Cylinder Head Gaskets: How to Measure and Order a Custom Replacement
If you've ever searched for a replacement cylinder head gasket and come up empty, you're not alone. Engine builders, small-engine mechanics, and powersports enthusiasts run into this constantly — especially with imported engines like the GY6, clone motors, and older or non-standard equipment. OEM gaskets for these engines are often discontinued, undersized for modified builds, or simply unavailable without a 4-week wait from an overseas supplier.
The solution is simpler than most people expect: custom-cut. Any cylinder head gasket can be reproduced from measurements in under 24 hours, shipped the same day, and cost far less than OEM pricing. Here's how to do it right.
What Is a Cylinder Head Gasket?
The cylinder head gasket sits between the engine block and the cylinder head, sealing the combustion chamber from the cooling passages and the oil system. It handles three simultaneous sealing jobs:
- Combustion pressure — preventing hot combustion gases from escaping
- Coolant — sealing water jacket passages if the engine is liquid-cooled
- Oil — keeping oil passages sealed from combustion and coolant
For air-cooled engines (like the GY6 and most small engines), coolant passages aren't a concern — you're primarily sealing combustion pressure and, in many cases, oil passages on the base gasket. This makes material selection and precise dimensioning the two most important factors.
How to Measure a Cylinder Head Gasket
You need four measurements to reproduce any cylinder head gasket. Take them from the old gasket if possible; if the gasket is destroyed, measure the mating surfaces directly.
1. Bore diameter (the main hole)
Measure the inner diameter of the combustion chamber opening — the large center hole the piston travels through. Use calipers for accuracy. Note: on modified engines this is often bored larger than stock.
2. Overall gasket diameter or shape
For round gaskets, measure the outer diameter. For non-round gaskets (most GY6 and clone head gaskets are roughly circular or D-shaped), measure the widest and narrowest points and note any flat edges.
3. Bolt hole positions and diameter
Count the bolt holes, measure each bolt hole diameter, and measure the distance from the center of each bolt hole to the center of the bore. If the holes are evenly spaced on a bolt circle, just record the bolt circle diameter (center-to-center). If they're irregular, measure each individually from the bore center.
4. Any additional passages
Oil passages, dowel pin holes, or coolant ports need to be reproduced. Measure their diameter and position relative to the bore center.
Use any drawing tool to generate a .dxf file and get your instant quote!
Common GY6 and Small Engine Head Gasket Sizes
The GY6 engine family (used in Chinese scooters, go-karts, ATVs, and pit bikes) has several common displacement variants with different bore sizes:
- 50cc GY6 — approximately 39mm bore, 62mm OD
- 125cc GY6 — approximately 52.4mm bore, 72-75mm OD
- 150cc GY6 — approximately 57.4mm bore, 77-80mm OD
- 172cc GY6 (big bore kit) — approximately 61mm bore, varies
After-market big bore kits change the bore diameter — sometimes significantly — which is exactly why OEM replacement gaskets don't fit modified engines. A custom gasket cut to your actual bore size is the correct solution.
What Material Should You Use?
For cylinder head gaskets, material choice depends on your engine type, compression ratio, and whether you're running stock or modified:
- Compressed fiber (CNAF) — the most common choice for small engines and GY6s. Handles temperatures up to 700°F, good conformability, seals well under typical head bolt torque. Ideal for stock or mildly modified engines.
- Copper sheet — excellent for high-compression or high-heat applications. Requires annealing before installation. Common on race builds and engines with modified compression ratios.
- Graphite composite — excellent chemical and heat resistance, lower compressibility than fiber. Good for engines that run hot or see frequent heat cycling.
For most stock and mildly modified GY6 applications, compressed fiber at 1/32" thickness is the go-to starting point. It replicates what the OEM gasket does without over-complicating things.
How to Order a Custom Cylinder Head Gasket Online
Once you have your measurements, ordering is straightforward:
- Open the quoter at fastestgaskets.com. Select the gasket shape closest to yours (most head gaskets are Ring or Flange type).
- Enter your dimensions — outer diameter, inner bore diameter, bolt hole count, and bolt circle diameter.
- Choose your material — fiber for stock engines, copper or graphite for modified.
- Get an instant price and add to cart. Most orders ship same day.
For non-standard shapes or engines with irregular bolt patterns, you can also upload a DXF file — a 2D drawing from any CAD tool. Free DXF software like LibreCAD or DraftSight can create this from your measurements in about 10 minutes.
When Custom Is the Only Option
Beyond GY6s, custom cylinder head gaskets become necessary whenever:
- The engine is discontinued or the manufacturer no longer supports parts
- The bore has been enlarged by a machine shop
- You're building a one-off or prototype engine
- You're working with an imported engine that uses non-standard metric dimensions not stocked by US suppliers
- You need a non-stock thickness for a specific compression target
In all of these situations, the OEM catalog is useless. Custom-cut from your measurements is the only path — and it's faster and cheaper than most people expect.
Get Your Custom Head Gasket Today
Whether you're chasing down a GY6 replacement or building something one-of-a-kind, we cut cylinder head gaskets to your exact specifications. No minimum order, no setup fees, instant pricing online.