Connecting rod length

[Roger]

I am  slowly rebuilding my spare engine, I have fitted a re-ground crankshaft and connecting rods.
I then fitted the new pistons without rings and the block to check piston height, as I have skimmed the block and re-bored it.
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.
Is this possibly because the Re-metaled rods have been machined to different lengths, or the throw of No. 1 & 4 is ground wrong?
I have moved the position of the rods and get the same result so it looks as though it is the rod length.
Does it matter that two pistons will be higher than the other two, as long as the pistons do not touch the head?
Would removing 0.020” off the high pistons be a good idea?

Roger

[Chris KC]

Hi Roger,

On 2-bearing engines it is fairly common practice to ensure the piston crowns on cylinders 2 & 3 are 10 or 20 thou below the block face, this protects against the pistons touching the head in case of a missed gear / excessive rpm (crankshaft whip). I can't quite follow what has been done by you and what by others, but is it possible someone else has set the piston crown height for you with this in mind?

The normal way to correct is by skimming the piston crowns, they should tolerate 20 thou or so off, depending on what you are using for pistons of course.

Otherwise if a lot has been skimmed off the block a shim or spacer can be inserted under the block to bring it up to height again, some of our 'cherished suppliers' sell the necessary shims in 20 or 30 thou thickness.

I'm afraid I can only guess where your height disparity comes from, but it may be worth having a conversation with the person who did the work. I take it you have white metalled rods? Otherwise I cannot imagine the rod lengths centre to centre could differ unless something is badly adrift. Is the crankshaft straight?

At any rate, the pistons 'must' be arranged to lie below the deck at tdc under all ordinary circumstances.

p.s. you did check it with a gasket in place, didn't you?

[Roger]

Chris,
Thanks for your reply.
I bought the crank and pistons all ready completed so do not know the history.
I machined a lot off the block top and bottom because of the rust pitting etc. I planned on using a thick shim which is why I did not use a gasket in the trial set up so I knew where I was starting.
As you say if the two centre pistons are low that is ok.
I will endeavour to set the high pistons to be below the top of the block.
I see 0.5mm & 0.75mm shims are available I may need slightly more but need to check again.
Yes my rods are white metal, I thought maybe the machining my have accounted for the difference in height although unlikely unless as you say to allow for 2 & 3 to be low.

Roger

[Chris KC]

I reckon as long as 1 & 4 lie flush you should be OK Roger. Though I personally would want to do a bit of measuring and find out for sure where the difference lies. If 2 & 3 pistons have been skimmed I'd have thought you would be able to see it.

[Roger]

All moving parts including pistons are new.
My suspicions are that the rod length varies, I doubt that the crank is at fault and if it is I have no way of checking.
I will continue to check and come back with my conclusions.

Thanks
Roger

[Geoff101]

Where did you get your rods Roger?

[Roger]

I bought the complete crankshaft and rods on !bay from Street area.
Roger

[Hugh Barnes]

if the pistons have been skimmed, the stampings would have been removed (ie 'FRONT' '+060' etc) That should be straight forward to confirm?

[Roger]

The piston are new.

Roger

[Howard Wright]

Hi Roger

I think that if you assemble the crank and rods out of the engine you should be able to thread a 1/2” silver steel bar through all of the little ends.  If you can’t then the rods are suspect.

However I doubt the rods themselves will be 20 thou out but the white metalling could have been bored incorrectly.

Sorry this check means you will have to disassemble the engine.

Cheers

Howard