Diagnostics
OTC 5605 Review: The Adapter Decides If It Reaches
A compression kit whose value is its adapters and its deep-well connector, not its gauge. Reaching a recessed plug is the part cheap kits fail at.
Before you buy anythingThis page also considers a cheap single-hose compression gauge, a cylinder leak-down tester and a borescope through the plug hole. Why, and when each is the better answer.
Contents
The gauge is the least interesting part of this kit. What decides whether a compression tester is useful on your engine is whether its adapter set reaches and seals your plug holes, and this one carries the piece cheap kits leave out.
A note on this assessment: researched from OTC's published kit contents, not run on an engine here. See how we test.
The problem it addresses
A compression test is the measurement that separates a tired engine from a healthy one with a fault elsewhere. It is the check worth doing before spending money on a car you are considering buying, and before spending money on a cylinder head you might not need.
The obstacle is rarely the concept. It is that the reading is only worth having if the adapter seals properly in the plug hole, and on plenty of engines the plug lives at the bottom of a narrow cast well where a standard fitting cannot reach or cannot seat square.
A leaking adapter produces a low reading, and a low reading is exactly what you were looking for — so the failure mode is a confident wrong diagnosis. The procedure that makes the numbers mean anything is in what a compression test tells you.
How it is specified
- Extra
- 8 in deep-well 14 mm connector for recessed plug wells; 5 in Ford Triton 16 mm adapter
- Gauge
- 2-1/2 in
- Hoses
- 12 in flex 14 mm standard and long reach; 25 in flex with quick coupler
- Model
- 5605
- Units
- Dual scale: 0-300 PSI and 0-2100 kPa
- Finish
- Nickel plated
- Feature
- Zero adjust
- Adapters
- 10, 12 and 18 mm
- Plug seats
- Flat and tapered
The adapters and the deep-well connector are the product. The gauge is a gauge.
What it gets right
Three thread sizes and both seat types. 10, 12 and 18 mm adapters covering flat and tapered seats is broad coverage, and seat type matters as much as thread — a tapered adapter in a flat-seat hole will not seal properly and produces the same false low reading as a leak.
The deep-well connector is the differentiator and it is the thing to check a cheap kit for. It is also the item most likely to determine whether the kit works on a modern-ish engine at all.
Two hose lengths and a quick coupler matter for a practical reason: you need to reach the plug hole and see the gauge while somebody cranks. The 25-inch flex hose with a coupler is what makes a one-person test possible on an awkward engine.
Zero adjust is a small thing that keeps a set of readings comparable, which is the point of the whole exercise.
What it does well
- Deep-well connector for recessed plug wells — the piece cheap kits omit
- 10, 12 and 18 mm adapters covering both flat and tapered seats
- Two hose lengths plus a quick coupler, so the gauge is readable while cranking
- Zero adjust, which keeps a set of readings comparable
- Dual scale, 0-300 PSI and 0-2100 kPa, which covers what a petrol manual quotes
- Nickel plated, for a tool stored in an oily box
What it does badly
- Petrol engines only
- Analogue, so nothing is recorded — write readings down per cylinder
- Three thread sizes covers common engines, not all of them
- Measures a symptom; the wet test is a separate step
- The test itself is a procedure, not a plug-in measurement
Ugly score 5, and it earns it
A gauge, a coil of hose and a tray of brass adapters in a blow-moulded case. It looks like every diagnostic kit and it lives in a cupboard between uses.
Five rather than higher because in use it is genuinely awkward-looking: a hose running out of a plug hole to a gauge propped on a wing, with the coil packs off and the fuse for the fuel pump pulled. Every compression test looks like a car mid-surgery, and that is the tool doing its job rather than a complaint about it.
Where it stops being the right tool
It measures loss without locating it
A low cylinder tells you something is escaping. It does not tell you whether that is past the rings, past a valve, or into the coolant. The wet test narrows it and a leak-down tester answers it directly by pressurising the cylinder and letting you listen for where the air goes.
If the intention is to diagnose rather than to screen, a leak-down tester is the more informative tool and this one is the cheaper first step.
The engine has to be prepared properly
Warm, every plug out, throttle wide open, fuel and ignition both disabled, and the same number of cranking revolutions per cylinder. Skip any of those and the numbers vary for reasons unrelated to the engine's condition, which is worse than not measuring at all.
It cannot be used once and forgotten
For a single check on one car, the cost is hard to justify against a cheap single-hose gauge — provided that gauge reaches. Where this kit earns its money is on several vehicles, or on one where the plugs are recessed enough that the cheap kit will not seal.
This page contains affiliate links. If you buy through them, Ugly Fixes may earn a commission at no extra cost to you. Products are selected independently.
OTC 5605 Deluxe compression tester kit
OTC
Petrol only. Check your plug thread size and whether the well is recessed before ordering — that is what decides whether any kit fits.
Check price and availability(opens in a new tab, affiliate link)Affiliate link. Price shown on the merchant's site.
Who should buy it, and who should not
Buy it if you look after older engines, if you are checking cars before buying them, or if your plugs are recessed enough that a basic kit will not seat. Screening an engine before a purchase is the strongest case — it is a cheap tool against an expensive mistake, which is the argument in checking a used car before you buy it.
Skip it for a one-off on an accessible engine, where a basic gauge does the same job. And skip it entirely for a diesel.
Would we buy it again?
Only if the engines in question actually need the adapter range. This is a kit bought for its fittings, and someone with one petrol engine and an accessible 14 mm plug is paying for coverage they will not use. Check the plug well first — that single measurement decides between this and something much cheaper.
The verdict
How well it solves the stated problem.
How much it offends the eye. Higher is uglier.
Worth the money, clutter and shelf space.
- Would we buy it again?
- Only if…
- Best for
- Someone diagnosing an engine they own, or checking a car before buying it, on a petrol engine whose plug thread is one of the common sizes.
If this is not right for you
- A cheap single-hose compression gauge
- A fraction of the price and adequate on an engine with accessible plugs in a common thread size, which describes plenty of older cars.
- A cylinder leak-down tester
- Localises the loss rather than measuring the total — it tells you whether air is escaping past rings, a valve or a head gasket, which compression alone cannot.
- A borescope through the plug hole
- Shows bore scoring, a damaged piston crown or coolant in a cylinder directly, which is sometimes the whole answer for far less effort.
Questions people actually ask
What does the deep-well connector actually solve?
Is the absolute number or the spread more important?
What is a wet test?
Does it work on diesel engines?
Why disable fuel and ignition?
Question this page did not answer? Ask it.