I can't verify the source, but am giving credit as it was forwarded to me.
Found this in A. Graham Bell's "Four-stroke performance Tuning"
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Many head gasket failures, however, are not due to inappropriate head gaskets being fitted, incorrect tensioning, or a block deck that is not perfectly flat. The problem, if it is an aluminium alloy head, could be that the head has been annealed when it was previously overheated and blew a head gasket. What happens is that the hot gases destroy the heat treatment and the head goes soft.
Consequently it will no longer hold a head gasket and it has probably lost its crush on the valve seats as well.
In this state the head also changes shape; it will be shorter and narrower.
Hence a head that does not easily drop down on the block dowels or is
jamming on the head studs is probably annealed. If you use head bolts rather than studs and you find trouble getting the bolts started into the block, it means the same thing;
the head has gone soft.
This can be checked with a Brinell or Rockwell hardness tester. On the Brinell scale a good head will read in the 95-plus range, and anything under 75 is too soft. When testing on the Rockwell B scale this translates to a hardness in the 48 to 60 range being ideal, while anything under 38 is soft. Obviously the place to test is in an area where the gasket has blown. Do not be content with a hardness test in just a couple of places. A casting flaw could throw the reading way off, so over an area of about a square inch test in at least 10-12 spots.
If the head is soft and you have a lot of money invested in it, it may be worthwhile to have it heat-treated afresh. This is not cheap and it may not be possible in heads where the cam runs directly on the aluminium. Ask around to find what specialists can do, or if it is a head from an after-market supplier, get them to give you a price. Basically the seats and guides are knocked out and it is then heated in an oven at 520°C for around 5 hours (some manufacturers specify up to 20 hours).
Then it is quenched in 80°C water and tempered (reheated) at 200°C for another 4 hours (up to 20 hours), then allowed to cool slowly in the air.
Following this the head will be machined for oversize guides and seats and the deck will have to be resurfaced. This all costs a lot, but if the head is a good one with lots of fancy port and chamber work, you will not want to throw it away. Note also that any time a head is welded close to a head
gasket fire ring line or valve seat it will also require a fresh heat treatment
Found this in A. Graham Bell's "Four-stroke performance Tuning"
-------------------------------------
Many head gasket failures, however, are not due to inappropriate head gaskets being fitted, incorrect tensioning, or a block deck that is not perfectly flat. The problem, if it is an aluminium alloy head, could be that the head has been annealed when it was previously overheated and blew a head gasket. What happens is that the hot gases destroy the heat treatment and the head goes soft.
Consequently it will no longer hold a head gasket and it has probably lost its crush on the valve seats as well.
In this state the head also changes shape; it will be shorter and narrower.
Hence a head that does not easily drop down on the block dowels or is
jamming on the head studs is probably annealed. If you use head bolts rather than studs and you find trouble getting the bolts started into the block, it means the same thing;
the head has gone soft.
This can be checked with a Brinell or Rockwell hardness tester. On the Brinell scale a good head will read in the 95-plus range, and anything under 75 is too soft. When testing on the Rockwell B scale this translates to a hardness in the 48 to 60 range being ideal, while anything under 38 is soft. Obviously the place to test is in an area where the gasket has blown. Do not be content with a hardness test in just a couple of places. A casting flaw could throw the reading way off, so over an area of about a square inch test in at least 10-12 spots.
If the head is soft and you have a lot of money invested in it, it may be worthwhile to have it heat-treated afresh. This is not cheap and it may not be possible in heads where the cam runs directly on the aluminium. Ask around to find what specialists can do, or if it is a head from an after-market supplier, get them to give you a price. Basically the seats and guides are knocked out and it is then heated in an oven at 520°C for around 5 hours (some manufacturers specify up to 20 hours).
Then it is quenched in 80°C water and tempered (reheated) at 200°C for another 4 hours (up to 20 hours), then allowed to cool slowly in the air.
Following this the head will be machined for oversize guides and seats and the deck will have to be resurfaced. This all costs a lot, but if the head is a good one with lots of fancy port and chamber work, you will not want to throw it away. Note also that any time a head is welded close to a head
gasket fire ring line or valve seat it will also require a fresh heat treatment
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