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So picked up this head the other day for £30, your typical rusty salvage head of unknown condition. However after derusting the head I can see what look like cracks between the valve seats on 4 of the chambers! It's a 248 head so it has the smaller exhaust valve which is even stranger. 

The thin is I'm not completely sure if they are cracks. The picture I've added is the most pronounced one of the set. I cant see any visible crack in the valve seat. It almost looks like a screwdriver mark, but there is a noticable height difference if I run my fingernail over the area.

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20210120_145744.thumb.jpg.a7b294958963c5df9ab41e6faa4a695d.jpg

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Most engine rebuild workshops have 'Magnaflux' (trade name) machines Magnetic Particle testing that could easily tell what is going on.

Or if you are near an airfield there is usually a woerkshop that may have similar  equipment.

Where about's are you. I'm in Hayes West London and could check it.

 

Roger

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That does look very suspicious to me.  Does it extend into the valve seat face at all?  Roger is correct (of course) about the definitive way forward.  That should also a give an indication of how deep they've gone and whether there is any chance of recovery with inserts.  You need to insert the exhaust side for unleaded anyway.

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59 minutes ago, RogerH said:

Most engine rebuild workshops have 'Magnaflux' (trade name) machines Magnetic Particle testing that could easily tell what is going on.

Or if you are near an airfield there is usually a woerkshop that may have similar  equipment.

Where about's are you. I'm in Hayes West London and could check it.

 

Roger

Thanks Roger, I’m in Hampshire. Cheers, I’ve asked my mate to have a look at it but with all this lockdown rubbish it’s difficult to move around. He’s got some maganaflux equipment so I think you’re right with that suggestion, probably the best way.

Jacob

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25 minutes ago, Nick Jones said:

That does look very suspicious to me.  Does it extend into the valve seat face at all?  Roger is correct (of course) about the definitive way forward.  That should also a give an indication of how deep they've gone and whether there is any chance of recovery with inserts.  You need to insert the exhaust side for unleaded anyway.

From what I can see on all the suspect areas none whatsoever. Lapped the faces a bit to be able to see the seat better and I can see nothing suspect. It’s just on the top, and they all seem to be in the exact same place across all the chambers more or less which is even stranger. 

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Hello Speedysix,

 

from the photographs that looks like a step from the milling work (You can see the cutter marks) If you can polish that area with a flap wheel or similar and get a better idea?

Alec

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It is odd.  If not showing even a hint on the seats then you could well be good.  4 cylinders affected on a head with small exhaust valves would be unusual to say the least.  Usually it's either the front or back cylinder affected.

Good suggestion from Alec.....

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5 minutes ago, 2.5piman said:

Hello Speedysix,

 

from the photographs that looks like a step from the milling work (You can see the cutter marks) If you can polish that area with a flap wheel or similar and get a better idea?

Alec

Alec, that is a very good shout, I’ll give that a try. 
Jacob

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3 minutes ago, Nick Jones said:

It is odd.  If not showing even a hint on the seats then you could well be good.  4 cylinders affected on a head with small exhaust valves would be unusual to say the least.  Usually it's either the front or back cylinder affected.

Good suggestion from Alec.....

Yeah this is what I’m hoping for and yes It is rather peculiar. I intended to mod this head so it’s much better to put this to rest before starting work! On another note I still need to do some CR calculations, doing that for the head is no problem but I’ve no idea how to do it on the block with dome top pistons, anyway that’s for a different thread!

I’ll give the suggestion a try in the morning and hopefully report back with a picture of the suspect area free of said line!

Jacob

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I think I've seen that mark before, and discounted it, as part of the manufacturing process, perhaps to mark the valve centres??

Certainly if I have, and ignored it, there has been no awful reckoning as my heads leak.

JOhn

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Ok I've had an investigation this morning with some Emery paper on the end of a file and have to say you where all right but JohnD gets the point this time! Turns out that mark is in all of the chambers and looks to mark the valve centres as John said! Better to have been safe in this instance so thanks for all the help from everyone. 

