Cobra Piston and Ring Inspection

Piston families and ring gap read year by year from all 83 factory manuals, 2005 to 2026, with the top end bulletins · Last reviewed 20 September 2026

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A used piston is the most honest diagnostic on the bike. It records what the engine has actually been doing, and most of that is legible before you throw it away. This page covers reading one, the ring end gap that decides whether the new set is any good, and which piston family your engine actually takes — which is not the same for every year, and where the expensive mistakes get made.

The short version

  • Ring end gap: 0.010″ (0.25 mm) to 0.015″ (0.4 mm). Measure it in the bore, on new rings as well as old.
  • An ECMU0276 will not fit an older bridged-exhaust CX50. Those take ECMU0060, which fits both. Check your year in the table below.
  • CX50 pistons come in A, B, C and D. CX65 pistons come in A, B and C — AB and CD were dropped in 2017, for the older bikes too.
  • An order for a CX65 ECC60208 may arrive under any of four supplier numbers. They are interchangeable.
  • Cobra publishes no rule for picking the letter. Read it off the piston that came out.
  • The crankcase stamp is for the base gasket, not the piston. Two different things.
  • New wrist pin bearing, new pin, new clips. Every time.

Ring end gap

0.010″ (0.25 mm) minimum, 0.015″ (0.4 mm) maximum. Cobra prints this twice in the CX50 manual, once as an inspection note and once as a caution during assembly, which is a fair indication of how often it gets skipped.

Put the ring in the bore on its own, square to the bore — push it down with the piston crown so it sits straight — and measure the gap with feeler gauges. Under 0.010″ the ring has nowhere to grow when it gets hot, and that ends in a seizure. Over 0.015″ it is not sealing, and that is the compression you were chasing.

Measure the new ones too

It is easy to assume a new set is right. Cobra prints the gap as a caution during assembly, not just as an inspection note, which is a hint about how often a new set turns out to be wrong for the bore it is going into — particularly in a cylinder that has had a few sets through it.

The 0.010″ minimum was restated in TSB0223 in March 2023, which raised it from the smaller figure Cobra published in 2019.

Piston kits by model and year

This is the part worth reading before you order. Cobra changed the CX50 piston in 2018 and the CX65 piston in 2017, and the new CX50 piston does not fit the old engine.

Model and years Piston kit Sizes Notes
King / SR 2005–2007 ECMU0060 one size Listed as one part number with no size letters.
King / SR 2008–2011 ECMU0060<letter> A, AB, B, C, CD, D Written as ECMU0060x in the manual, with AB, A and C given as examples rather than a full list.
King / SR 2012 ECMU0060<letter> A, AB, B, C, CD, D
King / SR 2013–2016 ECMU0060<letter> A, B, C, D
King / SR 2017 ECMU0276<letter> A, B, C, D Venom piston. Will not fit an older bridged-exhaust engine — see the warning below.
King / SR 2018 ECMU0060<letter> A, B, C, D The manual for this year prints ECMU0060A, while 2017 and 2019 both print ECMU0276. Cobra’s own inconsistency. The ECMU0060 is the safe order either way.
King / SR 2019–2026 ECMU0276<letter> A, B, C, D Venom piston. Will not fit an older bridged-exhaust engine — see the warning below.
Factory Works 2016–2018 ECMU0060<letter> A, B, C, D
Factory Works 2019–2026 ECMU0276<letter> A, B, C, D Venom piston. Will not fit an older bridged-exhaust engine — see the warning below.
JR 2006 ECMU0043 one size Listed as one part number with no size letters.
JR 2007 ECMU0060 one size Listed as one part number with no size letters.
JR 2008–2010 ECMU0060<letter> A, AB, B, C, CD, D Written as ECMU0060x in the manual, with AB, A and C given as examples rather than a full list.
JR 2011–2012 ECMU0060<letter> A, AB, B, C, CD, D
JR 2013–2016 ECMU0060<letter> A, B, C, D
JR 2017–2026 ECMU0276<letter> A, B, C, D Venom piston. Will not fit an older bridged-exhaust engine — see the warning below.
P3 2007 ECMOTE02 one size Single ring piston, one part number, no size letters.
P3 2016 ECMU0060<letter> A, B, C, D
P3 2017 ECMU0276<letter> A, B, C, D Venom piston. Will not fit an older bridged-exhaust engine — see the warning below.
P3 2018 ECMU0060<letter> A, B, C, D The manual for this year prints ECMU0060A, while 2017 and 2019 both print ECMU0276. Cobra’s own inconsistency. The ECMU0060 is the safe order either way.
P3 2019–2022 ECMU0276<letter> A, B, C, D Venom piston. Will not fit an older bridged-exhaust engine — see the warning below.
CX65 2007 ECMU0084 one size Listed as one part number with no size letters.
CX65 2008–2009 Not listed — This year’s manual lists no standalone piston kit, only the complete cylinder kit.
CX65 2010 ECC60389 one size Listed as one part number with no size letters.
CX65 2011 ECC60389<letter> AB, CD AB and CD are no longer stocked. Cobra moved to A, B and C in TSB1517, for these years too.
CX65 2012–2016 ECMU0184<letter> AB, CD AB and CD are no longer stocked. Cobra moved to A, B and C in TSB1517, for these years too.
CX65 2017–2026 ECC60208<letter> A, B, C May arrive under any of four supplier part numbers — see below.
An ECMU0276 will not go in an older engine

