All later factory produced minis were unleaded tolerant from this point on regardless of 998 or 1275 fitment. Factory unleaded 998 heads have inserts for all eight valves. Where as the factory 1275 heads have 4 inserts and maximum valve sizes liites to 33.2 x 29.4mm regardless of the application. The 1275 heads also have a slightly deeper chamber giving 22.5 to 23cc stick chambers rather than to more usual 20.5 to 21.5cc of the earlier leaded heads.
As to whether to convert or not that's up to you. Yes valve seat recession is very real and happens to the a-series cylinder heads. Heads can be converted by fitting inserts (I have 3 to do next week) however not a risk free process. Cracking is possible as is finding waterways lurking verycloseto where the inserts need to be fitted.
I fit inserts to all engines I build. I also fit inserts to all cylinder heads that are supplied by MLMotorsport and a good number of other customers. They are reliable when fitted correctly and can ensure your head casting is able to withstand modern fuel.
Using old used factory unleaded heads. Do not use and original head until it has been fully refurbished. Unleaded fuel causes extensive carbon putting on the exhaust valves and these will be well past there best as the newest ones are 16years old! Minimum work required to prepare these heads for use must be, check and replace valve guides and valves is worn, check for loose valve seat inserts, reface or replace exhaust valves, cut seats and check spring seated pressure before use. Failure to di this type of work will detract from the performance and reliability prospects of your replacement cylinder head.
I once worked ona mildly modified 1275 A+ engine many moons ago, standard 33.2mm unleaded head and a single hif44 carb fitted along with the usual st1 kit and a 266kent cam. We dynoed the engine at 48bhp @ wheels. The owner asked me what could be done to increase the power. I replied 'the leakdown test shows you need a valve job'. This we duly did. Refitted the head after a quality Dodd valve job and NO Porting and went back to the dyno. 54bhp @wheels. More than 10% power increase just from a valve job!
Valve seal means performance.......
Ac
Edited by ACDodd, 25 November 2016 - 01:22 PM.
