Is pressure testing a second hand head good enough or should I go the whole 9 yards and have it properly checked for cracks, dye etc etc

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Depends on what the head is , Mk1 cooper s definitely check as they crack , any others maybe . Do you know any of the heads history , is it off your previous car or did you buy it sight unseen ? did you pull it off the motor or was it sitting by itself ?

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With any old head its always a good idea to take it to your local reconditioner to chemical clean it, strip and check (and they will crack test it if you ask). Most mini heads warp over time, so skimming it flat is necessary and they will check and replace the valve guides for you as well as refacing the valves and the valve seats.

You might as well get the exhaust valve seat replaced with hardened ones for unleaded petrol at the same time. Cost all up is in the <$200 range.

I had a 1275 head I accidentally ported out thru the water jacket (as Vizard says happens) . After bronze welding it I bolted it to another head with a gasket, sealed the thermostat, heater and bypass connections, and hooked the garden hose to it. That is about 1,000KpA around here (142psi)- no leaks, she's a good-un again.
You could do this trick with heads suspected of being cracked- 142psi won't hurt the head, but should sure show if any cracks go into the water jacket.
If cracked the water will run out the ports.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Doc,
as you know, we are talking about a 295 head. I have a choice of 2. Niether have been used for many many years.
One is still attached to the block. My tendancy is for this one as I guess it would be lees likely to be warped.
Obviously both would require a lot of work and we have already talked about that.
Which one would you think would be better given they are both $300.00.

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Measure thickness with a vernier caliper- they were 2.750" when new.
If one is much thinner buy the thicker one. Even if warped (unlikely) there is plenty of metal to deck it clean.
If the exhaust valves have sunk into the head, best solution is get unleaded seats fitted.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

thanks Kev, I'll take some verniers with me.
Cheers

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if you can pop the valves out, a good idea might be to douse them in petrol and let it evaporate, if there are cracks the petrol will stay in them and show them up.... good idea anyone?

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did I tell you that I won a trophy?