20 replies to this topic

[Moshtaraq]

Hi again,

My block came back from the machineshop for a linebore and a valve job. I started to install the pistons on it and 3 of them cause no resistance at all on the crank but the fourth does. I can still turn the crank but not by the clutch axle. Another thing I noticed is the lack of end float on the fourth big end, all the other ones have a bit of float. Can I hone/sand the big end seat myself?

Thanks!

[Cooperman]

You need to find the reason before doing anything.

Was the crankshaft reground by the machine shop? If it was remove the rods and carefully measure the journal diameters. Mistakes do get made.

If it was not reground there must be another issue, but still do a measurement check. It should never be necessary to hone the big end aperture on rod/cap, especially if it is from an engine which was running.

Just a thought, but is that big-end cap on the correct way round? If the rod end will not slide across the journal when installed there is clearly a 'nip' on the]at journal. The rod cannot have changed in diameter and it is almost impossible for the bearing shell to be too thick.

Measure and find the problem, then have it corrected. Do not attempt tonhone the big end bearing seats yourself as that is just correcting one fault by making another.

[Moshtaraq]

The crank has not been reground. I'll measure for roundness and clearance. The cap is on the right way: tang to tang. I just can't afford another trip to the machine shop is all.

[timmy850]

Is the piston and rod in the right way? They can be fitted in 180 degrees out, but it will cause issues

[Moshtaraq]

What do you mean by piston and rod? The pin in little end?

[Rorf]

That is why it is so critical to do a trial build without  the piston rings in place, so if one piston and rod tightens proceedings up you can swop them around to see if the problem moves with that particular rod or it might be the journal.

 

It could even be a bent rod. 

 

The other reason for doing a trial build is to check whether the block might need decking to get the pistons flush with the deck.

 

Remove the rings and do a trial build.

 

Some builders call it a dry build, to ensure no tight spots when turning the crank.

Edited by Rorf, 07 April 2020 - 11:37 AM.

[Moshtaraq]

Yes this is my second trial build, first trial built exposed a problem with the mains so that's fixed with the linebore. Now I'm fitting the pistons without rings and I encountered this problem.

[Moshtaraq]

I measured the big ends and there is 0.02mm (0.0008 inch) difference between measurements 90° apart. The big end on the rod is 0.06mm difference. I measured the other ones and they are perfectly round but they turn without locking up or having hard spots so no suprise. Do I need that big end and rod machined ground and oversize bearings? Can I only do that one to cut costs or do I need to do all of them? How much would something like that cost?

[DeadSquare]

A line bore and valve job can not make any difference to the rotation being stiffer than it was before.

 

Try removing all the pistons and rods, check every thing, and start again with the duff one.

[Moshtaraq]

We narrowed it down to the 4th big end being out of round. Al the other components spin freely.

[DeadSquare]

How much is it out of round ?

[Moshtaraq]

How much is it out of round ?

See one of my previous posts: 0.02mm on the crank side and 0.06 on the rod side. All the other ones are round.

[DeadSquare]

In that case, it is a matter of how much of a perfectionist you are.

 

You could have a bit machined off the offending rod and cap and have it line bored, but that will leave that rod a bit shorter;  or you could have all 4 done, so that they are the same length;  or you could have another set of rods balanced.

 

Or, you could examine the old shells from that rod and take a calculated judgement to run with it.

 

BODGE:  .02mm -.06mm is an average ovality of about .0016" in imperial.  The foil wrapping on a Cadbury's Creme Egg is .0005", so try i, then 2 and 3 sheets of foil between the rod and the cap.

[Moshtaraq]

In that case, it is a matter of how much of a perfectionist you are.

Well I'm a perfectionist on a budget. But as it is now that bearing is way to tight. What do you mean by 'a bit shorter'? The machineshop said that my crank linebore moved it a couple of hundreds of a mm is that the same with the rods? How much do you think it'll cost me?

[DeadSquare]

If the bore of a split bearing is oval, by sufficiently shaving both halves, the two parts may be brought together and rebored true, as has presumably been done with your blocks main bearings.

 

It isn't by very much, but the block will end up very slightly lower.  In the same way, if the same is done to a con rod, shaving a "bit" off the rod and the cap, the rod will end up slightly shorter.

 

With care, in your case it should be less that .1mm.

 

I can't tell you how much a machine shop would charge,