Cylinder wear tollerance numbers?

[Rossdavies]

Day 2, start up from cold, had a little splutter and let out some blue smoke for the first 5 seconds, then drove perfect all day. Weird

[Rossdavies]

Just a quick start up from stone cold in garage today. No blue smoke at all. Although i am noticing a gurgly sounding engine with a bit of a rough idle, not that it effects anything too much, going to smoke test the intake system to see what's up. Hopefully will find a vacuum leak, and smooth it out some.

[ADEN]

Just a quick start up from stone cold in garage today. No blue smoke at all. Although i am noticing a gurgly sounding engine with a bit of a rough idle, not that it effects anything too much, going to smoke test the intake system to see what's up. Hopefully will find a vacuum leak, and smooth it out some.

After break-in you have to adjust valves clearance that might fix your rough idle.
Wish u luck.

[Rossdavies]

Valve clearances were set up by the machine shop when they did my guides. With it being an overhead cam, (and not having to remove cam/rockers to reinstall head), they should be right. But i will check, because i dont trust other peoples work too much. It is a little tappety for my liking.

[Rossdavies]

Rough idle turned out to be a combination of two things, badly set up valve clearances, too tight, so i adjusted to 0.25mm. Also a horribly rich mixture, which i am convinced is what killed my number one valve and rings. I sent my injectors off to be referbished and adjusted the airflow meter to lean out the mixture. Its smooth as hell now, with no horrible fuel smell out of the exhaust. Now i have noticed a piston slap when cold!!! But still no smoke or misfire. So it didnt work guys, i have ordered a 2 litre block to bore out to a fresh 2.5 litre. I Am going to run the engine until it smokes and knocks all of the time, because i just want to enjoy the car at the mo, and will update to see how long it lasts, but at the moment it idles super smooth, and pulls hard with no issues or smoke, just very irritating to hear my piston slapping the bore when cold.

[ForcedFirebird]

New stock pistons are supposed to have .02mm clearance. Yours were 10x past that! As you found out, it will run, but you are losing compression. If you had used a rigid hone and just took enough off to clean up the bores, it would have been fine. As long as the bores are straight and round, a little extra clearance won't cause a failure (in fact, you will probably run quite a while as-is, but you will start burning oil badly). Domestic manufacturers have far greater bore clearances than BMW (the Chevy 3100 has 91mm bore, but has .0028-.0061" [.711-1.5mm] clearance). The fact that the bores are no longer round is what is causing the issues (and probably worn ring grooves, you need to check with gauges).

As far as valve adjustment, it must always be done AFTER the head is installed. Clamping the head down with the head bolts/studs causes the head to flex and the dimensions will change - I always do them after bolting the head down, then again after 3-4 heat cycles, and they always have moved.

[Rossdavies]

Was worth a try, people can read this now and see why you can't get away with it. I will take the block to a machine shop, they have quoted £900 for sleeves, or we can do oversized pistons but i have a feeling the bores will be too far gone for that...

[ADEN]

I once tried boring b20 to b25 but machine shop said that the walls are too thin to be bored from 80mm to 84mm.My pistons had wear and at that time i couldn't afford new piston so i bored the new sleeves to match each worn piston to keep piston to bore clearances within specs.

[digger]

I once tried boring b20 to b25 but machine shop said that the walls are too thin to be bored from 80mm to 84mm.My pistons had wear and at that time i couldn't afford new piston so i bored the new sleeves to match each worn piston to keep piston to bore clearances within specs.

many people have bore the 80mm blocks to 84mm. IN the UK that is the favoured way for 2.8L OE stroker builds for some reason probably as you can get a fresh bore and use stock size pistons if you can find a good set of pistons which is getting harder and harder. There is heaps of meat to do this.

i would not add sleeves for 900GBP its not cost effective way to "only" end up with a stock setup. Oversized pistons will be less than 900GBP but obviously still need machining ($$$) but you get the option for much "better" pistons rather than OE and to use a different crank and a bunch of other things if you wished for not much more outlay.

[Rossdavies]

All i want is to repair for as small an outlay as possible. Im hoping i only have one dodgy cylinder, which was the one with no rings left, in that case would love to be able to find a matching piston and just sleeve that cylinder. But there are lots of different types and some have been superseded.

Anyways, im going to run it for now, because as i said, it runs perfect, fuel mix is bang on, theres no smoke, no lumps in idle, idle speed is perfect, and i held it at 120 mph for a while the other day and it never bothered it. I want to enjoy the car this summer as i spent all winter in the rain and snow building a purpose built garage for it, so im going to use everyday that the roads are dry. Then in the winter il decide if it needs doing/ gotten worse, and if so strip, take it to machine shop and see what we can get away with.

Its such a shame to have this problem as the car is like brand new, im guessing a sticking injector is what fucked number one cylinder as it seems like my intake valve stem and rings were being washed. The injectors have been overhauled now for peace of mind.

Lesson to be learned here is rich mixture is very bad!!

[ADEN]

You are absolutely right my new oem valve guides went out of specs after less than 2000km due to running rich my engine wouldn't idle well unless i go rich i didn't know that my cam was the reason for this.
Try to enjoy it as it is and plan your rebuild in the future.
My little advise to you for now:

1: Change oil earlier with filter.
2: Use oil treatment additive (less wear) you will be surprised how quiet it will sound.
3: Wait until the temp needle pass the blue area before you take off.
4: Valve adjustments every 5k to 7k.

Good luck..

[Rossdavies]

Thanks, will get some additive..

[ForcedFirebird]

The block "should" bore just fine, it's just a lot of work. I often use m50 block cores to build 3+l 24v's with oversize pistons, even (+2mm bore/hone). Since the m20 has the same bore spacing and there's adequate side wall thickness, shouldn't be an issue. Do the Euro 80mm m20 blocks have thinner walls on the sides than a b25? They are all Siamese, so the front/rear doesn't matter much.