Lots of smoke after M20 engine rebuild!

[Dammizi]

Hi guys, hopefully some one here can help me out with this issue!

At the end of last year I bought an e21 which had been held up unused in a garage for the past 15years with no previous history, it is a 320/6 model with a 323i engine transplanted into it by the p/o

decided to take the whole m20b23 engine apart and rebuild it from new which included:

honing the block
new goetze piston rings on 5 of the pistons and 1 piston completely replaced with a new one which came complete with rings installed (TRW) due to me being stupid and cracking the edge of the original piston...
polished the crank and installed new big and main end bearings
new seals throught and new head bolts etc.
cylinder head reconditioned by skimming the head and installing new valve stem seals but not the valve guides (stupid I know but somehow missed this out as was my first engine rebuild)
swapped over the k jetronic for Motronic 1.3 off a 2.0 and after initially starting up and having problems with heavily retarded timing due to having 2.0 fuel injection and a 2.3 crank and pistons, decided to swap the ignition over to the original distributor and got the engine to work seemingly flawlessly and pulling nicely through the whole rpm range...

this past weekend I finally decided to get it licensed and back on the road....

but after a few miles of driving I noticed that it was smoking quite heavily under acceleration (bluish white smoke which indicated oil was being burnt).
decided to keep on driving it to see if maybe it was due to the engine still not having run it. But after driving it for 2 days and doing almost a 100miles in it, seems like the smoke is not reducing at all and is quite bad as I can see large clouds of smoke coming out of the back.


upon checking the engine there seems to be quite a lot of valvetrain noise although the valves were all adjusted accordingly.
also the rocker cover breather seems to be blowing smoke and also from the dipstick guide.

today I took the engine top end apart once again to see what is happening in there and seems like all spark plugs are wet with oil and black except for the one above piston 4 which is dry and indicates a nice burn from it (this also happens to be the piston that was replaced!

now Im not sure if the problem lies within
-having worn valve guides( I can move the valves slightly to the side when not under tension)
-or having worn pistons / incorrect ring installation/ worn block bores!

I made the mistake of not measuring anything when installing all components back in the engine and left this up to the engineering shop to check out for me....

what do you guess is my problem here? having the replaced piston being the only one which is giving a clean burn seems to indicate having the other 5 pistons worn or with incorrect rings/ incorrectly installed, but the valve guides also seem to have a small amount of play in them....

here are some pics of the engine after I dismantled the top end today and wiped whatever was easily removable. - everything seemed to be covered in a black oily deposit apart from cyl 4 which had the replaced piston where everything was dry...

[Jeroen]

Several options like badly installed (or oldddd stock) valve stem seals, too much freeplay on the valve guides, but the differences between the spark plugs point us in the direction of the pistons indeed, or rather: the piston rings. If installed correctly it looks like they are the wrong size allowing too much oil past. Did you do a compression and leak test yet? And an oil pressure test? Wouldn't be surprised if your oil pressure would be way down as well resulting in the noisy valve train. As long as you don't know exactly what is happening, don't drive it. There's already a chance more damage has been done.

[Dammizi]

no compression or leak tests were done but oil pressure light went out immediately as soon as I would fire up the engine. before installing I had cleaned and tried the oil pump turning the drive by hand and it was pumping oil normally from what I could see.... havn't yet pulled the sump so that I can take out the pistons but i'm beginning to worry that I might have mistakenly installed the rings the wrong way round. would this cause this excess oil in the compression chamber? only thing i know is that the engine was pulling really well for what it was and I might have gotten a bit too happy with it and not allowed the running in to take place well. but there does not seem to be any scratches on the bores which would possibly indicate any broken rings...

is it possible to just measure the clearance between the piston and bore to see if it is out of spec or not with a feeler gauge? or do I have to pull the sump and pistons to measure it the actual bore and possibly also the pistons?

sorry for such seemingly stupid questions but this is my first time doing this kind of engine work and I already feel like throwing the engine out and using it as a boat anchor

[Jeroen]

Well could be, all three springs are different in shape and the upper two have a clear direction, size restrictions and the slots should be abt 120 degrees apart (making 0-120-240 deg).

The oil light doesn't really help, that switches off at abt 0.5 Bar I believe, barely enough for idle but that's about it.

[Dammizi]

rings were clocked correctly that im sure about. what im worrying is about the possibility that ring no.2 (middle one) might have been installed the wrong way round with the notch facing downwards instead of upwards ( forgot by now since it's been a while since I rebuilt the engine) could this possibly be causing the excessive smoke upon acceleration? it is also breathing smoke through the engine breather and dipstick tube

will be opening up the bottom end of the engine later on this week to check out the rings and confirm if I had installed them correctly or not....

also what kind of gap is allowable between the ring edges when compressed inside the bore? if I remember correctly there was over 1 mm clearance between the ring edges when compressed.

[Dammizi]

ok, so today I went ahead and pulled the pistons from the block, they were installed the correct way round but:

upon further inspection I can see 2 problems in the old pistons which I had reused using new goetze rings:

there is about .5mm of play in the top 2 compression rings upwards and downwards which is not present in the new piston which had been replaced,

and also fully closed the rings still have a gap of almost 1mm.

(these measurements were approximate since I had no feeler gauge at hand....

this seems to be way too much compared to the new piston. is this normal or is this most probably the cause of all this smoking?

should I buy a new set of rings with closer gaps? or should I just replace all the other 5 pistons?

btw, I also inspected the head properly and found that the Exhaust Valve guides are pretty worn and offer quite a bit of sideways movment so these are also going to be replaced before the head is reinstalled

[Jeroen]

Think your conclusions are correct. Don't have the tech documentation on hand to check the requirements so I can't tell what the requirements are for pistons and piston rings. Perhaps someone else can chime in?

[Dammizi]

if anyone can get me a copy of the clearances for an m20 engine, this would greatly be appreciated especially piston /bore size and clearance and the ring slot width. i'm contemplating just buying new pistons apart from replacing the valve guides due to quite obvious excessive play in them

[Dammizi]

ok, so I measured the piston ring side clearance (top and bottom of the top compression ring) and there is a 0.279mm approx in all pistons which seems to be way out compared to general max allowable clearances for this size of pistons 0.178mm

and the original pistons measured 79.91mm at the bottom skirt

compared to almost no gap between piston ring on the new piston and 79.98mm on the bottom skirt of the new piston.

I have yet to get an internal micrometer and measure the internal bore size of the block.

has anybody got the allowable clearances for an m20 engine??

[Jeroen]

Tried to find them but no luck in the E21 manuals I have here. Perhaps an E30 manual can be of help. Anyone?

[Dammizi]

thx jeroen,if anyone can get hold of the exact clearances required for the m20 engine that would really save me a lot of worrying about what I should do...

just got back to the garage and measured piston to bore clearance with a feeler gauge (I Know, this is not a good way of doing it but I dont currently have access to an internal micrometer...)

there is only a 0.102mm (0.004") gap which seems reasonable enough considering the pistons themselves are worn 0.07mm

the only things worrying me are the gap below and above the compression ring of 0.279mm (just checked with instructions on the packaging or the goezte rings I purchased and they mentioned a max of 0.13mm )

and the ring gap between edges:
top side of cyl bore: 0.559
bottom side of cyl bore: 0.61mm

is this really bad? what could the consequences be? and could this possibly be the cause of excessive smoke apart from the valve guides which are getting replaced? and should I replace the pistons or possibly find rings that are better fitting?