11 replies to this topic

[woody17]

i'm doing my head gasket and im getting my head skimmed while its off, because i think i have warped it.

someone said to me, ohh at least you will lower the compression, make it go faster!

this made me think--hmmm, what is the deal with skimming, disadvantages/advantages?

[tuktuk]

skimming it will raise the compression. it needs to be calculated properly, taking piston dish, ring land volume ect into account.

how much is being taken off ?

advantage - having a flat surface to get a good gasket seal

[Strontium Dog]

As long as you don't over do it skimming the head is a good thing. According to Mr. V it helps the A series head flow better and raises the compression which, unless you go to far, is also good. It provides a 'new' surface for the gasket to seat on and as long as the block is true and the head is torqued down correctly the head should not warp again. I always back of the head nuts a quarter turn or so and then re torque after a few miles. Let the engine get to temp. then cool down a few times and re re torque when cold!

[dklawson]

Let's pause for a second for some terminology:

Shaving the head: Removing a predetermined amount off a cylinder head (after combustion chamber volume measurements) for the sole purpose of raising the compression ratio.

Skimming the head: Removing a minimal amount from the head (typically after applying a colored indicator dye/fluid to the surface) so the resulting machined surface is restored to flat.

Yes, skimming will raise the compression ratio HOWEVER, the cut is so small the impact is minimal. Also, without the very careful measurements, the amount of the compression increase cannot be calculated. In short, you won't gain any significant power from skimming the head. The amount removed is kept to a minimum and if your head is not too warped and the machinist fixtures the head well... this can be as little as a 0.005" cut.

Shaving the head is a much more aggressive approach. The amount shaved off the head is calculated and it would not be unheard of to remove as much as 0.050" (10 times what you would do in skimming). Skimming is intentionally a minimal amount of machining... shaving is intentionally a calculated heavy cut.

[woody17]

ok, so what is the best for me to do?

[tuktuk]

get advice of exactly how warped the head is and go from there. or source another head

its only going to be unusable if its banana shaped.

[blacktulip]

to get a "banana" shaped head would only occur if it had been subject to serious overheating. if you didnt overheat it then a skim is probably all you will need, and thats if it even needs doing at all.

as said, the best thing to do is to get it checked first as you dont want to be spending out if not needed

[woody17]

after doin my head gasket, it has gone again, so a skim seems the only option

[Retro_10s]

Are you torquing it down correctly, and then re-torquing after a shake down in?

[Jordie]

68Nm or 50lb ft

Some engine builders say not to torque down after a "shake down". I was told to torque mine up and leave it and its been fine. Double check the bolts in order to make sure they are all the right torque.

Jordie

[woody17]

ok sent it off today, and he identified the problem immediately, one of them brass-looking-big-blob-type-rods-thingy-mi-bobs has been forced up, causing blowing between 2 and 3.

(not entirely sure that is the correct technical word)

[dklawson]

Yes, the thing-a-ma-bob or doo-flotchy is a brass widget on the cylinder head that does seem to "sink". I'm not sure what their purpose is. I've always assumed it was to plug various holes needed in the casting process. Hopefully the shop will be able to pull your old one out and either re-insert it or make you a new one and install it flush.