The V8 Owners Forum -

im rebuilding my engine in my 98 discovery 3.9 efi

the gaskets that were on are composite gaskets the new ones in my gasket kit are metal ones is it ok to use these it says landrover 3.9/4.2 efi on them

which are the best?

Thanks
Si

Not really.

The heads on a 98 RV8 are machined to take the composite gaskets. If you fit the tin gaskets you will raise the compression ratio, risk the inlet manifold not seating correctly, upset the lifter pre-loads etc.

This is normally OK if you are re-building a modified engine but for a straight re-build you should fit the composites - they are a much better gasket in any case.

If you have been sold the kit for your engine, having told them its a 98 rather than just a 3.9 Disco, then you have been wrongly supplied.

Russ

the tin gaskets are thinner, this gives yo a bit more compression which can be usefull, however you need to put shims under the rocker pedistals to allow for this otherwise you will have too much preload, as you would be setting this anyway it is not any extra work, just be aware that this is why you need more shims. If this rebuild involves a much higher lift cam you will need to check valve to piston clearance but if it is standard or near to it then the valves don't get close. remember to leave out the outer 4 head bolts on each bank.
Best regards
Mike

Don't use the tins! Even Rover junked them because they are more prone to blowing.

sorry to thread hijack but just a quick question.

I want to get the compression up on my 3.9 with 9.35:1 pistons. I have a set of early 3.9 heads and a set of serp 3.9 heads with smaller chambers. I obviously would prefer composite gaskets too. I would like at least 10:1 compression ideally so that's 0.65 I need to find in the first place, plus say 0.6 to compensate for the composite gasket, plus any polishing I do in the chambers.

How much can I realistically take off the bottom of the heads? A millimetre, two millimetres?

I reckon there's about 40 tho to take off the deck.

cheers for the info will stick to the composite type

have got the mains from landrover and they are wrong so thats a good start they were 160 too so im quite pleased hopfully it will want the cheap ones lol

Si

check the discount section of the forum mate,
can get parts a lot cheaper there mate.

ppyvabw wrote:sorry to thread hijack but just a quick question.

I want to get the compression up on my 3.9 with 9.35:1 pistons. I have a set of early 3.9 heads and a set of serp 3.9 heads with smaller chambers. I obviously would prefer composite gaskets too. I would like at least 10:1 compression ideally so that's 0.65 I need to find in the first place, plus say 0.6 to compensate for the composite gasket, plus any polishing I do in the chambers.

How much can I realistically take off the bottom of the heads? A millimetre, two millimetres?

I reckon there's about 40 tho to take off the deck.

Adam,

This can be quite complex as you have to know all the dimensions before you start taking large rashers off anything.

You need to know the volume of the piston bowl and an accurate dimension for the piston below deck height at TDC before you can start doing any machining.

The one piston i don't have data on is the 3.9

I do have some 3.9 pistons so could take an approx measure but this would not be 100% accurate.

If you can find out the piston volume and deck height i can make some suggestions, but the figures you mentioned removing would seem somewhat excessive.

Kevin.

Kevin.

kiwicar wrote: remember to leave out the outer 4 head bolts on each bank.

What is the benefit of doing this?

MAZVER wrote:

kiwicar wrote: remember to leave out the outer 4 head bolts on each bank.

What is the benefit of doing this?

The head gaskets won't blow!

The outer bolts that are level with the spark plugs do not have a coresponding bolt at the opposite side of the cylinder, because of this they put an uneven load on the head gasket. Rover dropped these bolts on later engines.

More on that. The head tilts, lifting it along the inner (valley) side. It then blows very slightly, and fills the oil with soot. If you don't change your oil often enough, the engine gets completely gunged up with tar. This is why you _must_ change your oil on these engines with tin gaskets.

Chris.

ppyvabw wrote:sorry to thread hijack but just a quick question.

