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BLOW­ING GAS­KETS

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.

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