17 replies to this topic

[dotmatrix]

Hello. How much is it ok for the block to be out of flat? When i hold a ruler over the block there is light under the middle. How big a space is ok?

[Chris1275gt]

Hi
How do you know the ruler is flat? If it is flat put a feeler gauge through the gap.

[dotmatrix]

Hi
How do you know the ruler is flat? If it is flat put a feeler gauge through the gap.

The ruler is flat Other places but between cylinder 3-4 there is a gap.

Do you know how big a gap is ok?

Edited by dotmatrix, 14 November 2019 - 10:36 AM.

[Chris1275gt]

Hi
Not that proficient yet but find the feeler gauge that goes through the gap snugly and post that measurement, loads of people on the forum will be able to tell you if it is ok or not and we'll both learn something.

[cal844]

I was told by a Ford race engine builder that 1 thou with a copper gasket or 2.5 thou with a composite gasket.

There was a time when said builder refaced an engine and head (head gasket failed) with a hard block and 80 grit sand paper.... checking every few strokes with a straight edge and feelers, the job was done in car, The engine ran perfect afterwards. From memory the gap was 4 thou, 2 thou either side of the gasket faces.


I dont know if that can apply to an A series.

Sorry I cant be of more help

Cal

Edited by cal844, 14 November 2019 - 11:18 AM.

[Rorf]

Good engineering practice and good engine building skills require it to be flat and parallel to the crankshaft centre line, like wise for the head. On the other hand for a botch job then anything goes

[DeadSquare]

Hello. How much is it ok for the block to be out of flat? When i hold a ruler over the block there is light under the middle. How big a space is ok?

 

If this is a1300, there isn't much metal between the bores to pinch the gasket, but with a 998 or 850, using Hylomar both sides of the gasket, you can probably get away with .005".

[Magneto]

I agree with Rorf, have it machined properly so that its flat and correct, then it will hold a gasket. The head needs the same attention......

[Spider]

Check using both sides of the ruler.

 

0.001" is the outside limit over the length of the block and 0.0004" (4 tenths of 1 thousands) over the width.

[ACDodd]

Have it machined anything else is just a compromise.

Ac

[dotmatrix]

thank you all for your answers. I know you are right. I have been looking at it tonight and both block and head had less than 0.002" of "room" everywhere (my thinnest feeler blade). The reason I don't want to get it skimmed is that this head has already been skimmed a lot and I don't want to raise the compression any more, plus once I cleaned it up (with a sharpening stone, I know, not exactly the correct way) the gaps got even smaller so I am quite happy with it.

 

I didn't check using the other side of the ruler but I will tomorrow. I think it will work. I will do some more checking for flatness but my impression was that it might be ok.

Edited by dotmatrix, 14 November 2019 - 09:36 PM.

[unburntfuelinthemorning]

The reason I don't want to get it skimmed is that this head has already been skimmed a lot and I don't want to raise the compression any more...

The compression ratio will go up a little if you skim the block but at the same time you increase squish which staves off detonation.

[dotmatrix]

 

The reason I don't want to get it skimmed is that this head has already been skimmed a lot and I don't want to raise the compression any more...

The compression ratio will go up a little if you skim the block but at the same time you increase squish which staves off detonation.

 

 

increasing the compression ratio will make detonation more likely I would think, why do you think it will help?

bringing the cylinder head closer to the piston will not help if it has a tendency to detonate regardless of compression ratio.

[unburntfuelinthemorning]

 

 

The reason I don't want to get it skimmed is that this head has already been skimmed a lot and I don't want to raise the compression any more...

The compression ratio will go up a little if you skim the block but at the same time you increase squish which staves off detonation.

 

 

increasing the compression ratio will make detonation more likely I would think, why do you think it will help?

bringing the cylinder head closer to the piston will not help if it has a tendency to detonate regardless of compression ratio.

 

When you skim the head you reduce the size of the combustion chamber (thus increasing the compression ratio) but the distance between the piston and the head in the squish area remains the same.

 

When you skim the block, again you reduce the size of the combustion chamber but the distance between the piston and head in the squish area reduces.

 

Reducing the distance between the piston and the head in the squish area increases the squish effect, this produces more turbulence of the air/fuel mixture which causes the fuel to be distributed more evenly in the mixture which then increases the speed of combustion.

 

Faster combustion means less time for the end gas to become heated to the point of detonation by the approaching flame front.

 

So while skimming the block increases the chance of detonation by raising the compression ratio, at the same time it reduces the chance of detonation by increasing the squish effect.

 

[dotmatrix]

 

 

 

The reason I don't want to get it skimmed is that this head has already been skimmed a lot and I don't want to raise the compression any more...

The compression ratio will go up a little if you skim the block but at the same time you increase squish which staves off detonation.

 

 

increasing the compression ratio will make detonation more likely I would think, why do you think it will help?

bringing the cylinder head closer to the piston will not help if it has a tendency to detonate regardless of compression ratio.

 

When you skim the head you reduce the size of the combustion chamber (thus increasing the compression ratio) but the distance between the piston and the head in the squish area remains the same.

 

When you skim the block, again you reduce the size of the combustion chamber but the distance between the piston and head in the squish area reduces.

 

Reducing the distance between the piston and the head in the squish area increases the squish effect, this produces more turbulence of the air/fuel mixture which causes the fuel to be distributed more evenly in the mixture which then increases the speed of combustion.

 

Faster combustion means less time for the end gas to become heated to the point of detonation by the approaching flame front.

 

So while skimming the block increases the chance of detonation by raising the compression ratio, at the same time it reduces the chance of detonation by increasing the squish effect.

 

now I am not a pro engine builder or educated in this stuff, I just follow my own experiences and the words of people I know that give me advice which is usually correct so I am not trying to argue with you :)

Your post is very informative and I see your point, I didn't think about that. But I will tell you the reasoning behind me writing "bringing the cylinder head closer to the piston will not help if it has a tendency to detonate regardless of compression ratio."

 

if the distance between piston top and cylinder head in the squish area becomes to small I have been told that this can give me local detonation in this area, which is part of the reason I don't want to skim the block besides raising the compression.