head skim

jaybe · Post by **jaybe** » Mon Sep 23, 2013 3:41 pm

hi all I am just wondering if anyone can advise me as to a safe amount I can have skimmed off my SD1 heads . I am not after huge power its just that I bought 2 x SD1 heads to go on my P6B engine and they have what looks like when rings have broke and ended up in combustion chambers on several cylinders . I still have the original high comp pistons fitted .

ChrisJC · Post by **ChrisJC** » Mon Sep 23, 2013 5:49 pm

If you take off 0.5-0.7mm you can use composite head gaskets and the compression ratio won't change.

Chris.

jaybe · Post by **jaybe** » Mon Sep 23, 2013 6:12 pm

well I have already bought full gasket set but the heads ones are the metal type . so can I use them or have to flash the cash again ? it was originally 10:5:1 comp with P6 heads and I thought I heard that putting on SD1 heads on lowers the compression ??

sidecar · Post by **sidecar** » Mon Sep 23, 2013 7:59 pm

My understanding is that Rover heads had two chamber sizes, 36cc or 28cc. Your old heads were 36cc as are the SD1 heads. In my humble 10.5:1 is really pushing the CR with todays modern piss poor fuel, if your cam is standard that will also push up the dynamic CR around peak torque which pushes the engine nearer to detonation, a CR of 10.5:1 with a wild cam would be OK. Your engine was designed to run on 5 star and will be right on the edge of detonation as it is. I would not skim the heads, I'd fit composite head gaskets to lower the CR to around 10:1 which would mean that you could run a decent advance curve with figure of 36 degrees all in at around 3000 RPM. I'd also not fit the outer row of head bolts as these cause the gaskets to blow into the valley area of the engine.

The only problem that you might have is that the lifter pre-load could end up getting messed up.

AJMHO!

ppyvabw · Post by **ppyvabw** » Mon Sep 23, 2013 11:51 pm

My humble (drunken) opinion:

I would say it depends on how much you have to skim off your new SD1 heads to repair the damage you mention. If it's just a skim to clean it up, then I'd just have that done and stick them on; if you're going to have to take a half a millimeter or more off, then certainly spend a few quid on thicker composite head gaskets to compensate. Your SD1 heads should have the same volume chambers as the P6 heads to my knowledge.

However, unless you know the history of the heads, you don't know for certain whether they have been skimmed before, or indeed if your block has been skimmed before, so to really know you ought to use a burette to measure them and calculate the compression accordingly. There are websites that do it for you like this one: http://www.csgnetwork.com/compcalc.html

sidecar · Post by **sidecar** » Tue Sep 24, 2013 4:34 am

Every 8 thou skimmed off the heads is worth 1cc out of the chambers.

mgbv8 · Post by **mgbv8** » Tue Sep 24, 2013 7:59 pm

Even if the rings have been in the chamber causing all sorts of damage, it should not have caused any damage to the mating faces. Your main concerns should be ensuring the valves seat ok, smoothing out the damage in the chambers and making sure the heads are flat.

If you want to use these heads and want to make sure everything is spot on then you will need to CC each chamber. But is this is for a road car you wont notice any problems running the engine with the gouge marks smoothed out.

We run the sdpare race engine with head that have damage like this. We took out all the sharp edges in the chambers gently with a dremmel / die grinder and they are fine. We run a 200hp shot of nitrous on these heads and the imperfections have not created any detonation.

jaybe · Post by **jaybe** » Wed Sep 25, 2013 4:42 pm

thank you all for your advise and will regrind valve seats and check them for distortion/warp , apart from the marks in head they look like they have done low mileage mind you I know that he has engines out of SD 1 dead banger racers