cylinder head over skim?

[johny]

There a MK1 Vitesse cylinder head going on ebay that has had a 2mm skim. It looks ok but out of interest is this a terminal amount of metal to have removed?

[NonMember]

It sounds quite a lot. Back of envelope calculation: stroke is 76mm, standard compression 9:1 and the combustion chamber occupies about 3/4 of the bore, so the original combustion chamber depth is around 11mm. Skimming 2mm thus raises compression by 20%, up to 11:1 ballpark.

[johny]

Isnt there also a risk of breaking into water/oil ways if taking too much off?

[NonMember]

Not oil ways, as I don't think there are any apart from the vertical one that feeds the rocker shaft. Water passages may be at risk but I'm not sure how thin the casting walls get. I'd expect the thinnest parts to be around the transfer holes through to the block, so enlargement or distortion of those would be the warning sign.

[KevinR]

need to be wary of core shift, it can result in very thin sections.  Best to get a head radio-graphed before and after serious skimming

[Nick Jones]

Question is..... do you need a cylinder head that has had a 2mm skim?

Might have some purpose if you are wanting to run a wild cam and need the compression ratio bumped way up to get the best from it. Otherwise, keep looking..... they are out there.

Not directly related but on the Mk2 heads, there are various thicknesses from the factory, the 2.5 ones being at least 2.5mm thicker than the (early) 2L ones. For reasons that seemed good at the time I modified a 2500S head, which is a whopping 4mm thicker than the early Mk2 2L head, for use on my flat piston 2L, plus I wanted higher CR. That was a BIG skim. I got away with it, though it’s scary thin under the spark plugs. However, I know others who were less fortunate and found water. One as an immediate hole at the outer edge of the squish area and another that showed as head gasket failure when the same area collapsed.

Nick

[Iain T]

I used this spreadsheet to skim my head to 10.5:1 

From standard it's 50 thou, roughly 1.25mm

192003111_CRheadskimcalculator_v3-2.xls

[JohnD]

Kevin and Nick are, of course, correct.    And the problem has always been there.

Kastner included an article on "Staking the Head" in his Triumph Preparation Handbook (p.47).      That referred to TR3/4 heads but the same is true of later sixes.   The technique is to drill through the face of the head, into the water jacket, but no further.    Tap the hole, and insert a bolt that bottoms on the far side of the jacket.   Then cut off the head of the bolt.  Kastner suggests filing the shank back to level with the head, which implies skill with a file that I don't possess!  I'd have it reskimmed.  He also  recommends an epoxy glue to help seal the threads - I wonder if a modern high strength threadlocker (Loctite 272?) might not be better today.

On how much skim is too much skim, Kaster says in the same article that more than 150 thou (nearly FOUR mm!!) is excessive.

Iain,   That spread sheet looks most useful, but I cannot see how you get the height "to mill off" from the "cc to remove".      This because in the version you posted, line 38 is hidden!    That includes the cell "cc per mm", which is labelled "Approx", never a good word to find in a calculation.   I don't disagree with your method, but that approximation must be corected for the spreadsheet to be useful, or accurate.

To be accurate, the Area that the combustion chamber presents to the bore must be measured.  The best way to this, IMHO, is to take a tracing of the chmaber edge onto fine graph paper.     A soft pencil, or a dirty thumb will leave a fine impression!     Then the labourious counting of graph squares will give the area, +/- 0.5 square mm.      Once this value is known, the the calculation of Volume = area x height may be done without the caveat of "Approx"!

I humbly suggest you adopt this method, and correct your spreadsheet!

John 

[Iain T]

5 hours ago, JohnD said:

Iain,   That spread sheet looks most useful, but I cannot see how you get the height "to mill off" from the "cc to remove".      This because in the version you posted, line 38 is hidden!    That includes the cell "cc per mm", which is labelled "Approx", never a good word to find in a calculation.   I don't disagree with your method, but that approximation must be corected for the spreadsheet to be useful, or accurate.

