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VALVE SEAT CONVERSIONS FOR UNLEADED PETROL Jim Whyman, Secretary There have been a few reports of valve failures on cylinder heads that have recently been converted with hardened valve seat inset for use with unleaded petrol. The FBHVC's Fuels Committee's specialist, Dr Matthew Vincent, made the following observations, and car clubs .are invited to pass these on to their members: Fitting valve seat inserts to older engines can lead to difficulties if care is not taken with choice of insert material. When the head is machined and an insert fitted, a thermal break is introduced. The chief means for exhaust valve cooling is through the seat, so when a thermal break is introduced, exhaust valve temperatures increase. Exhaust valve failure in heads with inserts is thus more likely. To counter this, the best inserts are manufactured using sintered powder metallurgy techniques. Heavy doses of copper can be introduced into the seat to improve heat transfer and thus counteract the tendency to overheat. It is also known that older cars frequently run significantly hotter as a result of wear and cooling system inefficiencies. These problems render such vehicles more likely to suffer valve burning or valve seat recession type failures. Consequently, it makes sense to check cooling systems and general engine health before spending money on an 'unleaded' conversion. It is also important to check that the conversion uses sintered metal inserts with good heat transfer characteristics. |
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