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QMy tuned Mini City keeps blowing the head gasket between the bores and the push-rod holes. Both head and block have been checked to ensure they are flat, and I have new Mini Spares head studs fitted. Any advice? Alex
AWhat you haven’t stated is what type of head gaskets you have been using, the size of the engine, or the actual spec, so we will try to cover all bases.
Firstly, there are a number of gaskets on the market now, particularly for small-bore, and not all of them good. People use them because they are cheaper than the genuine original type. We have seen some great results from alternative head gaskets, but have never been able to directly identify their origins. Consequently, our advice is to only ever use the genuine ‘Payen’ branded head gasket. From your description this seems like the most likely cause for such regular failure. As you have stated, the block and head
“How much the head has been faced by to increase the compression ratio after modification can be an issue”
face surfaces are flat, and the head studs/nuts in good order.
How much the head has been faced by in order to increase the compression ratio after modification can be an issue.
Machine more than 0.080-inch from the face of a small-bore head, for example, and the deck thickness of the head becomes very thin and moves/warps when hot. The result is that the
head gasket blows. There is also the possibility that the head has had so much machined from the face that the head nuts are bottoming out at the thread ends before clamping the
head down tight, so make sure there are plenty of threads below the level of the head, or head washers if used. If no washers are currently fitted, and the nuts are indeed bottoming out on the thread ends, then fitting some suitably thick washers will obviously help.
One last thing. Some companies started using aluminium for their rocker posts when manufacturing roller-tip rockers. BMC used aluminium posts in the very early days, but stopped when it became apparent they were the cause of blown head gaskets, because the material under the head nuts collapsed after a few heat cycles. This meant the original and essential clamping force exerted by the torque value used was relaxed. So check yours with a magnet if anything other than standard rockers are being used. People still buy and fit second-hand tuning parts on a regular basis, so it may be that a set have found their way on to your particular motor.