Furnace Brazing of Four-stroke Cylinder Heads

On four-stroke cylinder heads, cavitation or corrosion in the exhaust valve seat bore is a fairly common occurrence. If the damage is severe, then the o-rings cannot function anymore. The consequence is water leaking into the combustion space.

Such corrosion or cavitation can be rectified by machining the valve seat bores and installing over-size valve seats. But as the maximum allowable over-size usually is limited to around +2.00 mm, oversize valve seats may not always solve the problem. In such a case the installation of repair bushes by furnace brazing can offer a permanent solution.

Furnace brazing as a process is well established and has been used for various jointing applications in a range of industries. Its adaptation to cylinder heads has been pioneered and internationally patented by QuantiServ.

Cylinder head after refurbishment, inlet valve seats installed (right), exhaust valve seats not yet installed (left) to show the quality of the refurbished bore
Cylinder head after refurbishment, inlet valve seats installed (right), exhaust valve seats not yet installed (left) to show the quality of the refurbished bore

During the last few years QuantiServ has carried out reconditioning of hundreds of cylinder heads of many different types by this process successfully.

As the name suggests, furnace brazing is done at high temperature in a vacuum furnace, where a bonding metal is evenly deposited between the cylinder head and the repair bushes through capillary suction. This results in an extremely strong and durable bond. Our process is approved by Lloyds Register and is internationally patented. Class certificates are available on request.

We do not recommend cast iron welding because of its unreliability and high failure rate. Furnace brazing has many critical advantages over other repair methods, such as cast iron welding:

  • a high strength repair with uniform hardness throughout
  • no porosity or risk of cracking as is often the case on welded cylinder heads
  • critical dimensions are restored to nominal so that standard valve seats can be installed
  • no risk of catastrophic failure due to inserts coming loose
  • increased life time – the alloy bushes that we install resist corrosion and cavitation better than cast iron does
Severe corrosion in the exhaust valve seat bore
Severe corrosion and cavitation in the exhaust valve seat bore
Refurbished head, prior to installation of valve seats
Refurbished head, prior to installation of valve seats
Newly fabricated repair bush prior to installation into a cylinder head
Newly fabricated repair bush prior to installation into a cylinder head
Pre-machined cylinder head, with repair bushes ready to be fused in
Pre-machined cylinder head, with two repair bushes ready to be brazed in
Corroded exhaust valve seat bore prior to repair
Another corroded exhaust valve seat bore prior to repair
Furnace brazed cylinder head, prior to installation of standard size valve seats.
Furnace brazed cylinder head, prior to installation of standard size valve seats
The same exhaust valve seat bore after furnace brazing
The same exhaust valve seat bore after furnace brazing
One needs to look closely to see where the head ends and the bush begins
One needs to look closely to see where the head ends and the bush begins
A seriously corroded exhaust valve seat bore prior to repair
A seriously corroded exhaust valve seat bore prior to repair