But if you read the note in cell B35 it does give a fuller explanation from measured data...

If you want to be really OCD about it I think the plastic sheet with a hole in sealed against the head combustion chamber and the volume measured with liquid from a burette is the way to go! Old skool but it works, I did it on a Jaguar head some 30+ years ago, how time flies!

Iain

[JohnD]

Measuring the chamber volume by buretting is also essential,  but if V=AxH, and you have only an "approx" idea of A, then H will also be a guestimate!

http://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/

 

[Iain T]

John, 

It does assume V=34.7 which unless measured on your head may not be correct.

I measured the thickness of my head and confirmed it was standard and unskimmed, I then put the various data in the spreadsheet and compared the Vhead new to the values in the Triumph Tuning Guide which states for a 2litre 10:1 is 32.5cc and 11.5:1 as 27.2cc. The values the spreadsheet calcs are 32.2 and 27 so pretty accurate, certainly for me. 

Iain

[Gadgetman]

I did a 30thou skim on a 1600 head and after measuring its 10:1  Although you are better to skim the block and deck the pistons... I learnt this after I skimmed the head!

[JohnD]

3 hours ago, Iain T said:

John, 

It does assume V=34.7 which unless measured on your head may not be correct.

I measured the thickness of my head and confirmed it was standard and unskimmed, I then put the various data in the spreadsheet and compared the Vhead new to the values in the Triumph Tuning Guide which states for a 2litre 10:1 is 32.5cc and 11.5:1 as 27.2cc. The values the spreadsheet calcs are 32.2 and 27 so pretty accurate, certainly for me. 

Iain

You measured the thickness of the head and assumed that the chambers were unchanged.    I'm sorry, Iain, that makes you spreadsheet worthless.    Please read my article, which is by far NOT the last word on head skimming, but at least does insist on measurement.   As you yourself say, measuring the chamber volume is not difficult, just fiddly.     You cannot achieve a raised CR safely  without measuring.

[Iain T]

John, we agree measuring is the accurate way to go. However if you are not worried about getting your compression to within close limits but just want to raise it, providing your valve heads are flush with the chamber top and not proud or recessed (then that should be addressed), this is a good general spreadsheet to calc a head skim. From my interweb searches it does appear a standard chamber is as near as damn it 35cc. Therefore going by the spreadsheet a 1mm or 40 thou skim from a standard thickness head will raise the CR but not so that it causes any problems. 

For those owners who do not want to measure their chambers this spreadsheet is probably good enough. Of course with the caveat that your chamber hasn't been modified. But then that would lower the CR!

Conclusion, measuring is best but if you are not mechanically minded ask someone who is! 

Iain 

[JohnD]

Iain,

Would you set your carburettors to "good enough"?     Or your toe, front or rear?   Would you tolerate a steering wheel that wasn't centralised, headlights that aren't set correctly?   

If all you want is to raise the CR ' a bit' - unlikely to give a significant benefit, but anyway - then perhaps it is sufficient to muddle through.   But with 99 octane fuel, the highest that is safe is a bit less than 11:1.     By my no doubt to some, 'OCD' approach, I aim at 10.5.      That I achieve that was shown the next spring, when the old fuel in the tank 'pinked' like mad which was worrying, until a tankful of fresh stopped it at once.   The ocane boosters in fuel are highly volatile.  If you want to get any benefit from raised compression accurate measurement is essential.

[Nick Jones]

10 hours ago, Iain T said:

providing your valve heads are flush with the chamber top and not proud or recessed

Valve heads should stand proud from the chamber roof, not flush and certainly not recessed. And you raise an important point. Valve heights have a significant effect on chamber volume and thus CR.  This does mean that direct measurement of volume with a burette or syringe is much the safest way.

CR on a Mk2 2L is already marginal for modern fuel and doesn’t want to go up unless using an “interesting” cam with lower dynamic compression.  Injection, giving more even fuel distribution, and electronic management of fuel and ignition help the margins.

Nick