Cylinder wear tollerance numbers?

[Rossdavies]

Hi everyone, had heavy blue smoke, misfire and bad oil fouling on cylinder one sparkplug and after a failed compression and leak test, found a few problems. Intake valve on number one was worn on shaft, so had it replaced with all new guides and seals done at the machine shop. Also, number one piston rings are worn very bad, oil ring was nothing left, no ridges left. The question is, (have already ordered new standard 84mm rings), how much bore wear is acceptable to get away with fitting standard rings. I have a ridge at the bottom of the bores, (only front and back of each bore) but not really noticeable ridge by feel at the top of the bores. I've measured with a caliper and at worst, my worst cylinder is worn 0.2 mm. Is this going to be ok?

[TobyB]

If the ring came apart, nope. Or rather, you're going to want to measure the bore pretty carefully.

Likewise, that piston is going to be a mess in the ring land area.
BMW blocks are very durable.
The pistons don't usually wear too badly in diameter.

What DOES seem to happen is that the ring lands on the pistons wear significantly.
So you put new rings into the old gaps, and the new rings fail pretty quickly because they get
twisted too much.

But measure. That's the only way to know for sure.

t
seldom reuses pistons.

[digger]

Overbore and new slugs I’m afraid

[Rossdavies]

Right, i have some pictures and an update for anybody who's interested, i found some scoring at the top of the bores upon cleaning and further inspection, found a way to get rid of them although after alot of people saying if a bore is scored so far you can feel it with your nail its scrap. I took to it with a glaze buster wound up as tight as it would go, dry, with no oil. Almost wore the stones out. I decided to use the glaze buster to take down bore material as it would take material from the whole circumference evenly, hey, what was there to lose right? After i was happy that i could no longer feel scratches (although still visible) i tried a new ring in the bore and my end gap was fine! Im calling it fixed, but yet to run the engine as im waiting for parts. Heres before and after pics. Before pics aren't very good sorry.

[Rossdavies]

[Rossdavies]

And here's after i hit it with glaze buster im pretty happy

[Rossdavies]

If the ring came apart, nope. Or rather, you're going to want to measure the bore pretty carefully.

Likewise, that piston is going to be a mess in the ring land area.
BMW blocks are very durable.
The pistons don't usually wear too badly in diameter.

What DOES seem to happen is that the ring lands on the pistons wear significantly.
So you put new rings into the old gaps, and the new rings fail pretty quickly because they get
twisted too much.

But measure. That's the only way to know for sure.

t
seldom reuses pistons.

I think i caught it just in time!! Rings did not break, but as you can see pretty badly shot, i think cylinder one has been eating valve guide material for a while, but its all nice and new now. Im not too concerned about the scratches no more, should be running like clockwork now. In the third picture you can see the scratches, after I've honed the bejesus out of them, what's your opinion??

[digger]

you said it was worn by 0.2 mm that is way too much to get good life with a dangle berry hone and plus there is a ridge. its one thing to have the surface uniform but there is a reason why people measure the bores for out of round, taper, size etc

[ADEN]

My cylinder bores had bumps in them and worn i had to do sleeves.
As mentioned above ring grooves were out of specs my machine shop fabricated new rings that fit the worn grooves lol.
Piston skirts rarely wear.
Take your measurements and do it right.

[Rossdavies]

We will see, i bet it will run like clockwork, people are a bit overkill me thinks, at the end of the day if it don't work, the extra cost of trying it this way first is minimal, so it seems like common sense to me, as opposed to spending £1500 on boring and oversized pistons like a bull in a china shop, but will let y'all know either way as it will be rebuilt by end of next week. Will give updates as time goes on as to whether it fails again over time.

[moatilliatta]

Thanks for pleasing my curiosity on what not to do. Will be excited to see the outcome and how long it last.

But fresh bores with piston like the others said. Make sure your hone job has the proper cross hatching.. A ball hone will probably just make the cylinders more uneven while trying to clean up the rough areas.

Just gonna hone it!

[Rossdavies]

Thanks for pleasing my curiosity on what not to do. Will be excited to see the outcome and how long it last.

But fresh bores with piston like the others said. Make sure your hone job has the proper cross hatching.. A ball hone will probably just make the cylinders more uneven while trying to clean up the rough areas.

Just gonna hone it!

I didn't use a ball hone i used the 3 legged solid block type and scraped the shit out of it. Should take it down nice and round. Do you know what? There's no info on people trying this out. Everybody's like 'oooh you have to go and spend loads of money otherwise it wont be right'. Would be nice to see if people have tried and succeeded, to give people the chance not to waste money at the machine shop. Ofcourse the machine shop is going to say you need work, otherwise they'd close.

[ADEN]

Honing scraping =taking material off. Take your measurements before and after and see. Cylinder to piston gap is ok m20 is steel and expands more when warm.
I think what matters more is piston grooves wear.My old stupid mechanic ignored the piston grooves wear and installed new stock rings and it smoked on the first start.

[Rossdavies]

My piston grooves are fine rings are nice and snug in them

[Rossdavies]

So i put the car back together today, just new rings, a head gasket and bolts. It started on first few revolutions and was misfiring, blue smoking from the off. I kept the revs at 1500rpm as recommended for seating in new rings. And slowly the misfire disappeared as did the blue smoke. Kept the revs at 1500 to 2000 for a good 20 mins whilst allowing to rev up/down within that range, all to avoid immediate glazing during the wearing in process. Once up to temp, I took the car out to put under load, and the smoke was present only at the top of the rev range, the more I drove it, the further up the rev range the smoke got, until it finally disappeared altogether. Now, not a single misfire, the car idles very cleanly and pulls very hard. Im still a dubious, due to the cylinder wear but it seems ok at the moment, having only used for around an hour. I have hardly used this mtech 1 sport now since i bought it a year ago for an eye watering 18k, so am very happy its back for me to use. My ring gaps were, i'd say, around 0.7 mm (out of tollerance for use they say), just goes to show people are too quick to throw money away when a bit of give or take can be applied. But we will see i might end up eating my words in a few weeks/ months. Will update. Hope this is helpful to people.