i think phil it totally correct, with cossies we all seem to always be looking for problems

with a N?A runabout ya run it on the cheepest fuel ya can buy, check the oil when the little oil check light comes on when taking corners

with a cossie if theres a funny smell out side in the country ya pull over and call the AA out to be told its the farm over yonder

 

So If the height of head is below 138.68mm on std pistons, will there be any valve piston contact?.. or is it simply the valve seat area which becomes the issue?

 

stu / karl

early cosworth group a 2wd pistons pa1079/6.2cr i think, had only very small squish assisting raised area on top. also they stopped about a mile short of the block face. whats going on here then, does a lower static cr mean less squish required?

excellent tread BTW

 

Not quite, i think you are getting confused about what the where the squish area is: (this is how i've understood it so far)



The red blobs on the leftmost diagram show the squish area, as the piston travels up the bore, the mixture is guided towards the centre of the combustion area. Now if you skim the head alot (middle diagram) or remove the squish area, there is alot less (sometimes nothing) squish area to guide the mixture to the centre of the combustion area. Adding a decompressin plate does nothing to aid this, as it is just an extension of the bore, it makes more space between the top of the piston and the cylinder head, but doesn't put back your squish area

 

In teh picture above, in the middle example where you have skimmed a slice off, in reality all you will have done is dropped the whole head down "X"amount. The point where the piston meets the flat squish roof will be in EXACTLY the same place. The top of the bore (or block if you like)

When you add the plate in picture 3, you actually RAISE the head by an amount.

Does that help?
Maybe if you see where the squish area is on a YB cylinder head?

 

Jees, we all came back at once

 

a VERY rough sketch of the squish area if it helps..

 

wes, I agree 100% with you, your drawing is indeed more accurate and representative.

Now, if the decomp. plate reproduced the slope (As it was suggested above), would that be ok ?

As for worrying... I worry that if my engine blows up I will have to spend ŁŁŁŁ's putting it back together, so I'd rather make sure it's as good as I can get it whenever it's in pieces.

If that means a new head, rather than taking chances with 16 valves or holes in pistons, a new head is the cheaper long term alternative - it's not worrying, it's financial planning...

I am actually in the process of putting my other half's car back together, and have two heads that are both below min. thickness... (hence my keen interest in this post ). Now do I take a chance with either and cross fingers, or, do I fix it properly now ???

 

We did all come back to this together didn't we ? Had enough of the muppet room for the evening ?

100% with you here.

what if you raise it by the X amount you took off above ? That's what I don't really get.
Only thing that's clear in my mind is that the physical shape of the bit you take off when you skim is not the same as that of the decomp. plate.

 

To get surface hardness you must have to heat treat the head? i don,t know of any alloy filler rod that will work harden with what ever surface finish/ prep you give it.

I weld the cylinder bores on 2 stroke cylinders prior to the surface being Re plated ( Ceramic coating) and know only to well how this affects the strength of the substrate alloy.

 

but also

I think it would be quite difficult to reproduce the squish area by shaping the decompression plate, manufactures spend millions on design so these things work properly! I would scrap the decompression plate idea, too much to go wrong.

If the head has been extensivley worked on then i'd ask Karl about the welding and maching to get it back to original thickness, if it's a bog standard head (apart from the skimming) then again either talk to Karl or source 2nd hand ones, and keep the skimmed ones for a rainy day (when all the good yb heads run out )

 

I think I have scrapped the decomp. plate idea last night... still keen to understand why it's a bad idea though... Just being curious...

 

Well lets just simplify it and say its hard enough to get a YB to seal with one fooking head gasket, let alone two of em.

Anyone care about squish given that comment?

 

Nope

 

 

BRILLIANT thread!

Stu - you say that you get a better burn if the fuel goes to the centre of the piston therefore create a bowl in the piston rather then skimming the tops of the pistons so would it be worth doing this to a mahle in a cvh engine rather then skimming them flat!?

Karl - can you also weld onto a cvh head too? also are we still on for saturday please say yes

 

I think my Question was answered on page 1!

Might be a good thread for the technical essay section?

 

i have reclaimed head by various methods , but would be seriously sceptical about having the face completely welded and re machined simply because of hardness issues thats not to say it cannot be done , karl, you must have done this succesfully otherwise you wouldnt mention it , just one of those things i wouldnt have done till id tested it myself

though i must admit ive rewelded stuff and machined it back to good as new that alot of people said was scrap

 

Could you elaborate ? I am in need of ideas and solutions that do not involve buying a new casting...

 

1) rarther than a decomp plate, would one of those copper head gaskets (is it ferriday make them? ) be a viable alternative, as there is no gasket as such, and the squish area could be built back in by shaping the gasket?

2) does the squish area on a 2wd and 4x4 head vary? I know they are diff in combustion chamber design, ie pent roof and rounded.

3) do valve cutouts in pistons promote hotspots and det?


awsome topic, keep the tech info coming best read i've had 4 ages

 

From what I have read on their website, you have two options.

-Copper headgasket, only up to a certain thickness.
-Decompression plate, thicker than H/G, but then you need a "normal" i.e. non-copper head gasket to go with.

Their copper headgasket has fire rings (or something else) machined in it, so they can't make it too thick.

