Rebuilding an m20b25. Got questions.

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Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Sun Jan 01, 2012 9:08 pm

So i have decided i want to have a spare m20b25 rebuilt.

I want to do a basic rebuild but don't want to cut corners as i will boost it in the future.

I plan on replacing main/rod/intermediate shaft bearings, honed, planed.

Anyone care to give me some advise? I still haven't spoken to the engine builder but id like to be prepared.
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Sun Jan 01, 2012 9:36 pm

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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Mon Jan 02, 2012 7:01 pm

I have seen the thread, even have your spreadsheet :P.

But it lacks bearing clearances(unless i missed it)

Also im more interested in the actual machining advice. What should i have done to the motor ect..
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Mon Jan 02, 2012 7:42 pm

You need to plastigauge the crank journals to get clearances which will tell you what bearing to order. Mine was actually within stock spec so I ordered original size bearings. The Bentley should have the clearances for the factory bearing clearances.

As far as machining goes, it really depends on how far you want to go with the engine. You can have the rods lightened and balanced, knife edge the crank, full port match and polish, weld the head, oring the block, have the rotating assembly blueprinted, over sized valves, etc...the limit really lies in your budget.

Minimum is a hone or factory overbore depending on cylinder wear, get the rods line honed, check valve spring pressure and shim back to stock if its low. Check valve guides and seats, a simple 3 angle valve job is a good idea since your in there. A good machine should be able to tell you all this too.

Unless you plan to go high boost don't let them talk you onto going crazy and stick with a clean factory rebuild. The m20 isn't worth going crazy on a NA build, the return on investment isn't worth it. I've seen 10k NA builds making 220hp. For low boost (10psi or below) a totally stock rebuild is fine. People have pushed 22psi on totally stock m20s with a GOOD tune. These engines seem like they were built to boost.

Just be smart where you spend your money and you will be happy with the result.
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Tue Jan 03, 2012 4:55 am

For now im planning on running 12PSI max.

Should i have the block/head decked while im in there? How much can be taken off before i have to deal with a thick head gasket or adjustable cam gear?

I want tight tolerances, i plan on adding 3mm to the oil pressure relief to increase oil pressure(as peerless did in one of his posts). Is there any e30 oil pan baffles? or can i get away with a IJ crank scrapper?
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Tue Jan 03, 2012 2:05 pm

12psi should be fine with a stock rebuild, but if you plan to turn it up on the future getting the block oringed may be worth while. I would deck the block or head any more than what's required to get it flat. The CR on a b25 is nearly perfect for boost at 8.8:1, so raising it by decking is just going to ruin your squish area and reduce combustion efficiency. Oring and run a stock gasket with head studs before decking to increase CR. This is what I meant when I said spend the money in the right places.

If you run a stock gasket you shouldn't really need an adj. Cam gear unless you go to a crazy cam, which isn't always a good idea depending on the intent if the build. For my initial setup I plan to run the stock cam which is pretty goos for boost, and if I want to step it up i'll probably just get a cam that matches the oe duration and overlap with a bit more lift.

IE makes a crank scraper, but be prepared to do a lot of measuring and grinding to male it fit right.

Nothing wrong with bumping oil pressure, though its not really needed if the system is set up correctly. May want to look into reducing the spring pressure on the oil cooler thermostat instead so your not pushing more pressure through the turbo, just beginong to cool it sooner.
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Tue Jan 03, 2012 8:22 pm

Well i will be running a restrictor on the turbo to begin with.

How much can be removed before ill have valve clearance issues?
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Tue Jan 03, 2012 8:37 pm

Do you mean how much over size can the valves go? Biggest I've seen is +1mm intake and exhaust. Those you can buy off the shelf. You can still use the factory valve seats with those as well. Im sure you can go a little bigger with custom valve seats, but I don't k ow how much before you get valve interference.
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Wed Jan 04, 2012 7:42 pm

I mean piston to valve clearance :P
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Wed Jan 04, 2012 9:12 pm

Oh, not sure but I think people run 272s on stock bottom ends. Im still running the stock cam.
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Re: Rebuilding an m20b25. Got questions.

Postby Dontrusthise30 » Wed Jan 04, 2012 9:48 pm

I'm doing the same thing as you and I just dropped my spare engine off at the machine shop. It might be more expensive here in the keys since business is probably slow. Prices:

45 to Jet Clean
85 to bore first cylinder and 45 for each additional cylinder
They're going to check the surface as well and only resurface if necessary. So that right there is already 355 bucks! I don't know what the charge would be just for a hone though. No where near as much as boring the block.
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Re: Rebuilding an m20b25. Got questions.

Postby SkiFree » Wed Jan 11, 2012 4:50 am

In regard to a baffle placed down around the oil pump pickup, go for it. It doesn't have the same function as the scraper (think of them like Batman and Robin). The scraper's (even the fancy teflon lined ones) main purpose is going to be fighting the crank spinning oil-tornado. The baffle, however, fights oil sloshing up out of the pickup area once the oil is already there, as well as oil creeping up the walls in those long corners (thus potentially starving the oil pump). If someone claims that a baffle is not nessesary because the scraper does the same thing (stops oil creep in corners) isn't figuring that if the oil is creeping all the way up to the scraper in the first place, then it could already be too late.

Baffles can be found from VAC (super fancy), TC motorsports (functional), or IE (functional). I'd highly recommend welding them into place, the less potentially loose bolts inside the engine, the better.

M20_fever - I've played around with the drawings on the IE scraper and believe all the kinks are worked out on the new batches. If you hear of any issues from here on out, please let me know.
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Wed Jan 11, 2012 4:19 pm

Will do, wasn't bashing just stating my experience. One it was properly fitted it was a very nice piece. What software do you guys use?
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Re: Rebuilding an m20b25. Got questions.

Postby toointoxicated62 » Thu Feb 09, 2012 12:35 am

Dropped the block and the machine shop. Has some surface rust on one cylinder. They are going to let me know if it will clean up with just a hone.

Then im going to have it hot tanked. Rings, main and rod bearing and intermediate shaft bearings replaced, planed if the blocks needs it.
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Re: Rebuilding an m20b25. Got questions.

Postby M20_fever » Thu Feb 09, 2012 1:09 am

If they hot tank the block replace the intermediate bearings even if they look good.
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