Connecting rod length

[Roger]

Howard,

Thanks for your reply, 
I did that when I had the assembly out of the crankcase it went through all four but No. 4 was a bit tight because the small end bore was a good fit.
So that must mean all the rods a are the same length within the white metal?
The crank case top face is flat now I have taken the bumps out round the studs.
The block is parallel within a few thou.
It looks like it all checks out ok but I must have missed something.

Roger

[Reckless Rat]

If you hadn't had the block faced would you have noticed? I suspect the anomaly has arisen due to the reduction in block height, not the rods. A shim should sort it.

[Robert Foreman]

I would now number the pistons 1 to 4. Then remove them from the rods and measure the crown height from the gudgeon pin hole with a micrometer. They should all be the same.
  If the block isn't finally fitted you can also remove it and measure with a micrometer again to check the machining that has taken place.

[John Cornforth]

Hi Roger.

The "rod through all the small ends" test will show up bent con rods. But I don't think it confirms they are all definitely the same length.

Say No 1 and No 4 are equal but long and No 2 and No 3 are equal but short. If you look endways on at the crank and rods out of the block, the little end eyes can still be swung into the same line and a rod passed through, but the triangle formed by the rods will not be an equilateral one.

On the other hand, if you reassemble the same rods so that No 1 and No 2 are long and No 3 and No 4 are short, they will form two triangles of different heights. The rod can be passed through No 1 and No 2 but you will find it offset when you attempt to pass it through No 3.

[Roger]

If you hadn't had the block faced would you have noticed? I suspect the anomaly has arisen due to the reduction in block height, not the rods. A shim should sort it.

Most probably not, but how much effect does the different height have on compression for example?
It’s just a road car so am I being too critical?

Roger

[Tony Press]

This still appears strange-

"I found No.1 & 4 pistons 0.020” above the top of the block No. 2 & 3 flush with the top face of the block."

Surely with the block skinned properly the pistons should all be at the same level.

I understand you said changing the rods around didn't affect this.

Wouldn't it indicate the crank was causing the 20 thou difference between 1 & 4 and 2 & 3 ??

[squeak]

Further to Tony's suggestion re bent crank or improper journal centre grind,  try taking 1 assembled rod and piston (any one) and try it on each crank pin in sequence. If any discrepancy of piston crown height is found, the crank is at fault. 
If no discrepancy follow Robert's advice and measure all pistons for pin to crown height.
If still no discrepancy, the BE white metal bore is suspect.

I must add  I claim no expertise in engine building

[Zetomagneto]

Hi, how are you checking the Piston heights? How are you determining TDC? I would use a dial gauge on a magnetic base ,with the anvil on the centre of the piston crown, needs to be in the centre because the Pistons will rock in the bore at TDC, turn the crank back and forth to get the highest reading, note the reading, and do the same on the other 3.
Are you using a LC head, if so Pistons won't hit the head.
Does anyone know the compressed thickness of a silicone base gasket?

[Bob Culver]

1 1/4 cc, about 2.5% of c.r, not ideal but the mixture mass and strength varies far more than this cyl to cyl.
If not all machined by same person at same time mixed rod lengths are likely. Operatives generally guess from the old rod so exact 6 inches not always acheived. Nice to have highest cr adjacent the main bearings!

[Roger]

Thank you for all your replies,
Tomorrow I plan to check the crankshaft by using one rod on each journal to check variations.
Check the length of each con rod
Check each piston top to gudgeon pin height.
The cylinder block is true and parallel.

Roger