Engine & Parts

Engine parts for your motorcycle

Engine parts cover everything behind the cylinder wall: pistons, rings, crankshaft, connecting rod, valves, guides, gaskets, covers and the wear parts that keep combustion sealed. When compression drops, or the engine starts using oil, it is almost always one of these parts that has reached the end of its life.

An engine works to tolerances of hundredths of a millimetre. A piston in the wrong oversize or a valve with a worn guide alters compression, cooling and oil consumption all at once.

That is why choosing engine parts is not just a question of finding something that physically fits. It is about hitting the right bore, the right ring type and the right material for the loads your engine actually runs under.

This guide goes through measurements and markings, the difference between cast and forged pistons, gasket materials, valve guides and shims, plus how to read wear before it gets expensive.

You also get an overview of which type of parts suit motocross, road, touring and classic restoration — and which brands in the range cover each area.

We stock engine parts from Athena, Prox, Vertex, Wiseco, Wossner, Hot Rods, Cometic, James Gaskets, Kibblewhite, Kibblewhite Precision Machining Inc., S&S Cycle, MCS, Moose Offroad and Moose Offroad Hard-Parts.

Most of the range is in stock for next-day dispatch, and there is free shipping on orders over €200.

Measurements, markings and construction

The figures you need in order to order correctly are the bore in millimetres, the stroke, the piston pin diameter and — on four-strokes — the valve stem diameter. If they are not in the manual, measure them directly off the parts you have removed.

Bore, stroke and oversizes

The Bore, Stroke and Stroke Engines series cover exactly the measurements that determine displacement and compression.

• Standard bore: the same size as from the factory, used when the cylinder only needs a light hone
• Oversizes: typically steps of 0.25 mm and 0.50 mm after boring the cylinder
• Piston pin: diameter and length must match the small end of the rod exactly
• Compression height: the distance from the pin centre to the piston crown governs the compression ratio
• Big bore: larger bore than standard, requires a new or bored cylinder

Valves, guides and shims

On four-strokes the valve side is where wear shows up first, and where Guides, Head and Valve Shims belong.

• Valve stem: often 4.5 mm or 5.0 mm on modern single-cylinder engines
• Valve guides: pressed into the head and must be reamed to the correct clearance after fitting
• Shims: typically available in 7.48 mm and 9.48 mm diameter in steps of 0.05 mm
• Springs and retainers: must match the lift of the cam you are running
• Valve seats: should be checked and if necessary cut when the guides are replaced

Always measure twice, and write the figures down before the parts go off for boring. One measurement noted wrong costs a whole cylinder.

Choose engine parts to suit your riding

The choice depends on whether the engine runs short hard loads on track, long steady revs on the road, or low load in a restored classic. The table below brings together the four typical scenarios.

Riding style	Type	Advantages	Disadvantages	Brands in the range
Motocross and enduro, four-stroke	Forged piston and uprated valve kit	Handles high revs and a tendency to detonate	Needs more cold clearance, can be noisy when cold	Wiseco, Vertex, Kibblewhite, Hot Rods
Motocross and trail, two-stroke	Piston kit with ring, clips and bearings (Engine)	Quick to change, low cost per service interval	Short service life, must be changed by hours	Prox, Vertex, Athena, Wossner
Road and touring	Standard oversize, complete gasket set	Reliable, seals over long distances	Not built for tuning or high compression	Athena, Cometic, Prox, MCS
Harley cruiser and bagger	Big bore and cam kits (Milwaukee Eight, Cam)	More torque low down	Considerably more expensive, requires matched mapping	S&S Cycle, James Gaskets, Cometic
Classic and restoration	Replacement parts in original dimensions (Replacement)	Keeps the engine's original character	Limited range for older models	MCS, James Gaskets, Moose Offroad Hard-Parts

Racing is about shortening the service interval rather than building the engine stronger than it needs to be. A forged piston left in too long costs you a head and a cylinder, not just a piston.

For road and touring, sealing and heat stability matter more than peak power. Here a complete gasket set and correct torque figures are more important than an extra oversize.

On Harley engines a Cam upgrade moves the torque curve, but it has to be backed up by the right spring and lifter package. Otherwise the gain stays theoretical.

If you ride a mix — a bit of track, mostly road — choose for the hardest load the engine sees, not the average.

Materials and variants explained

The choice of material determines how much heat and detonation a part can take, and how much cold clearance it needs. That applies in particular to pistons, gaskets and valve components.

Cast and forged pistons

Cast pistons have a dense grain structure and low thermal expansion, which gives small cold clearance and quiet running. They suit standard engines running in their intended rev range.

