3.9 V8 needs a rebore - options?

I'm sure badger will be able to give some much more educated advice than me - but i thought you could skim quite a lot off the heads the get the compression up? Did on mine to keep it the same after changing to composite gaskets

Yes, that's always an option. I was kind-of more thinking about alternative pistons, seeing as I'm going to be buying a set in any case. Spending anymore on getting the heads skimmed would probably get me into trouble with the financial controller at the moment! ;-)

Cheers,

AndyC.

I'd have thought that a head skim would be cheaper than special pistons, and probably wouldnt hurt the heads either. Cost me about £30/head to get mine done IIRC.

There are a few places selling stroker kits! ;)

Ok, if they are skimming the block anyways, then that is the way to go to get the c.r. up a touch - much more effective than head skimming due to the small head chamber. IF, and only if, it has tin head gaskets then you can replace them with composites and remove an absolute maximum of 35 thou total, from either head or block or a combination of both. Any more and the inlet manifold bolts (and gasket!) won't line up with the heads! Preferably get the inlet manifold faces skimmed at the same time, by half the ammount taken off the head. When skimming the block be aware of piston protrusion above the deck face at TDC - no more than 0.005" protrusion but preferably flush. You'll need to stick in a couple of pistons (without rings is ok) and the crank to see where the piston height is at the moment then skim the block accordingly. If you take, say, 0.010" off the block then take a further 0.020" off each head (leaves a margin of 0.010" for any future skimming required) then take

0.015" off each face of the inlet manifold it'll all go together nicely with composite gaskets and ought to give a c.r. of something in the region of 10.1:1, about the max for this engine. The max c.r. any given engine can run is just as much to do with combustion chamber shape and design as it is a function of octane rating. If you plan on running LPG, this c.r. increase will do it the world of good, but don't run on LPG for the first 1000 miles or you'll get pretty bad bore glazing! Badger.

Good point about the cost of a skim - maybe I can get a bulk discount! ;-)

Cheers,

AndyC

That's great, thanks Badger.

The engine is a 1993 3.9 so does have tin gaskets, but I was planning on replacing them with composite ones, so a skim of 10thou on the block and 20thou on the heads, along with 15thou of the inlet faces sounds like a plan.

I've got new pistons and rings on order, 9.35:1 from Real Steel, so once they arrive I'll head around to the engine shop with them, the heads and the crank to see what goes - I'm sure they'll love me for that! ;-)

Cheers,

Andrew.

If I'm reading you right Badger, you're saying that 10:1 is possible with stock 9.35 pistons and composite gasket by judicious head and block skimming?

I didn't realise that, any particular reason for it?

AJH

Yep. Use the formula for calculating c.r. c.r. = (sv+ccv)/ccv and work it backwards from a known cr (comp. ratio) and sv (swept volume) to find the ccv (combustion chamber volume), then deduct the volume removed from the ccv by decking the block (v = 3.1406 x radius squared x height), do the same for the thickness removed from the head but take approx. 70% of this figure due to the chamber being smaller than the bore, add the two together and subtract from your original calculated ccv and re-work the formula remembering to add the extra capacity for the rebore. Subtract 0.4 from the new cr figure to allow for the composite gaskets and that is your result. It might not give exactly 10:1, but the way I've suggested is a safe method. The deck can be skimmed by 0.025" if there is no piston protrusion, the heads can also be skimmed by 0.030" to compensate for the thicker gaskets (reducing the chamber volume at the same time) and figures of near-on 10.7:1 are theoretically possible.

Apparently, the correct (not rich!) wet mixture of petrol/air actually aids the transfer of lubricant around the compression rings whilst bedding in, whereas gas doesn't and glazing can occur. That's what I was told by someone with considerably more years experience than myself, a person I respect and trust, and it seems to be the general concensus of opinion so I'm happy to go with it.

Badger.

That's great, thanks Badger.

The engine is a 1993 3.9 so does have tin gaskets, but I was planning on replacing them with composite ones, so a skim of 10thou on the block and 20thou on the heads, along with 15thou of the inlet faces sounds like a plan.

Check with pistons installed, you may well be able to take a bit more off the deck if you wish, and the heads, see my other reply below. Also, check the fit of the inlet manifold before skimming it as some castings are machined to looser tolerances than others!! Badger.

3.1416... not that it makes much difference.

Exactly the same as I've been told and also experienced.

On or around Wed, 23 Apr 2008 23:06:19 +1200, EMB enlightened us thusly:

Mmmmmm... Pie...

Yep, typo pie at that! It's actually 3.1415927...... for some reason I typed 0 instead of 1...!

Badger.