1 reply to this topic

[Purple Tom]

Hi

I recently came across a 12G295 head in my travels, and snapped it up. Its in good nick, complete with valves, but it has no springs or rockers. What rockers can I use? Will the later forged rockers be ok, or am I better to find a set of the earlier pressed ones? Also, can I use double valve springs, or am I better to stick to single ones?

Will having the head converted to run on unleaded cause possible damage to the valve seats during the work, or am I best to leave it as standard and run an additive, Valvemaster or similar?

Thanks in advance

Tom

(oh yeah, how much do you think 5 off 4.5x10" Cooper S reverse rims are worth? They're not in bad nick, could do with a sandblast and respray)

[Dan]

Well, since the process of converting a head to unleaded consists of using a dirty great milling machine to chew the old valve seats right out, yes it will damage them! The valve seats are the parts of the head which can't cope with unleaded fuel, so you cut them out and shrink in some new ones made of a harder material which will resist the erosion caused by the exhaust gasses when the lead isn't present. You also need new valves on this head as the standard valves aren't unleaded compatible either. It will be cheaper in the long run to do this than to keep using a substitute and a lot more convenient. They will also fit new guides first (to centre the cutter properly) so you will have a nice, good condition head.

Also the 12G295 will need to be skimmed by a fair bit to raise the compression if it is standard currently. It will need the combustion chambers measured (which is called buretting), as standard on this head they are 28.3cc which gives very low compression on a standard 998 (the Cooper engine it was designed for had ultra high compression domed pistons). You need to knock about 3cc off this to fit it to a standard 998, but obviously you should measure your engine exactly to determine the volume required to get good compression. Then the machine shop will shave the head down till it's the correct volume.

I would steer clear of double valve springs on the 998 unless you are going for extreme levels of tune, as they will lead to a big increase in internal losses.

Regarding rockers, well the late sintered type are OK and the pressed ones are good but very rare. The forged Cooper version are very good indeed as they have good geometry and are the only standard rocker which maintain a 1.3:1 lift throughout the full range of travel AFAIK. They are also very strong and don't flex and have a good heel surface shape so reduce side loading on the valves somewhat, though not as well as roller tips. Mini Spares Centre have had these reproduced and they are the ones I run on my 12G295 998. You need the rockers and their own type of adjusters and locknuts. Also make sure when you pick them up that the bushes in each of them have the correct oil holes in, as I have had some before with no oil holes at all and that isn't good. A thick wall rocker shaft is a good idea too.