Motorcycle Valve Job
(text and pictures by Mark Trotta)
Proper valve seating is crucial to a strong-running engine, and the quality of any valve job will be co-dependant on valves, guides, and springs.
Engine valves have three functions:
- Supply air/fuel mixture to the combustion chamber.
- Remove unwanted fumes from the combustion chamber.
- Remove unwanted heat through the seats and guides.
To do all these things effectively, valves must properly seal every time they open and close.
Disassembly And Inspection
With the head(s) removed from the motor, disassembly is straightforward. Removing the valves releases the valve springs and retainers.
A valve spring compressor is necessary to remove valves.
Read: Motorcycle Engine Tools
Check/Replace Valve Seats
Once the valves are removed, check the valve seats for wear and/or damage. Check carefully for cracks.
If the head has had several valve jobs in the past, the seat may be "sunk" into the head. This means the top of the valve will stick out taller than the others, and that changes valve train geometry.
For either of these scenarios, seat replacement is the only option.
If the seats are only worn, they may be cut or ground to restore the sealing surface. This is done only after the valve guides are checked and/or replaced.
NOTE: Some older heads do not have valve seat inserts, the seats were ground right into the head. If that's the case, and the seat is bad, some serious machine work is needed to repair it. It's easier to just replace the head.
Check/Replace Valve Guides
Valve guides need to be checked for wear. If they are worn and out of spec, they should be replaced, or sometimes can be renewed by knurling.
Read: How To Remove Valve Guides/Measuring Guide Wear
Read: How To Install Valve Guides/Knurling vs Replacing
Can't I Just Lap My Valves?
Many weekend mechanics, faced with old and worn valve seats, decide to just lap the valves and reassemble the cylinder. Obviously this is done because of time and money constraints.
Remember that valves help transfer heat out of the engine. If you decide to just lap the valves and the seat is too narrow, excess heat builds up inside the head. We all know the dangers of running hot, especially an air-cooled engine.
If the seat is too wide, you're robbing the engine of power.
At best, valve lapping will clean up very light wear on a low-mileage head. Anything more than that, lapping is not effective.
Valves need to be checked for wear, pitting, burning, and hair-line cracks. Any signs of these and you'll want to replace the valve. A little pitting is OK, but not on the face, margin, or stem.
Carbon build-up is common on valves. Be careful removing it, you don't want to scratch them.
Since exhaust valves run much hotter than intakes, many engine builders will replace exhaust valves automatically. This is cheap insurance against possible failure.
Check/Replace Valve Springs
Valve height affects valve train geometry and guide wear. Manufacturers will specify a minimum valve spring height. Replace if there's more than an 1/8" of difference.
Old valve springs can be tested to make sure they are capable of maintaining proper pressure. A special machine is required for this.
Valve Spring Shims
In a non-performance motor, slightly worn valve springs may be shimmed to restore proper valve height. This is sometimes necessary when valves and seats are machined.
Aluminum Cylinder Heads
Granted, they're lighter and easier to repair than cast-iron heads, but they're also more fragile. If an aluminum cylinder head is warped or cracked, additional repair steps will be required.
Proper Valve Seat Width
Many factory shop manuals specify an optimum seat width of .050". Some engine builders will cut them a little thinner than factory settings. This gives slightly better performance but slightly shorter life.
Before the old valve spring retainers and keepers are installed they should be inspected for any unusual wear. If anything looks questionable, replace it.
Note: Some cylinder heads are designed to have valve stem seals, others aren't.
The use of three-angle seats in cylinder heads has been an industry standard for decades. This is done by cutting relief angles above and below a 45 degree center contact surface.