally what "gear marking compound" (or "diff marking compound" on a differential) refers to. Painting the drive gear and rotating the mesh reveals whether contact sits centred on the tooth face or is skewed toward the heel, toe, top or root — the standard diagnostic for backlash and mesh alignment on diffs, gearboxes and PTO drives.
- Valve and valve seat work — engine reconditioning and industrial valve refacing both rely on it to confirm a valve is seating on a continuous, even line rather than just part of the face.
- Flat and machined surfaces — surface plate work, hand scraping, and checking flatness before line-boring or resurfacing a mating face.
- General fault-finding — spotting uneven wear on legacy or reconditioned machinery, confirming alignment during reassembly, and picking up surface defects that would otherwise only show up once the part's back in service.
It's a cheap, low-tech check that heads off a very expensive mistake: fitting a bearing, gear set or valve that looks right but isn't actually making contact where it needs to.
Frequently Asked Questions
What is Prussian Blue fitting compound used for?
It's a non-drying marking paste used to find high spots and confirm real contact between two precision-machined or close-tolerance surfaces — bearings, gears, valves and mating faces are the classic uses. Paint it on one surface, assemble the parts, then read where the colour transfers.
What's the difference between Prussian Blue and engineers blue?
In the trade, they're used interchangeably for the same job. "Engineers blue" is the generic name for this whole class of high-spot marking paste; "Prussian Blue" specifically refers to products — like this Permatex compound — formulated with genuine Prussian blue pigment for the colour. On the bench, they do exactly the same thing.
Is Prussian Blue the same as gun bluing?
No. Gun bluing (or steel bluing) is a controlled chemical process that forms a black-blue oxide layer on steel for corrosion resistance and finish — completely different chemistry and job. Prussian Blue fitting compound never reacts with or coats the metal; it's purely a temporary marking aid that wipes off.
Is Prussian Blue the same as layout fluid or marking ink?
No, and it's a common mix-up. Layout fluid (often sold as marking blue or layout ink, Dykem being the best-known brand) is a fast-drying dye used to coat raw stock so scribed cutting lines show up clearly before machining. Prussian Blue fitting compound does the opposite job: it never dries, and it's used to check contact between two assembled parts, not to mark lines on one.
Does Prussian Blue dry or harden on the part?
No — it's specifically formulated as non-drying, which is the whole point. It stays a workable paste so it transfers cleanly during the check and wipes off completely afterwards, with no residue left behind.
How far does a 22ml tube go?
A long way. You only need a thin film for each check, so a single tube covers a large number of fit checks over time — a low-cost item worth keeping on the bench.
Is it safe to handle in the workshop?
Treat it with the same care as any mineral-oil-based workshop compound: avoid prolonged skin and eye contact, work in a ventilated area, and wash your hands after use. Always check the current Safety Data Sheet before use, particularly for repeated or high-frequency handling.
Where can I buy Prussian Blue fitting compound in Australia?
AIMS Industrial stocks the Permatex Prussian Blue Fitting Compound (PX80038, 22ml), available for trade and workshop use across Australia.
