Minimum cossie head thickness????
28-02-2005, 09:21 PM
#4
PassionFord Post Whore!!
Join Date: May 2003
Location: Wiltshire UK
Posts: 3,795
Likes: 0
Received 0 Likes
on
0 Posts
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)
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)
28-02-2005, 09:27 PM
#6
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??
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??
28-02-2005, 09:35 PM
#7
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.
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.
Trending Topics
28-02-2005, 09:39 PM
#10
PassionFord Post Troll
Join Date: Apr 2004
Location: West Sussex
Posts: 3,174
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by dave cos4x4
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.
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.
that the brass thingamybob can be knocked in with a drift so it should be ignored as proof of thickness.p,s cheers stu
28-02-2005, 09:41 PM
#11
Originally Posted by brad
Originally Posted by dave cos4x4
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.
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.
that the brass thingamybob can be knocked in with a drift so it should be ignored as proof of thickness.Have heard similar things.
WTF is it anyway...lol.
Dave.
28-02-2005, 09:56 PM
#16
10K+ Poster!!
Thread Starter
iTrader: (1)
Join Date: May 2003
Location: Lancashire
Posts: 12,748
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Stu @ M Developments
Daz,
Depends on application. Is it that depth on both sides?
Depends on application. Is it that depth on both sides?
The car is going to be built as a 7.5:1 for T34 and 30 psi on greys...
28-02-2005, 10:04 PM
#18
0-60 in 17 seconds (eek)
Join Date: Apr 2004
Location: Berkshire
Posts: 6,717
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
28-02-2005, 10:06 PM
#19
0-60 in 17 seconds (eek)
Join Date: Apr 2004
Location: Berkshire
Posts: 6,717
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Stu @ M Developments
Originally Posted by frog
Stu, what do you mean by diagonally skimmed ? I don't get it 
Set the machine up so you take more meat from the exhaust side than the inlet side basically.
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) ?
28-02-2005, 10:16 PM
#23
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
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
28-02-2005, 10:22 PM
#27
just finding my feet
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
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
28-02-2005, 10:27 PM
#31
just finding my feet
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
) and the arrow and a line to tell me what im looking at
28-02-2005, 10:33 PM
#35
0-60 in 17 seconds (eek)
Join Date: Apr 2004
Location: Berkshire
Posts: 6,717
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Stu @ M Developments
A very talentless illustration of the area that becomes too thin....
Things are much clearer now thanks - ginge how about you ?
Bowl the head out ???
28-02-2005, 10:38 PM
#37
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.
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
brad,
Stu do you ever finish workin m8 ?
Stu do you ever finish workin m8 ?
28-02-2005, 10:41 PM
#38
Originally Posted by dave cos4x4
Stu,
could you not resit the valve seats further in and lob a bit off the valve tips.
Dave.
could you not resit the valve seats further in and lob a bit off the valve tips.
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
28-02-2005, 10:44 PM
#39
0-60 in 17 seconds (eek)
Join Date: Apr 2004
Location: Berkshire
Posts: 6,717
Likes: 0
Received 0 Likes
on
0 Posts
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" ?
28-02-2005, 10:45 PM
#40
0-60 in 17 seconds (eek)
Join Date: Apr 2004
Location: Berkshire
Posts: 6,717
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Stu @ M Developments
Deffo a discussion i must have with Karl though if i can ever get hold of him
Anyway, time for bed now, thanks for the educational session stu