head skim

rscob · Post by **rscob** » Sun Jan 03, 2010 4:19 pm

how much do i need to take off when having the heads skimmed,its a rover 3.9 v8 9.35 cr and i'm going to fit the composite type gaskets,btw happy new year everbody. rob

unstable load · Post by **unstable load** » Sun Jan 03, 2010 4:44 pm

Rob,
There is no minimum amount to remove. The machinist should check the warpage with a straight edge prior to setting the head/s up on the mill then he will machine until they are uniformly flat and all the "old" metal is removed.
There is however a MAXIMUM limit that cannot be exceeded when skimming and it is dependant on the manufacturer's specifications. This is to prevent breakthrough into the water jacket and or oil galleries if too much meat is removed. If the heads are a first skim then you will be fine.

SuperV8 · Post by **SuperV8** » Sun Jan 03, 2010 5:21 pm

There are two machined register points on the head at each end you can use to take a measurement from. The tolerance is in the workshop manual. I have it for the 4.6 but not the 3.9.

Although I was told by my machine shop that these aren't always that accurate.

ChrisJC · Post by **ChrisJC** » Sun Jan 03, 2010 5:31 pm

I believe Rimmers state that a tin gasket is 0.5mm and composite is 1.2mm.

Chris.

sidecar · Post by **sidecar** » Sun Jan 03, 2010 6:19 pm

Going to comps is a good move, taking about 20 thou off will get the CR back that you would have lost due to the thicker gasket. As you CR is only 9.35:1 you could take about more off to up the compression. Maybe 30-40 thou.

rscob · Post by **rscob** » Sun Jan 03, 2010 6:33 pm

40 tho sounds good,but what sort of compression ratio would that give me,it also has a piper 270/2 cam fitted. rob

sidecar · Post by **sidecar** » Sun Jan 03, 2010 7:07 pm

rscob wrote:40 tho sounds good,but what sort of compression ratio would that give me,it also has a piper 270/2 cam fitted. rob

That would take a bit of working out, I'll do it tomorrow if nobody else works it out first.

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

Do you know if your heads are 28cc 10 bolts jobbies or are they 36cc 14 bolts heads?

Really the "proper" way to do all this is to burette the heads and the volume above the pistons but I think that you will be OK as long as you are not going for the absolute max compression.

I run a 4.6 with a piper 285 cam and 10:1 CR, my heads were 36cc ones, they've had 70 thou skimmed off them!

Pete

kiwicar · Post by **kiwicar** » Sun Jan 03, 2010 7:25 pm

roughly speaking the gasket change is reconed to result in a change in CR of about 0.7:1, as that change in thickness is about 20 thou of an inch, taking a further 20 thou off should put the final CR at about 10.05:1, probably be a little more as the chamber tapers in toward the roof but only a little. As 10.5:1 is about the max you want with a standard cam and you are using one with a little more duration you should be OK. However you will need to check the valves don't hit the pistons, I think they are a fair way away on the rover, however you will have made 2 changes to the engine that will bring valves and pistons closer together so you would be advised to check first. Also you will need to check lifter pre-load and space the rocker shaft posts to compensate for the skim.
Best regards
Mike

sidecar · Post by **sidecar** » Mon Jan 04, 2010 8:58 am

OK I’ve worked a few things out.

The total volume above your pistons now when using tin gaskets is 58.9 cm3 in order to give you a CR of 9.35:1 (I worked this out by using a cylinder volume of 492 cm3) The actual head volume is not important, it is the TOTAL volume above the piston that matters. In other words the head volume + the gasket volume + the piston dish (also taking into account how far down the bore the piston is at TDC)

I thought that comp gaskets were 1.0mm thick but as someone has posted up saying that they are 1.2mm thick I’ve worked out some CR’s for both.

The gasket volume of the “tins” is 3.4cm3, it is 8.3cm3 for the 1.2mm comps and 6.9cm3 for the 1.0mm comps. (94mm bore)

Everything that I’ve worked out relies on the fact that Rover are telling the truth when they state that your current CR is 9.35:1 on tin gaskets.

If you just plonked on the 1.2mm comps and did not skim the heads at all the volume above the pistons would be 63.8 cm3, this would give a CR of 8.71:1

If you just plonked on the 1.0mm comps and did not skim the heads at all the volume above the pistons would be 62.4 cm3, this would give a CR of 8.88:1

If you fitted 1.2mm comps and skimmed 40 thou off the heads (5cm3 reduction) the volume above the pistons would be 58.8 cm3, this would give a CR of 9.36:1

If you fitted 1.0mm comps and skimmed 40 thou off the heads (5cm3 reduction) the volume above the pistons would be 57.4 cm3, this would give a CR of 9.57:1

To get the CR up to 10:1 the TOTAL volume above the piston would have to be 54.4cm3.
On 1.2mm comps you would have to skim of 75 thou, with 1.0mm comps to get the same CR you would have to skim off 64 thou.

