Anybody tell me what the minimum cossie head thickness is?

Thank you please.

 

138.68

 

how much does a skim generally take off then.?

How many skims roughly per head.?








Dave.

 

Start off new about 139.2mm dave, so 0.5mm or so (20 thou) potential to take off.
Unless you get lucky, that will be a couple of skims.
I just had mine done, it was unskimmed to start with and not warped, but just to get the face clean and free from the effects of corrosion on the surface round waterways due to an ageing gasket took just over 8 thou (0.2mm)

 

does the thickness of the head matter if your running pistons with valve pockets in ?? if so why ? (genuine question by the thick bloke )

 

Brad,
Its a misconception that the head becomes "Too thin, or weak" and also that its about the altered "Compression Ratio"

The issue is the area around the valve seat becomes dangerously thin after 138.68mm. As a last resort the head can be diagonally skimmed if the damage is around teh exhaust side, but a new casting is of course the proffessionals prefferred choice.

Im sure someone can pop some pics on here to illustrate a meaty head and a fooked one??

 

Is this all talk too about the brass thing in the head.

What is the best thing to use to clean the crappy bits of gasket off too, need this done on mine as fitting a metal gasket.

Don't want to get it skimmed just for the sake of a clean.



Dave.

 

So a head at 138.4 is pretty much scrap then!

 

Daz, could you not use the thicker gaskets.

Could be used for a low comp engine maybe, or , a normally aspirated engine.



Dave.

 

i heard from a reliable source that the brass thingamybob can be knocked in with a drift so it should be ignored as proof of thickness.


p,s cheers stu

 

Have heard similar things.

WTF is it anyway...lol.



Dave.

 

Stu, what do you mean by diagonally skimmed ? I don't get it

 

Yeah, lets see some "bent like a banana" heads.

 

Brad,
Thats deffo reliable info as i know a twat who does it and then flogs em

Dave,
Read the reason again why 138.68 is the ideal minimum to answer your own question...

Daz,
Depends on application. Is it that depth on both sides?

 

Sorry Greg, missed ya there.
Set the machine up so you take more meat from the exhaust side than the inlet side basically.

 

Not sure Stu. I have told the lad to measure both ends.

The car is going to be built as a 7.5:1 for T34 and 30 psi on greys...

 

Hope hes not using a 50p plastic ruler and rack of eye...

Mentioning no names cos he uses PF

 

Here are a few examples

this one is 138.48 mm



this one is 138.12 mm






I have no idea about that one though, but I believe it had only 5thou skimmed off form new.

 

Thanks stu, now I get it

Now, what about decompression plates ? Are they any good to make up the lost thickness (assuming there's enough meat around the valve seats as you said above stu) ?

 

All those look buggered

 

You're really a man of few words Stu!!!

 

You wanted examples of bad ones, so 100% success there !

 

If theres enough meat aroundthe valve seats, decompression plate is pointless and uneeded.

Anyway, its a poxy idea at best and totally ruins a carefully calculated squish area, ofton promoting det almost as well as lopping the top off of your pistons

 

what am i looking at

i just see shiny heads or dirty heads

wheres the circles ad arrows for the other members who may know know what there looking at

 

mmmmm, don't get it again !!! Technical essay time ???

 

138.8mm:



139.2mm:

 

no no no, just need circles and red arrows

stu essays are confusing and i have to phone uo and ask kenny to explain in a way i can understand

hes answer is usually ill leave a note for stu on his desk and if theres a problem he will ring ya

 

how can u get the comp ratio lowered without getting the pistons decked ?

 

Ginge, can't remember what the circles were for, these are just pictures I kept. I'd love to understand more about squish and stuff though

 

A very talentless illustration of the area that becomes too thin....



Hope it helps

 

i actually ment stick circles around what im looking at ( as i dont see what the difference is ) and the arrow and a line to tell me what im looking at

 

Bowl the head out to increase the clearance volume.

 

see thats for all the people that never knew

the other one looks thicker that richm showed at 139.8

 

taaaaaa muchly
right then children , lesson over
stu do you ever finish workin m8 ?

 

Not a bad drawing late at night - and not one you prepared earlier either
Things are much clearer now thanks - ginge how about you ?

Bowl the head out ???

 

Stu,

could you not resit the valve seats further in and lob a bit off the valve tips.



Dave.

 

Its a tad late to get into a technical essay about squish, but very briefly:

The squish area is the thinest area between piston and cylinder head and is usually engineered to be the full circumference of the piston.

The purpose of this very close area is to force as much of the potent fuel mixture to the center of the clearance volume as possible, so when the burn is initiated we have maximum progression from the kernel of the mixture with a nice even and progressive burn outwards.

Having loose pockets of fuel in the area around the cylinder walls can result in a mixture that can, when the cylinder pressures rise, spontaneously combust, and start to destroy the surrounding metal, much the same as is evident in some of the above pictures.

Often, removing squish will be deemed to have "Fixed" an awkward engine, when in fact, the lowered CR was in fact the alteration that had the effect. This lower compression cylinder with higher squish clearance is actually more prone to det than one left alone and bowled in most cases, due to the aforementioned reasons

The moral?
Get a Pro to BOWL out your pistons, not hacksaw teh fookin tops off

This is pleasure for me pal, i enjoy chattin shit to folks, its better than watching tv etc IMO.

 

Dave,
A very experienced machinist could indeed do this with some reshaping of the chambers, yes. However, ive seen a few done over the years and they tend to be a bit poor IMO with some valve seat failures recorded after havng it done. Thankfully none of my customers.

Deffo a discussion i must have with Karl though if i can ever get hold of him

 

So from the above, a dished piston is better than one that has four little pockets (from a combustion point of view), cos the one with the four pockets would appear "domed" rather than "bowled" ?

 

Surely worth it, aren't we going to run out of heads one of these days ?

Anyway, time for bed now, thanks for the educational session stu