 

I think people are still not following what part of the head makes up the squish here, but i cant think of a good way to put it into words....

 

I've been trying too Stu.. seems so easy, eh..?
I thought my dodgy sketch may make it clearer..



if you have a spacer plate with a 90-odd millimetre circular bore (ie to the gasket line on the picture), the shaded areas of the head (the squish area) will be spaced apart from the block face (and obviously the piston crown)

 

I'm understanding that Rich

So, if the plate is made to include the bits you have shaded, rather than just round holes, does that make it OK?

(back to what I originally asked )

 

lol.. i've been refraining from suggesting that..
yeah, in theory, but the unsupported area would just not be stable enough IMO.. and we are then missing the point - the primary problem isn't a 'dangerous' compression issue (makes around 0.2 CR increase at minimum thickness or thereabouts) but rather the issue of breaking into the valve seat inserts.. so the plate would not help that at all..

 

Here's a question...........

If the head has been skimmed to it's limit, does this mean that where the squish area was dome shaped, but now has a flat step at the valve seat edge, that yo're gonna be more prone to hot spots in that area?

 

Rich,
How do you mean 'dome shaped'?
The squish area on the head is flat in both cases, the skim makes very little difference to the squish area, it gets marginally wider towards the valve seats as you machine into the sloped chamber wall, but very little changes..

138.8mm:



139.2mm:



you can see the flat squish area above the inlet valves in these pictures is almost identical..

 

I think I followed all that and finally managed to understand the squish thingy..
Have made a drawing in power point... so asking the man to covert it into a .jpeg or .gif to post it...

 

I know its quite a bad drawing but the way I see it is the first drawing is a "normal" head, never been skimmed, with red area being the squish area.

Second drawing, head been skimmed and the white bit is the part of the squish that has been "lost" in the skim. Am I right there?

Third is with a decomp. plate added (red). As the plate is around the valve area only and does not replace the squish that is missing(white area), the head has just been raised... still ok?

Fourth is I guess the sort of plate that would be needed to get the squish back to normal....

Am I almost right or shall I get back to my kitchen???

 

Yes you've got it pretty much right, however the decomp plate shaped to the squish area will require 2 headgaskets, so there will also be a space between the plate and the head, which would lead to even futher problems!

 

why need 2 gaskets wes? the plate needs sealing to the head, but doesn't need a gasket as such..

 

Which other method of sealing could you use that would not leave a gap between the plate squish sections and the head?

 

Good, you've made my day... thanks!!

As Greg (my other half) said before, we're in need of a working head. So really, what's the best thing to do? Get a new head now while they are still available or try to have ours reconditioned?....

Because from what I understand, there is no way once the head has been skimmed to actually get it back to absolute "normal" like it was when new, whichever solution is chosen. I guess that would alsomean the car would need to be remapped then to compensate for the "loss of squish"...

How would that affect the performance and reliability (or lack of... ) of the car?

 

it's common practice with Zetecs for example to use a shim plate to reclaim compression ratio after machining, the shim is sealed to the head with a thin coating of metal gasket sealant which mainly serves to seal the fluids and compresses to virtually nothing when torqued up, and a gasket fitted as normal between the shim and block..

essentially the same as we used here (although we did the other way up, plate to block and gasket between plate and head):
http://www.rsrich.force9.co.uk/zetec...090014_jpg.htm
http://www.rsrich.force9.co.uk/zetec...090015_JPG.htm

 

If you can get hold of a brand new head then that is the best starting point, when i had my engine built i bought a new one, it was then machined to grp A spec, i didn't want to spend the money on the work to be done and then find the head would be scrap if it needed one skim at any point in the future. Next best option is to source a 2nd hand head that hasn't been skimmed ever.

You really should speak to Karl 1st though about the welding and refinishing, as a new head is about Ł1000 - Ł1500 + VAT!

 

Has anyone seen the little insert on page 10 of Performance Ford this month (april 2005) ?

There's something in there about Ferriday engineering being able to CNC machine the head's combustion chamber for a head that's been skimmed.

Can someone who knows what they are talking about check this for me please, better still, give them a call (mike 01785 621 710) and find out what the deal is ?

There's no point me calling him, whatever he'll tell me will sound like gospel cos I haven't got a clue

CheeRS guys.

 

for the morning crew.

 

just as a reminder... may be useful to get back to an early stage of the discussion that we've maybe strayed from..
-------------------------------------------
Quote from Stu:
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.
-------------------------------------------

the compression change isn't so much the issue as the machined face breaking into the valve seats... (compression increases to about 8.2:1 for a 0.5mm material loss).

spacer plates and chamber machining won't change this at all..

 

Rich, I agree the discussion is very interesting from a technical point of view, but I think a few valid points were made regarding resurrecting otherwise scrap heads. This is probably of significant interest to quite a few people since decent quality heads are rarer than hen's teeth.

At which point does the increase in C/R become an issue on a stage 3 engine running an off the shelf chip ?
Would live mapping be required to counteract the increase in compression ratio ?

 

Yes!! agree entirely! although IMO Karls welding reclamation is the only one relevant due to the seat exposure issues, which any form of decompression plate or chamber machining will not address.