To be honest I would not want to push the CR up into the very high 9’s or 10:1 when just relying on Rovers 9.35:1 as a starting point. I would be buretting the piston dish volume at TDC and the head volumes as well. You can certainly go for 9.5:1 or even slightly higher without having to worry about measuring volumes.

Also bear in mind that even though you might take 1mm off the heads the valves won’t end up 1mm closed to the pistons because the gaskets are thicker. (They maybe approximately 0.5mm closer)

You might have to use shims to sort out the pre-load, personally I don’t like shims and have used adjustable pushrods on my engine but others have used shims and have found them to be OK.

HTH,

Pete

sidecar · Post by **sidecar** » Mon Jan 04, 2010 9:57 am

sidecar wrote:Going to comps is a good move, taking about 20 thou off will get the CR back that you would have lost due to the thicker gasket. As you CR is only 9.35:1 you could take about more off to up the compression. Maybe 30-40 thou.

Just a thought....Your engine IS on "tins" at the moment???

If it is already on comps then everything that I've worked out will need doing again!

The reason that I ask is that the 3.9 engine is not in my area of knowledge, I had a 3.5 and now have a 4.6 so they are the lumps that I know about.

Pete

rscob · Post by **rscob** » Mon Jan 04, 2010 6:46 pm

wow great reply[you must have a lot of spare time on your hands],one more question how do i know if the heads have been skimmed already,before i go and get 50 tho off and find the valves hitting the pistons.thanks for the reply . rob

ChrisJC · Post by **ChrisJC** » Mon Jan 04, 2010 6:58 pm

Get a copy of the Land Rover V8 Overhaul Manual, and it gives you the datum points on the heads for both tin and composite gasket heads, or:

The datums are areas at the end of the heads that look like the obvious place you'd clamp the head if you wanted to mill it. They are taller on the inlet side.
For the 4.0 / 4.6 engines (composite), the dimensions are:
22.94mm and 62.56mm with a reface limit of 0.5mm.
For the 3.5/3.9/4.2 engines (tins), the dimensions are:
23.9mm and 63.5mm with a reface limit of 0.5mm

Chris.

sidecar · Post by **sidecar** » Mon Jan 04, 2010 7:45 pm

rscob wrote:wow great reply[you must have a lot of spare time on your hands],one more question how do i know if the heads have been skimmed already,before i go and get 50 tho off and find the valves hitting the pistons.thanks for the reply . rob

I worked it all out whilst I was at work!

Like others have said, there is a register on each head that you cam measure to determine whether the heads have been skimmed.

Are they 10 bolt heads?

If they are then it is very likely that they have been factory skimmed so the lump that you need to measure maybe less than some of the figures quoted. From memory the 36cc heads have a lump that is 930 thou thick. I GUESS that the 28cc ten bolt heads have had around 60 thou taken off them at the factory so the lump will be around 870 thou thick. Like I said I'd rather trust a burette!!

Also (and I may have said this!) the real 'proper' way to do this sort of stuff is with a burette.

Pete

rscob · Post by **rscob** » Mon Jan 04, 2010 8:58 pm

sidecar wrote:
rscob wrote:wow great reply[you must have a lot of spare time on your hands],one more question how do i know if the heads have been skimmed already,before i go and get 50 tho off and find the valves hitting the pistons.thanks for the reply . rob

I worked it all out whilst I was at work!

Like others have said, there is a register on each head that you cam measure to determine whether the heads have been skimmed.

Are they 10 bolt heads?

If they are then it is very likely that they have been factory skimmed so the lump that you need to measure maybe less than some of the figures quoted. From memory the 36cc heads have a lump that is 930 thou thick. I GUESS that the 28cc ten bolt heads have had around 60 thou taken off them at the factory so the lump will be around 870 thou thick. Like I said I'd rather trust a burette!!

They are 14 bolt heads,i don't know what gaskets were previously fitted as the heads were off for inspection of the bores etc when i bought the engine,i have put a steel rule across the heads and both are okay[ ie can't get the feeler gauges under any part of the heads]can anybody post a picture or diagram of the part of the head i need to measure to check if its already had a skim,thanks rob.

sidecar · Post by **sidecar** » Mon Jan 04, 2010 9:09 pm

rscob wrote:

They are 14 bolt heads,i don't know what gaskets were previously fitted as the heads were off for inspection of the bores etc when i bought the engine,i have put a steel rule across the heads and both are okay[ ie can't get the feeler gauges under any part of the heads]can anybody post a picture or diagram of the part of the head i need to measure to check if its already had a skim,thanks rob.

If they are 14 bolt heads then they must be 36cc chambers, I guess you need to check the size of the lump of ali that sticks out of the side of each head, if it is 930 thou then they have not had a skim carried out by anyone. V8 Development can confirm the 930 thou size, they told me the figure (or a figure) a couple of years ago. (Sorry, I don't have a picture)

I also guess that the 9:35:1 that Rover specified is for tin gaskets as the 28cc chambers were meant for engines that used the comp gaskets.

I guess that you know not to use the outer head bolts even if your block is drilled and tapped to take them. (Dunno if 3.9 blocks have the holes tapped, like I said 3.9 lumps are not "my thing")