Rebuilding an m20b25. Got questions.

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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Mon Mar 05, 2012 8:17 pm

Got the block back the machine shop. It had slight rust on cylinder one and two. Machinist told me that a hone could probably get it completely out. When i went to check out the block i thought it was completely removed but i defiantly missed it. I doubt that its going to be a issue. I think its more visual then anything, when i use my fingernail it doesn't hung up or anything. Also have a pic of the polished crank and the piston with the worst scoring. I have another m20 block that i will pull pistons and rods and check to see which are the best.

Thoughts?

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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Mon Mar 05, 2012 8:37 pm

I wouldn't worry about the cylinders, but I think those piston skirts and crank are questionable. Hard to tell from the pics, but the scoring on them looks kinda bad.
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Mon Mar 05, 2012 8:45 pm

no scoring on the crank, its due to shitty lighting. I had just hit it with WD40 also.

I think the camera make it look worse then it really is, my fingernail doesn't get stuck on any. Im going to take higher quality photos when i go back to the storage unit.


Also the machinist is asking 325 for a valve job. Seems rather high to me. Thoughts on that?
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Mon Mar 05, 2012 9:00 pm

That's more than I paid for a 5 angle valve job fwiw.
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Mon Mar 05, 2012 9:04 pm

gonna go to another machine shop first then
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Thu Mar 15, 2012 2:56 am

I had a spare m20 with some bad surface rust in a few cylinders.


I pulled the pistons out and they are defiantly in better condition then the ones from the other motor. The other motors pistons had scoring on both sides of the pistons. These have slight scoring on a single side.

I may go to the yard and spend a day pulling pistons to see if i can get a set with no scoring as i dont want to spend the cash on getting brand new ones lol.
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Re: Rebuilding an m20b25. Got questions.

Postby Yoshi » Thu Mar 15, 2012 11:41 am

pulling pistons at a junk yard... fun!
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Thu Mar 15, 2012 4:28 pm

As long as the heads are off its fairly quick! lol
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Re: Rebuilding an m20b25. Got questions.

Postby Yoshi » Thu Mar 15, 2012 4:41 pm

don't you have to yank the oil pans too?
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Thu Mar 15, 2012 6:03 pm

Buy used pistons off the forums, way easier lol.
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Valve/Piston intercourse; how much damage is "too much"?

Postby moapys » Mon Oct 27, 2014 1:50 pm

Hi - I just suffered an "interference situation"; not a belt breakage - but an idler shaft shearing...same effect - valve/piston collision. My issue: Beyond the usual bent valves (all intake <?>) and broken rockers (all six intakes <!>), I have one badly nicked piston top.

I'd say that the depth is as much as a 1/16" in a few locations. The question: If I grind/polish out the high spots, is there much of a chance of "burning through" the injured piston?

Thanks for your opinions. Paul
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Re: Rebuilding an m20b25. Got questions.

Postby Yoshi » Mon Oct 27, 2014 5:50 pm

Um... post pictures of the piston. I've run pistons with some nice pits in them for years with no issues. I think the biggest issue will be compromising of the skirts and rings from the hit. Not necessarily the piston top. You'll never know how bad till you rip apart the motor. But by that point, you might as well rebuild the motor.

Honestly, simpler to just get another M20.
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Re: Rebuilding an m20b25. Got questions.

Postby 87e30 » Mon Oct 27, 2014 10:52 pm

Seems a lot of that going around these days. I had my tensioner pulley break off a few weeks back. Timing belt only skipped 1 tooth, but have a horrible valve tap now.
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Re: Rebuilding an m20b25 head (piston crowning)

Postby moapys » Wed Oct 29, 2014 1:35 am

Per the guidance above, I had already inspected the bore; the cylinder walls still look good (e.g. no shards-scraping). The linked photo - was not allowed to save at any resolution - describes the impact as well as possible...the top ridges seem easily flattened with a Dremel. The gashes would remain, but I've had some feedback that I can likely endure if the gouges aren't too deep, and not at the piston edge.

https://www.dropbox.com/s/42zo8b897nuzc ... 7.JPG?dl=0

I appreciate the thoughts. This is a low dollar project car, so no new m20, nor bottom-end work; it will need to suffice with Craigslist/eBay parts...or leave my life on a flatbed :cry: !

Much obliged, all.
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Re: Rebuilding an m20b25. Got questions.

Postby 87e30 » Wed Oct 29, 2014 12:20 pm

Here's your picture.
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DCFC0057.jpg (208.15 KiB) Viewed 2703 times
Schwarz 5-speed 87 325is (newest project)
Arktisblau 5-speed 86 325e (current DD)
Schwarz 5-speed 87 325e (parts car/LS1 swap?)
Gazellenbeige automatic 85 318i (70,000 miles)
Alpine white 5-speed 87 325is (my first e30, possibly for sale)
99 Suburban

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