I want to get the compression up on my 3.9 with 9.35:1 pistons. I have a set of early 3.9 heads and a set of serp 3.9 heads with smaller chambers. I obviously would prefer composite gaskets too. I would like at least 10:1 compression ideally so that's 0.65 I need to find in the first place, plus say 0.6 to compensate for the composite gasket, plus any polishing I do in the chambers.

How much can I realistically take off the bottom of the heads? A millimetre, two millimetres?

I reckon there's about 40 tho to take off the deck.

Assuming that your engine was running on “tins” and the heads with the larger combustion chambers and the CR was 9.35:1 then the piston bowls must be 12cc. Are you sure that the 3.9 used the larger chambers?

The CR is the total swept volume of the piston plus the volume above the piston at TDC divided by the volume above TDC.

I have just used the theoretical figures to work this out, in the real world you should measure it all. (Any mistake will either cause your engine to under perform or blow up! )

Swept vol of one cylinder = 3900/8 = 487 (is a 3.9 lump actually 3900 cc?)
Deck height = 1mm = 6.9cc (bore = 9.4cm)
Tin gasket vol = 3.4cc
Combustion chamber = 36cc
Piston bowl = 12cc

So the total volume above the piston at TDC is 6.9+3.4+36+12 = 58.3

The CR is (58.3+487)/58.3 = 9.35:1

That is also assuming that the deck height is 40 thou (1mm)

Once you know what the piston bowl and deck height is you can work out how much has to come off the heads but in your case as you have some smaller chamber heads you have more options.

OK, if you just plonked the large heads on with the comp gaskets the volume above the piston would go up another 3.4cc (the comps are about 1mm thick)

The CR would be (61.7+487)/61.7 = 8.89:1


If you used comps along with the smaller (28cc) heads the vol above the piston would be 53.7cc

The CR would be (53.7+487)/53.7 = 10:1 (In other words you would not need a head skim at all!)

If you wanted to use the comps along with your old 36cc heads then you need to lose 8cc of volume above the piston. I happen to know that every 8 thou skimmed of the heads will lose 1cc of volume, therefore you would need 64 thou off the heads. (Mine have had 70 thou off them). Note that I said HEADS, skimming the DECK 8 thou will lose about 1.7cc

Bare in mind that I know nothing about your engine, I have made loads of assumptions! I do not even know whether as standard your lump runs tins/comps, large chambers or small chambers

I would never put a head in for a skim without measuring everything first.

For this you need a burette, a TDI for setting the pistons at TDC, a piece of Perspex and Vaseline to seal the deck height when measuring the volume in the piston and block. You also use the Perspex to seal the combustion chambers.

Finally, Like others may have said, you must bare in mind what all this will do to the fit of the manifold and the lifter pre-load!


EDIT...Just found out that the 3.9 is actually 3947cc, each cylinder is 493cc, not 487!

Oh well, someone else can work it all out again!

Pete

Pete,

That always the problem, until you measure an individual engine accurately it all purely guesswork, the comp ratios Rover quoted can only be an average figure, if you look at the Landrover engine spec sheets the actual crank stroke, rod lengths, and piston compression heights can all vary by quite a few thou.

My data also shows the comp gasket for the 3.9 and later engines as having a compressed thickness of 46 thou. which equals 8cc. the 3.5 comp gaskets are 40 thou for approx 7cc.

Adam get it all measured and you can work it out using pete's formula.

kevin.

CastleMGBV8 wrote:Pete,

That always the problem, until you measure an individual engine accurately it all purely guesswork, the comp ratios Rover quoted can only be an average figure, if you look at the Landrover engine spec sheets the actual crank stroke, rod lengths, and piston compression heights can all vary by quite a few thou.

My data also shows the comp gasket for the 3.9 and later engines as having a compressed thickness of 46 thou. which equals 8cc. the 3.5 comp gaskets are 40 thou for approx 7cc.

Adam get it all measured and you can work it out using pete's formula.

kevin.

The Devil is in the detail!