Piston height variation

[Jono]

I am building a fast road engine based upon a 1300GT.

I've had the block bored to 1310 and crank to 10/10 undersize. I am using Minispares Nural pistons. I have dry built the short engine and tightened up but not fully torqued the big ends and mains (but have done them up tight).

Measuring from nr 1 to 4 with a dial gauge I get -10, -6, -5 and -8. I am quite surprised at the variation - is this fairly normal/acceptable or does it signify something?

The crank spins nicely in the mains with no signs of tightness.

Cheers

Jon

[Jono]

P: I have used A+ rods which I understand to be a matched set.

[CooperTune]

I do a dry build on all engines I build. I find TDC #1 and lock the crank. I then check all piston/rod assemblies in the #1 hole. This gives me some idea of difference in assembly length. Make a note of each assembly piston to deck. Then I pick one assembly and try it in every cyl moving the crank to get TDC 2 and 3. This will give you and idea of how square the block is. If there is a little taper to the block you can select fit and get close. It is possible to shorten rods a little by reconditioning. I have a jig for machining the tops of flat top pistons. On race engines I shoot for zero and street units I leave .005 down the bore. I also have a piston pin I had polished to slip though a press fit rod. This way I can move pistons and rods around to get a correct length. Steve (CTR)

[Spider]

The variation you have sounds pretty good - for factory rods!

Most of the variation will be from the rods, though you'll usually get some with those pistons too.

On stock type builds, I try to match up the rods and pistons to optimise what's already there, sometime you win out and sometimes not. Floating pins are a blessing at this pint of the build!

[Jono]

Thank for the comments chaps.

So, is this something I should live with or should I be looking to correct or optimise it?

The problem is the pistons are fitted now and the opportunity to swap around has been missed.

Is it safe to assume that the deck is going to be 'true' and that the difference is in the pistons/rods? Could I have the pistons shaved while fitted to the rods to get them all the same and then deck the block?

...or am I getting too hung up on what is just going to be a nice (hopefully) torquey road engine?

Jon

[mk1]

On a road engine a variation of 5thou is perfectly acceptable. In my experience this is almost always down to variations in rod length.

M