Rebuilding an engine: pistons installed, crank rotation "very" stiff

Question

As I'm assembling my engine back (Skoda Estelle 120, 70s, RWD, rear mounted engine) I have the following issue: installed crankshaft, camshaft and distribution chain, torqued as manual recommends, it turns freely by hand (a little bit stiff). Then installed pistons: as soon as I put #1 I notice the engine won't spin freely but needs a wrench in the pulley nut...installed the rest of the pistons and now I need the wrench with a little pipe to assist it. Spinning it for a while seemed to soften it a bit. As I apply oil, and as it drips to the stand's pan, I see it gray like something is being rubbed.

Cylinders got sleeved, I'm using the same pistons the mill shop guys used for re-sleeving.

I have lubed and torqued the installed parts so far. Left the rings gaps as recommended, however found this issue: the oil ring, which came with the spring kind of inner ring doesn't seems to fit well in the piston grooves, because it seems that the spring ring is too thick. To make it fit, I stretched spring ring a lot until it got less thickness, then the oil ring with the spring ring could fit and the piston could get into the cylinders (it needed some blows with the back of a hammer, but when spinning for tests, I don't see the cylinders being scratched or marked)

So far it is not "seized" but won't spin without wrench + pipe.

Question: is this normal or at least acceptable at this stage? What it needs to soften out? More manual cranking? Or put the whole engine together and crank it via starter but without actually firing it?

Answer 1

I am concerned with you description of the oil ring - the ones I have fitted come in 3 parts : 2 scraper "flat" rings and a "top hat" mounted on its side that locates the other two. Fitted correctly there should be no excess length, unless you were given the wrong ones...

As for turning the engine by hand, a long wrench sounds about right. The starter acts using the "lever length" provided by the flywheel and produces 6kW (assume 600A and 10volts...). Once I had engines re-built ready to start then I would spin the engine without sparkplugs until the oil pressure light goes out or oil pressure shows on the gauge (you can fit one temporarily).

A note here, a common error is to lubricate the surface of the bearings as they are fitted but the wrong surface : the bearing surface that touches the block must be fitted dry and clean. The surface that mates with the crankshaft bearing needs to be lubricated. I used to use graphogen as it stays still during the rebuild process and does not drain away as oil does. Also, it does not spoil the oil, but I would do an "early" change on a rebuilt engine.

Answer 2

Ok mystery solved: I have two piston sets, both are identical in look, dimensions and part numbers/marks...only that they are not :) I took the ones I installed and carefully checked measures and visual inspection against the other set. The difference is that the stiff ones have the oil ring groove 1 mm less deep than the other, this explains why the coil ring couldn't get into there enough and left the oil ring impossible to get into the cylinder. Fix was simple: I just installed the other pistons set and now the engine spins by hand and a little rag at the crankshaft pulley end.

However, the pistons have other difference: they may look the same and share same measures, etc, but they also differs at the crown: one is flatter than the other set. The ones I have to use are flatted, so when installed they are 1.4 mm below cylinder top edge. The other set I can't use have a more domed crown, so at its top it levels with the cylinder. Oh well...