The V8 Owners Forum -

Hello all I have finally managed to get hold of a 4.2 to put in my Capri my first forts are to get the bottom end right before I put it in the car. I wont to raise the compression to around 10.5 my forts are to use the pistons available on ebay for £355 which also increase the engine to 4.3 but will this require new liners or just a re-boar or can anyone recommend a moor cost effective way of raising the compression many thaks Andy[/img]

Hi, and welcome to the forum.

Ok, firstly the only way to increase capacity is to increase either the bore or the stroke.

Secondly the 4.2 is actually closer to 4.3 but Land Rover just called it a 4.2

Thirdly the easiest and cheapest way to up the compression ratio is to change the composite head gaskets for the older type tin gaskets or fit 4.6 heads. Either of these is a better way of doing this as the +0.040" rebore needed for those pistons leaves you no room for further boring of the block. If something went wrong and the block needed a rebore your only choice would be to have it relinered.........anyone know how much that costs....Coops?????

http://cgi.ebay.co.uk/ROVER-V8-ENGINE-4 ... 45efc7a5a8

Just wondering what make they are?

Andy,

Here you go Pair of 4.6 heads £100.00 inc postage and probably not far from you.

http://cgi.ebay.co.uk/Rover-v8-cylinder ... 2306cbc07c

Make sure they are actually 4.6 or 4.0 which are the same with 28cc, chambers instead of 36cc chambers of the earlier heads.

If you buy the DJE pistons you have got to do a rebore which is another £250 so your out £605 before you start and the engine may need rebalancing unless the new piston weights are exactly the same as the original ones.

You would get a better performance gain by having some headwork done on the later heads.

You will need to use tin gaskets to get the increase in compression.

Kevin.

Spending money on pistons a few thou over isn't worth it. Being a V8 the machine work is much more expensive being harder to set the block up and obviously twice as much as your average 4 pot, a bore would cost more like £600 I'd have thought Kevin, thats what I was quoted a while back but did include plateau honing too.

Spend the money on some decent heads, inlet manifold & eddy carb (or injection), nice headers and ARP's if you are thinking about nitrous or using it hard.

Don't forget +.020 pistons are not available for the 4.2. If you end up down that route you will need +.020 3.9 low comp pistons then machine some off the height and the skirt.
Otherwise you are into +.040 piston territory like those from DJE.
I always thought +.040 was a no no.
I think the guys are right, the easiest way to increase CR is later 28cc heads with tin gaskets. But I don't think that on its own will get you to 10.5:1. I've lost my spreadsheet that gave me all the calculations for different combinations.
Mike

TC

£250 is what I paid for a 36thou overbore and plateau honing to fit Chevy 305 pistons.

Kevin.

Thanks so much for all the answers it seams 4.6 heads are defiantly the way to go. Can you also skim the heads as well to increase compression then shim the rocker posts to get a higher compression[/img]

That seems cheap Kev! If you fit 4.0 or 4.6 heads with tin gaskets the compression ratio will be higher than standard and you will probably have to shim the rocker shafts to get decent geometry anyway. I think you would get better results from some headwork and fuel delivery than just upping the compression ratio (past a point).

Shimming the rocker posts will only affect the valve gaps / pre load /geometry as Tom says!

The only reason I mention shims is if the heads are skimmed this will take away some of the clearance so shims should get the clearance back with in tolerance .I have not said much about head work yet as my plan for the engine is to rebuild it with arp bolts and a fast road cam kit , webber 500 carb and manifold with a nice set of headers.
With the heads I am just planing to use a standard set of 4.6 heads which I will polish and port mach as well as possible and run it like that for a year or so then I can buy some stage 3 heads when I can afford them

if you are going to overbore I would go the chevy piston route, there is a far better choice of pistons and you can go for chevy small journal rods (slimmed down) while you are at it ending up with a better bottom end for the money spent.
Mike

I asked to my local machine shop to modify stock low comp 3.9 pistons to be used with 4.2 crank.

Normally I will have it next week. Post pics.