Aprilia CM278203 Cylinder Base Gasket 0.5 mm – RS, RS4, RX, SX and Tuono 125 4T

The Aprilia CM278203 cylinder base gasket is the sealing gasket between the crankcase and the cylinder of the Aprilia 125 4T single-cylinder engine. The official catalog name is "CRANKCASE/CYLINDER GASKET THK. 0.5": the thickness is 0.5 mm, and this is the data you need to check before ordering, because the base thickness determines the compression ratio and valve clearance. It replaces the previous code CM157503. It covers RS 125, RS4 125, RX 125, SX 125 and Tuono 125 from 2011 to 2026. New original spare part.

✓ Strengths

• Original spare part, not universal – this is the gasket with the manufacturer's code, die-cut to the exact shape of the crankcase: oil passages and fastening holes are where they should be.
• Declared thickness 0.5 mm – on base gaskets, thickness is not a minor detail: changing it shifts the compression ratio and squish clearance.
• Multi-layer construction with sealing beads – metallic support with raised elastomeric screen printing around the sleeve and passages (observed from the product photograph).
• One code for fifteen years of Aprilia 125s – from the 2011 RS4 125 to the 2026 RS and RX-SX E5+.
• Replaces code CM157503 – if your exploded view still shows the old reference, this is the correct replacement part today.

Technical specifications

Vehicle compatibility

Complete list from the official Aprilia spare parts catalog: 8 model families and 33 versions.

Can't find your version, or not sure about the thickness installed on your engine? Contact us for more information: with the VIN number we will check the exact code required before you order.

Who it's perfect for

It's the spare part for those who are opening the engine of their Aprilia 125: cylinder head overhaul, piston or piston ring replacement, troubleshooting oil consumption or a leak at the cylinder base. It's one of those few-euro parts that are never reused and, if missing on the workbench, stops your work halfway: it's better to have it before you start, along with the head gasket and sealing rings. It's also the spare part for those taking an early RS4 or RX towards one hundred thousand kilometers and wanting to reassemble with original parts.

Frequently Asked Questions

How do I know if I need the 0.5 thickness?
The safest way is to measure the one you're removing with calipers before discarding it. Alternatively, send us the VIN number: we will check the catalog to see what thickness is specified for your engine. Installing a different thickness changes the compression ratio and the clearance between the piston and valves.

Is this the head gasket?
No. This is the base gasket, which sits under the cylinder against the engine crankcase. The gasket between the cylinder and the head is a different part number: if you are doing a complete cylinder head overhaul, you will need both, write to us and we will put them in the same package.

My exploded view shows code CM157503: is it the same part?
Yes. CM157503 is the previous reference for the same gasket: today the correct code to order is CM278203.

Do I need to apply sealant?
No. This is a gasket with pre-formed elastomeric sealing beads: sealant will mix with them and worsen the seal. Clean surfaces, dry gasket, and correct tightening torques.

Can I reuse the old one?
No. A base gasket has already undergone its permanent deformation during the first tightening: reassembling it will not reseal it. On an open engine, this is the wrong way to save money.