Cooling Cap or Compression Adaptor? Which Fits, Why Tests Mislead

The tester won’t seal on the tank, the glow plug adaptor has the wrong thread, or coolant keeps vanishing with no leak in sight. Match caps and adaptors to your engine and kit, and know where each test can mislead.

The Testing & Diagnostic Tool Spares range at PartsForMachines has 140 Sykes Pickavant spares, including 34 cooling test caps for cars, vans, trucks, buses and tractors and 20 glow plug compression adaptors.

Which Cooling System Test Cap Fits My Vehicle?

The one made for your make, model and year. Each numbered test cap has its own applications list: Cap No 15 (33156800), for example, covers the Ford Transit from 2006 and the Volvo C30 from 2006 to 2012.

Caps are screw-on or bayonet, so check which your tank takes: the 315 car kit, for example, has 16 threaded and 3 bayonet adaptors. The XL caps cover trucks, tractors and plant, such as the 33141800 XL cooling cap for bayonet-cap Massey Ferguson and New Holland tractors, JCB backhoe loaders and many John Deere 6000-series tractors.

Losing Coolant With No Visible Leak: What Test Comes Next?

A combustion leak test, after a pressure test. The pressure test raises the cold system to running pressure to show leaks from hoses, the radiator or heater core, and the gauge should then hold steady for a few minutes. If it falls and nothing drips, a combustion leak test can show whether a head gasket fault is to blame.

A combustion leak tester draws gas from the cooling system through a reaction fluid that changes colour if exhaust gas is present. Refills such as the 33194200 standard reaction fluid, or a sensitive version, suit the matching Sykes testers.

Glow Plug or Injector Hole: Which Compression Adaptor Do I Need?

Use the route your engine and kit are designed for. Many car diesels are tested through the glow plug holes: the 314200V3 car kit holds 12 dummy glow plugs (glow plug adaptors) in M8, M9 and M10 sizes. Each must match the plug’s thread and pitch, such as M10 x 1.25.

Where the injector is easier to remove, a common rail kit adds a dummy injector and clamps. The 31424700 TD5 compression tester goes in the injector bore, not the glow plug hole, held gas-tight by the engine’s own injector clamps. It works with an existing compression kit on five-cylinder Land Rover TD5 engines from 1998 to 2005.

Compression Reading Off the Book Figure: What Should I Check?

How you test can move the figure, so check your method first, then compare the cylinders with each other as well as with the book. The workshop manual gives the specified pressure and how far cylinders may differ.

Test with the engine at working temperature, the battery fully charged and all glow plugs or injectors out. The test runs on the starter, so a weak battery that slows cranking can upset the result. Two neighbouring cylinders both reading low most likely point to the head gasket between them.

What Should I Check Before I Order?

• your kit’s series or part number, and whether it has quick-release or threaded connections
• the vehicle or machine make, model and year, to find the cap number
• whether the tank takes a screw-on or bayonet cap, and the neck size if you use expansion plugs
• the glow plug thread, pitch and adaptor length, such as M8 x 1.0
• glow plug or injector access for the compression test

Browse the Testing & Diagnostic Tool Spares range, or contact the PartsForMachines team by WhatsApp, telephone or email with your kit number and vehicle details. A photo of the filler neck or the old adaptor helps.

Product options, delivery options and returns terms are shown on each product page. A 30-day return policy applies to eligible items.

Frequently Asked Questions

What pressure should I pump the cooling system to?

No higher than the rating marked on the radiator or expansion tank cap, or the vehicle maker’s test figure. Too much pressure can damage the cap and other parts of the system. The 33150100 hand pump’s gauge reads 0–2.5 bar (0–35 psi).

Should I test the radiator cap as well?

Yes. Workshop procedures pressure test the cap as well as the system: a sound cap holds its rated pressure, and one that won’t should be replaced. Check whether your kit includes a cap test adaptor.

Will my caps work with another pump or an older kit?

Older and heavy-duty pumps can take 315 car caps with an adaptor: the 33142500 314-to-315 adaptor for a 314 Series XL pump, or the 03186870 for an older 318 hand pump. For another make of pump, check the coupling or ask us first.

My vehicle isn’t on any cap list: what can I use?

A universal expansion plug kit. The 319 kit (33190000) uses tapered rubber plugs that fit radiator and header tank necks from 21mm to 50mm, and its retaining clamp helps stop a plug slipping, especially on plastic expansion bottles.

Should I buy a full cap kit or just the caps I need?

If you already have the pump, buy caps for the vehicles you work on most and add more later, as caps are sold singly. Not every piece in a kit is sold on its own, so check the part number first.

Can I buy a single 8mm glow plug adaptor?

Yes. M8 x 1.0 adaptors are sold singly, such as the 31427770V2 M8 glow plug adaptor, which is 107mm long. Check the thread pitch and length, not just the diameter, before you order.

The adaptor has the right thread but won’t engage or reach: why?

Length matters as well as thread: the M10 x 1.0 adaptors 31422070 and 31425270V2 are 63mm and 93mm long. Order the one listed for your engine. For glow plugs set deep in the head, an extension such as the 31434900 adds reach.

Can I turn the engine over by hand for a compression test?

No. The test is made by cranking the engine on the starter, and cranking speed affects the reading, so turning it by hand won’t give a usable figure.

Is it safe to pressure test my own cooling system?

Yes, with care. Only remove the cap once the engine has cooled, follow the kit and vehicle instructions, and use the pump’s release valve to let the pressure off afterwards. Sykes designs its 315 kit for serious enthusiasts as well as workshops.

Can I compression test a tractor or plant diesel?

Yes, with a kit for commercial diesels. The CV kit (31430000) suits industrial, plant, marine and agricultural engines and comes with end adaptors and clamping plates. Its 0–60 bar gauge (31420100) and 90-degree elbow (31473070) are sold as spares.