Need to know engine measurments and tolerances for an m20

[Dammizi]

Hi, do any of you know what the max clearance allowed for between piston to bore on a bmw m20b23 engine(same bore as m20b20 80mm)? after taking the block for honing and re installing everything, I now realised I am at 0.102mm (0.0039") of clearance between piston skirt and bore, and also the clearance of the top compression ring upwards and downwards is at 0.279mm where the goetze rings I have installed reccomend a max of 0.17mm max.

should I just get new pistons/ tighter fitting rings or risk it and see If I get any more problems with the engine?

I have no previous history of the engine apart from the car when I purchased it had not been running for the past 15 years and had a bent connecting rod and severely worn exhaust valve guides which I am replacing now after installing the engine again after the rebuild to find that it was smoking heavily under acceleration if I let the engine idle for a few seconds.

thanks for your help guys

[reggid]

D) Swap a running engine in.

[motormanmick]

Hello Dammizi,
clearances below suggest that .0039 is too much when they recommend .0020-.0025 for street and .0030-.0035 for race.
Bent rod suggests water was in the chamber on the compression stroke, it sounds like you have already spent some $ on the engine, a re bore and new pistons/rings or as above, swap in a good running m20b25

http://www.metricmechanic.com/pdf/Pisto ... -Chart.pdf

[Dammizi]

problem is that here in malta where I am, to get a classic rating on my vehicle I need to have an engine that came out on my car originally ( e21 ) so swapping in an m20b25 is not really an option. and have alreay spent close to 1500euro on rehoning my engine and replacing all necessary parts including:

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/ gaskets etc... 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 and are being done now)
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...

so I guess I have to just rebore and replace pistons? or could I possibly find better fitting rings and risk the 0.0016" oversize of the bore? how bad could this be?

also another thing to consider is that the pistons are worn 0.003" which is contributing to the fact of the slight over clearance between the bore and piston. could I possibly get away with only a piston swap? in fact where I had replaced the 1 piston there is no oil deposit around the bore unlike the other 5 cylinders...

if so do any of you know where I can purchase an additional 5 new pistons for the engine? or maybe if not just a complete set of 6. the seller I bought the other piston new from only has 4 left in stock so this leaves me with 1 more needed :/ and cant seem to find anything else available unless custom forged, and this is way to expensive!

do any of you know of a good cheap m20b23? might consider swapping out at this point since im getting fed up playing around with this engine

[motormanmick]

Know how you feel, I'm rebuilding mine on a tight budget, I wouldn't mix pistons, different manufactures probably have different specs, ie weight, safest course is to buy a full set. Below is an Australian manufacturer with a couple of sets of pistons for the m20b23 and our dollar being what it is against the euro it might work out for you.
Company has a good reputation in Aus, cheers.


http://mail.precisionintl.com/engine.as ... 61&EID=321

[Dammizi]

thanks for that link motormanmick! been searching all over the web for someone with these pistons in stock, just sent them an email asking if they ship worldwide since from the site they only seem to offer national postage!

also another thing i'm not sure about is that they only seem to have the raised top pistons currently available, while mine are flat topped. are these interchangeable? and will they cause any clearance issues with my head? '731'?

couldn't find much info regarding the difference in these pistons....

[motormanmick]

best to email them, I think I saw on their website a couple of options, so best to ask them. Years ago I had a partnership in performance mods to cars and engines, in some cases we used to machine off some of the raised top of pistons and balance to suit what we wanted, again ask them if it is possible with their pistons. If you go that way, you trial fit the piston at tdc with plasticine on the crown, fit the head with valves down and an old head gasket, check the plasticine for clearnce