TSB0518, 4 January 2018: the ECMU0276 is the Venom piston and has no oiling holes on the exhaust side, because the engine it is built for has no exhaust bridge to oil. Cobra states it will not work in an older engine with a bridged exhaust.

The older ECMU0060 fits both. If you are buying for an engine you have not positively identified, the ECMU0060 is the safe one.

Rings, wrist pins and clips are interchangeable between the two. The rings look different — there are letters on the top face — but Cobra says the shape and material are equivalent.

Cobra bulletin TSB0518 figure comparing the old CX50 piston with two oiling holes on the exhaust side against the new Venom piston without them
The difference that decides whether it fits. Two oiling holes on the exhaust side of the older piston. None on the Venom. Tap to enlarge.

CX65: four numbers, one piston

Supply problems mean an order for ECC60208 can arrive as any one of these, and only one is available at a time. They are all the correct piston.

Part number Supplier Coating
ECC60208 Vertex uncoated
ECMU0084 Athena uncoated
ECMU1084 ProX moly coated
ECMU1184 Athena moly coated

From TSB0625, 20 September 2024.

The CX65 wrist pin got longer in 2017

When Cobra moved from a forged piston to a cast one, the wrist pin grew by about 2 mm. Cobra says the new pin is not a service replacement part for the old one. Rings went from ECEX0005 to ECC60221 and circlips from ECMUSR00 to ECC60222 at the same time. TSB1517 also corrects page 30 of the 2017 CX65 manual.

What the size letter means

Cylinders do not all come out of the machine at exactly the same bore. The differences are tiny, but a two stroke piston runs a clearance small enough that they matter, so Cobra sorts pistons into size groups and letters them.

Cobra publishes no rule for picking the letter — not in any manual, not in any bulletin. It is not an upgrade path and a bigger letter is not a better piston. Read the letter off the piston that came out. If the cylinder is new, it will tell you which one it wants.

The crankcase stamp is not the piston size

The digit stamped in the case is the base gasket. Two different things.

Cobra CX50 piston with the ring and ring groove shown for inspection
New wrist pin bearing, new pin, new clips. Every time. The arrow on the crown points to the exhaust. Tap to enlarge.

Do it interactively

The piston size selector does the table above for one bike: pick your model and year and it gives you the kit number, the sizes Cobra still supplies and the warnings that apply to that engine.

Parts for this job

Rings on their own are the 15 hour job and are sold separately — ECMU0155 for a CX50, ECC60221 for a CX65. A full piston kit is the 20 hour job. Check which piston family your engine takes before you order — see the table above.

Which diagram you need

Once you are on your own year, these are the schematics to open.

  • Engine — top endWhere the piston kit, the rings, the wrist pin bearing and the clips all live, on both bikes

Open your model, then your year

Every model page below lists every year it was built. Pick your year, then open the diagram named above — part numbers change between years, so order from your own year, not someone else’s.

Part numbers change between years. Open your model above, pick your year, then open the diagram named here. Piston kits are sold by size letter. The letter is stamped on the piston. Read the one that comes out before you order, and if it is unreadable, call us with the year and model rather than guessing.

Sources

  • Every part number and size letter comes from the Engine — Top End parts list of the manual for that model and year. All 83 manuals from 2005 to 2026 were read.
  • The fitment warning is TSB0518, 4 January 2018, including Cobra’s statement that ECMU0276 pistons will not work in older engines with a bridged exhaust. An earlier version of this page listed ECMU0276 for all CX50 models and all years, which would send a pre-Venom owner a piston that does not fit.
  • The CX65 change to cast pistons, the dropping of AB and CD and the new ring and circlip numbers are TSB1517, 12 January 2017.
  • The four interchangeable CX65 suppliers are TSB0625, 20 September 2024.
  • Ring end gap is Cobra’s, printed as both an inspection note and an assembly caution. The minimum was restated in TSB0223, 24 March 2023.
  • The 2008 and 2009 CX65 manuals list no standalone piston kit, only the complete cylinder kit. The table says so rather than inventing a number.
  • Cobra does not publish what bore range each size letter covers, in any manual or bulletin. This page does not guess one.