Forged pistons in the Forged series have higher ultimate strength and cope better with detonation. In return they expand more, so they need greater clearance to the cylinder wall from new.

Gaskets and gasket materials

Head gaskets are available in multi-layer steel and in composite. Multi-layer steel handles high compression and repeated heat cycles, while composite is more forgiving of slight irregularities in flatness.

Cometic and James Gaskets cover most of the gasket side, and Athena supplies complete sets with head, base and Cover gaskets for the same engine.

Valves, guides and rings

Steel valves take more mechanical abuse, while titanium valves save mass and allow higher revs. Titanium wears faster on the seat and needs closer checking.

Ring sets in the Rings and Ring series come both as complete sets and as individual rings. The compression ring and oil control ring must always match the piston they sit in.

Wear, service life and maintenance

Engine wear almost always announces itself before it becomes critical: falling compression, oil consumption, blue smoke, a discoloured spark plug or new noises at idle. Take the signs seriously while they are still cheap to put right.

Piston and rings

On a two-stroke, the piston and ring are wear parts changed by hours — typically every season with ordinary trail riding and considerably more often in competition. If you measure the ring gap larger than the manufacturer's limit, the time has come.

On four-strokes, wear shows up as oil consumption and blue smoke on the overrun. Grooves in the piston skirt and vertical scoring in the cylinder wall mean the cylinder must be measured before new parts are ordered.

Valve clearance and valve guides

Valve clearance is checked on a cold engine at the manufacturer's interval — on a competition four-stroke often every 15 to 30 running hours. Clearance that gets smaller over time means the valve is sinking into the seat.

Sideways play in the valve when you check the guide gives oil consumption and poor sealing. New guides from Kibblewhite must always be reamed to the correct clearance after being pressed in.

Crankshaft and connecting rod

Radial or axial play in the big end shows up as a dull knock at low revs under load. The parts in the Crankshaft and End series cover both complete crankshafts and rod kits.

Hot Rods and Prox supply both ready-built crankshafts and rebuild kits. If you change the rod, the crank must be trued and checked for runout afterwards.

How to choose and fit correctly

Start by establishing whether the cylinder can be used as it is, or needs boring. Only then do you order a piston in the correct oversize — never the other way round.

Use the Find your bike function on Partsdenmark.com to filter the range down to the parts released for your exact year and model. That removes most mis-orders.

Order gaskets, seals and bearings at the same time as the major parts. An engine lying in pieces waiting for a single gasket collects moisture and dirt.

Clean all faces completely free of old gasket residue, and check the flatness of the head and cylinder with a straight edge before you assemble. No gasket can save a warped face.

Always torque to the manufacturer's figures and in the specified sequence, and use new bolts where they are specified as stretch bolts. Lubricate the piston, rings and bearings during assembly.

If the engine has been running lean or rich, find the cause before the new part goes in. Go through carburettors and intake rubbers for air leaks and wrong jets.

Run the engine in as specified, change oil and filter early, and check the valve clearance again after the first few hours. The Filter series covers oil and air filters from Moose Offroad among others.

Frequently asked questions about engine parts

Below you will find answers to the questions we get most often from customers who work on their own engines.

Which oversize should I choose?

Measure the cylinder — or have it measured by an engine machinist — before you order. The oversize must match the bore the cylinder is machined to, typically in steps of 0.25 mm.

If the cylinder is Nikasil plated, it often cannot be bored, but has to be re-plated at standard size instead. In that case the piston must be standard bore.

Do I have to change the crankshaft when I change the piston on a two-stroke?

Not necessarily. Check the play in the big end and the axial clearance, and only replace if the figures are outside the limit values.

How often should valve clearance be checked?

Follow the manufacturer's interval. On competition four-strokes it is often every 15 to 30 running hours, while road engines can go many thousands of kilometres between checks.

Adjust with Valve Shims in 0.05 mm steps, and make a note of which thicknesses you fit. That makes the next check quicker.

Can I reuse the head gasket?

No. Both composite and multi-layer steel gaskets bed in after the first torque-up and will not seal reliably again. Always use a new one.

How do I know the part fits my motorcycle?

Use the Find your bike function and cross-check against the measurements you have taken off the engine yourself. If you are unsure about the bore or valve stem, measure before you order.

If you are still unsure, contact us with the frame number and year, and we will find the right part number for you.

If you need the rest of what goes with an engine rebuild, you will find complete sets under gaskets, and fresh spark plugs for running in under spark plugs.

All engine parts on Partsdenmark.com are covered by 2-year right of complaint, and we offer a European price match if you find the same item cheaper at a European dealer. There is free shipping on orders over €200.