On to porting now...

Jacob

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Agreed, cheers once again Alec, redrooster, Nick and John. Whilst this is going, does anyone have a link to a thread or the info on CR calculations? I know how to do them on the head side but I dont know how to get the value with domed pistons in the block? Was hoping there would be a sticky for it or in some of the head modifying stickys

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Measuring the volume of a dome.
              A circular dome is a sector of a sphere.  Imagine a horizontal slice from the top of an orange.  At the centre of the sphere, the circumference of the sector forms an angle with the ‘North-South’ axis of the sphere.  If the sector is half the orange, the angle will be 90 degrees.  If the sector is smaller, so will the angle.

 Where R is the radius of the sphere, the volume of a sector of a sphere is:

Volume=  Pi x R^3 x [2 - 3xCos(angle) + Cos (angle)^3]
                      3
             The size of the angle is measured in degrees.  If the radius of the sphere is in centimetres, the volume will be in cubic centimetres, or millilitres.

              That's fine, but how to measure the radius of this imaginary sphere of which our dome is a sector?  You can, by measuring the dome, and applying some more trigonometry.
              The diagram shows a side view of the sphere and sector.  D is the sphere's centre, BC the plane that divides the sector from the rest of the sphere.  AD and CD are radii, the first forming a right angle with the plane, BC.  By measurement, we know the diameter of the sector or dome, which is twice BC, and the height of the dome, AB.  We need to calculate the angle at the centre, which is twice the angle ADC, and the radius of the sphere, AD.

         First, consider the right angled triangle ABC.  Calculate AC, by Pythagoras.
                        AC = SQR(AB^2+ BC^2)

1950179344_domeonpistoncalculation.jpg.7699872304d19950cd274710c0097a21.jpg


           Staying in triangle ABC, now calculate the angle ACB, using trigonometry.  Call the angle 'Theta'
              Tangent Theta = AB/BC.
           Then draw another radius, DE, so that it cuts AC at a right angle.  AE will be half of AC.
  The two triangles, ADE and ACB are  "equivalent"; they have the same angles, though their sides are different in length.
 (Proof: the angles DAC and BAC are the same angle; in each case they are right triangles so the third angle of each must be the same.)
           Therefore, the angles ACB and ADE are both Theta degrees.
  (If ADE is Theta, then angle ADC, which is the angle at the centre of the sphere, is twice Theta.)

          Staying in triangle ADC, we know Theta and we know AE.
          Sine Theta = AE/AD
               So AD = AE/Sine Theta.  AD is the radius of the sphere.

           We can now apply the formula for the volume of the sector
Volume=¬Pi x R^3 x [2 - 3xCos(Theta) + Cos (Theta) ^3]
                   3

              In the case of the GT6 dome, this calculation gives a value for the volume of the dome of 8.66 millilitres.  The original GT6 head had combustion chambers of 45.5 mls, and a CR of 9.5.  If you skim your head to achieve a CR of 10.5 and forget the dome, you will have a theoretical CR of 13.5!  The actual dynamic CR will be less than this, but you could expect a bit of pinking.

 

That's theoretical, you may prefer to measure the volume of the dome!    Place a domed piston in the bore, with ring in place, but add smeared vaseline all around the rim, to seal it.     Place the same glass plate you use to measure chamber volumes over the bore, and measure the volume in the bore by buretting.       Measure the depth of the piston head, and the bore.

Now, the apparent volume of the bore is PiR^2H.

But the buretted volume will be less.      The difference is the volume of the dome.

Haven't written this for a long time, but QED!

JOhn
 

Edited by JohnD
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I removed No. 1 piston on my engine during the week to check whether the dome is solid or hollow. I was toying with the idea of having the domes milled off so that I could fit a 517528 head on my block..... as you correctly point out , they are hollow. I'm still on the hunt for a 248 head. 

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