Mk3 Compression ratio following skim

[Sandy Gibson]

I am going to get my head skimmed following blown gasket issues.

It is casting no. 515972 with  a head depth of 2.96” which I believe is standard and should be giving 9:1 ratio.

My question is how much to get skimmed?  All I want is a flat surface but should I suggest .040” and would that still be within sensible limits?

Bit in the dark on this one. Any suggestions would be most helpful.

Thanks

[johny]

The shop should tell you how much they need to take off to achieve the correct surface and this will depend on any corrosion damage and/or distortion of the head. Now if you want to increase CR then thats another matter as it runs the risk of breaking into an internal waterway or ending up with too high a ratio....  

[Pete Lewis]

yes i echo that    let them make a suggestion    upping the CR too far will bring a string of "now what" problems 

remember CR  timing fueling are all a ballanced act  move one and it upsets all the others 

Pete

[johny]

Not to say, more likely to blow head gaskets!

[Sandy Gibson]

I appreciate that the head castings will all be individual, but if the shop were to suggest .040” would that be a safe punt?  Don’t want to risk creating more problems than I’m trying to solve!

 

[Pete Lewis]

to just skim to resurface  0.010  would be more practical    0.040 is a big chunk to take  off

Pete

[johny]

If they say that is the minimum necessary to achieve the desired finish you havent got much choice, well except try another place I suppose....

[johny]

Also if they say thats whats needed and you give them the go ahead the responsibility if theres a problem will still be yours but its got to be done hasnt it...

[Sandy Gibson]

Thanks for that folks.

I have a known good spare head I can swap over if it all goes pear shaped so I’ll probably go for the minimum the shop suggests and keep my fingers crossed!

[JohnD]

Sandy,

Better than taking a blind punt at some  random skim is to measure the chamber volumes, and do the calculations!   All the kit you need is a square of glass and a burette, the first from the waste bin of a local glazier and the second from eBay.

See my article at https://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/

John

[Iain T]

On 13/08/2022 at 18:12, Sandy Gibson said:

My question is how much to get skimmed?  All I want is a flat surface but should I suggest .040” and would that still be within sensible limits?

Sandy, the best and most accurate way is to use JohnD's method but if you want to calculate this is a useful spreadsheet. You do need to know if your bores are standard size and according to my Haynes book your engine should as standard be 9.5:1. It's for a 1500 engine but the math is still valid. I had 40 thou skim from a standard thickness head.

Iain

192003111_CRheadskimcalculator_v3-2.xls

[Nigel Clark]

John's advice is the gold standard, and essential if you're determined to extract maximum performance on track.

However, for a road engine, a straightforward skim to get the gasket face level and clean is all that's needed. Usually 5-10 thou is enough. Start raising the CR significantly and yes, there are performance benefits but as Pete has said, timing and fueling will need to be optimised for the new setup. Plus the engine will only be happy on 99 octane fuel.

Nigel

[Pete Lewis]

if you wantfor some reason  to raise the CR  then the calc's  are vital     

if you just need a minimum skim  to  clean the  face then they are not 

question  why do you need a skim ..is there some face corrosion ????or  what  

Pete

[johny]

I think theres been blowing head gasket issues so maybe warped although wouldnt that be unusual seeing how 'solid' our heads are?

[Sandy Gibson]

I bought the engine to rebuild as a spare. PO advised it had broken rings and this proved correct (badly ridged cylinders)

I had block rebored and fitted the original head after cleaning it and reseating valves etc.  Faces looked fine and could not measure any distortion with feeler and straight edge.

After 5 minutes running, got a sump full of mayo.  
 

Have check all the bottom end and can’t find any issues so now I’m looking at the head.  That’s the background and where I am now.

Sandy

[Pete Lewis]

very unusual for a sump full of mayo from a head gasket  

or did you use the correct gasket   if its a flat deck you must not have a tab at the rear end of the gasket 

you only use  a  tabbed gasket for recessed bores  mix them incorrectly  and you get instant disaster

some rocker cover studs break into the coolant passage  but thats a small leak

you must have had a gusher   which doesnt sound like a head face problem

Pete

[Sandy Gibson]

Pete.

Totally agree with you.  Gasket was the correct Payen (non recessed) and torqued up correctly.   Don’t think the problem is a cracked block as the original oil was in the sump when I bought the engine and it was black.

Really stumped with this one and now looking at the head.

I also have another option to fit another known good head and see what happens.  If I get the same problem, at least I’ll know the issue is somewhere in the bottom end.

Sandy

[Sandy Gibson]

This was how the gasket looked when I lifted the head.  

[Pete Lewis]

there s not a lot of the signs of gasket being compressed  its a bit flat all over  no distortion around the ports/holes  

Pete

[NonMember]

It may just be the photo but it really looks like the fire rings are proud of the gasket, which makes it a recessed block type regardless of what it was sold as.

[Sandy Gibson]

Getting about 041” on the rings and 038” round the outside.  About 003” more compared with another used Payen I had.

Dont know what a new one would measure.

Gasket definitely didn’t have the tab on the end and the head studs were all in the correct way up.

I tend to agree with Pete that it’s unlikely I could have lost a significant amount of water from a gasket leak in 5 minutes running but this is new territory for me, any help appreciated!
 

Thinking of trying a leak test on the block.  If I seal the top water channels with some plastic / silicone then lightly torque the head back.  Then compressed air through the bleed valve.  Given that I seem to have a gusher ( good description Pete)  do we think that might show up the source?

 

 

[JohnD]

Sandy,

Something has been inside the front chamber! Rolling about in a spiral pattern!

And, you'll need to have those pistons out, so inspect the bore walls very carefully, even getting a crack test dye kit or a local machine shop, who will have a kit, to look for cracks in the block.  You didn't say how much you had it rebored, but 'core-shift' in original manufacture can leave some blocks intolerant of any overbore, as the wall is left too thin.

John

PS Ah! you're already thinking that way! Sad if it is cracked, as then it's scrap.

[Sandy Gibson]

John

Rope trick marks!!

Block was rebored to .040” which I know is getting close to limit but there was certainly not visual evidence of problems.

Do you thing a compressed air test on the block would show up problems?  Obviously the block would be cold and I appreciate that wouldn’t help.

Sandy

[Chilliman]

If you've got a cracked block allowing that much coolant out - the fact it's cold wont make a huge difference, a pressure test will still manifest itself somewhere, although I do doubt that as the problem unless the block has had some serious abuse. 

[Pete Lewis]

we wait with bated breathe      still reckon theres a gasket problem it just looks like its just come out the packet  

theres not much sigh of it being compressed 

Pete