Block Deck and Piston Height

[cap'n crunch]

Good Morning,

Is it normal for a piston to sit proud of the deck on a Twin Cam? the outside of the piston sits just proud of the block (less than 1mm) with then the central raised section of the piston obviously higher still.

Also for the piston to be higher than the deck towards the front of the engine then flush or lower at the other side?

I will try and get some pictures to help explain later.

Thanks!

[Elanconvert]

sounds wrong to me!!!!
causes? block skimmed incorrectly? crank bent? main bearings not seated properly?

rohan will know!!!

fred

[rgh0]

It should not sit proud. If you have standard crank and rods and pistons then it sounds like the block has been decked or line bored to reduce the crank centreline to block top height.

cheers
Rohan

[CBUEB1771]

Neither situation is correct as you can see from the other responses. I am sure that both problems are the result of the block having been surfaced by a machinist who did not bother to set up properly on the main bearing bores centerline. I went though this once only to have the fine machinist inform me later that he would have had to charge more to set up on the centerline rather than to just clamp the block down on the sump mounting surface. This begs the question as to parallelism of the sump mounting surface and the mains centerline on a standard block. I would have thought these were parallel to one another but I don't know the sequence of machining operations that Ford used.

[cap'n crunch]

here is a picture of Number 1 Piston at TDC...

[CBUEB1771]

here is a picture of Number 1 Piston at TDC...[/attachment]

Wow, that appears to be quite a bit of difference in deck height going across the diameter of one piston. Is this condition consistent across all four cylinders? If so it seems that your block has been surfaced very badly, being significantly out of parallel with respect to the main bearings centerline.

[Elanconvert]

some years ago I had the block of my imp emgine skimmed, which brought [all four] pistons proud at tdc.......

solution was simple...skimmed 8thou off the top of the pistons......

fred

[RotoFlexible]

here is a picture of Number 1 Piston at TDC...[/attachment]

Wow, that appears to be quite a bit of difference in deck height going across the diameter of one piston. Is this condition consistent across all four cylinders? If so it seems that your block has been surfaced very badly, being significantly out of parallel with respect to the main bearings centerline.

Are all four pistons proud of the block by the same amount, and all four higher in the front by the same amount? If the block surface was out of parallel with the crank centerline and lower at the front of the engine, you would expect #4 to be somewhat recessed in its bore, #3 less so, and so on. If all four pistons are consistently positioned in their bores at TDC, then the block is OK and something is going on with the pistons, e.g., they are canted in the bores, or they were machined incorrectly.

[john.p.clegg]

..or a twisted small end...

John

[cap'n crunch]

2nd Piston - Similar to the first above the deck at the front and flush at the back.

3rd Piston - Slightly less, just above deck at front and lower than deck at the back.

4th Piston - Just lower than deck at the front and just above deck at the back.

Which all adds up to how is that possible? And what now?

[2cams70]

Hi,

I have the same issue with my engine where pistons are level with block with standard rods. Suggest you use a composition type (eg. Adjusa) head gasket in lieu of a standard copper gasket. These are around .020" thicker than a standard gasket hence you will end up with a squish clearance and compression ratio around where it should be assuming head hasn't been skimmed too much. Interested if you could get the dimension front and rear of the block from the main cap mounting face to the top of the block. Should be able to determine from this whether block has been decked or machined out of parallel with crankshaft axis (I doubt it's a case of out of parallel machining given your observations).

With regards to the piston height being uneven with respect to the deck face it may be due to piston clearance in the bore although if this were the case I would expect the unevenness to be perpendicular to the crankshaft rather than in line with it as it appears to be in your case. (Maybe it's worn big ends allowing the conrod to rock along the crankshaft axis?) Suggest you remove a couple of pistons and check the skirt wear pattern as a bent conrod will usually show up as an uneven wear pattern and whilst there check the big end bearings for excessive clearance as well.

Regards