What is a cylinder head bolt, what do they do and why shouldn’t you re use them? The cylinder head is a vital part of any motorbike engine. The cylinder head bolts secure the cylinder head in place and keep it attached to the engine.
The cylinder head bolts in a Kawasaki ER5
The cylinder head bolts pass from the cylinder head, down though the cylinder barrel casing and into the crankcase. They clamp the head to the engine and stop the engine oil, coolant and fuel/air mix escaping.
Note – there can be a different number of bolts depending on the size of the engine (there are 8 bolts in the engine used for this blog).
Cylinder head bolt loosened from the engine. They are quite long (to go from the head to the crankcase).
Why shouldn’t the cylinder head bolts be reused if the engine is dismantled?
Cylinder head bolts are known as ‘stretch bolts’ (also called ‘torque to yield fasteners’), which means they deform (or stretch/get slightly longer and thinner) when installed in the engine.
This also means they shouldn’t be reused as once they’ve been used they lose their ability to stretch (which can result in leaks from the engine, loss of compression or the bolts working loose).
Cylinder head bolts removed from a motorbike engine
They are mainly used in motorcycle (and other) engines due to the following reasons:
- They have more ‘clamping force’ (how much force they apply when bolted in place) then regular bolts, meaning less bolts have to be used, which saves weight (and increases the power to weight ratio)
- Less bolts and not having to use thread lock means less labour when manufacturing the engine
- Stretch bolts give a more consistent clamping force
- It’s easier for manufacturers to work out how long they will last
Drawbacks of using stretch bolts to secure the cylinder head:
- Stretch bolts SHOULD NOT be re used, they SHOULD be replaced once they’ve been loosened
- They should be replaced if any of them are additionally tightened, loosened or damaged
- They are more expensive then normal bolts
Note – Although they are more expensive then normal bolts, they are not normally massively expensive so should be replaced (as having to dismantle the engine again due to leaks of loss of compression is a total PITA!).
Cylinder head bolts removed showing the holes the bolts go into.
Note – Cylinder head bolts should not be confused with the head cover bolts (which are visible on the top of the engine). Cylinder head bolts are not visible unless the cover (and possibly other engine parts) are removed.
Disclaimers:
The information provided on this page is ‘to the best of our knowledge’ and should not be taken as 100% accurate!!
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