Rebuilding an M20.. do I REALLY need to get a machine shop involved?

[hotsaucewizard]

Title states it all. I've got the motor completely stripped, new (used) crank has arrived, and I'm basically ready to put it all back together.

All of the machine shops near me have a 2 week wait minimum before they can even get to it. The motor was running "fine enough" before I pulled it all apart... the original plan was to have everything cleaned up, decked, honed, etc.. have the crank inspected, pistons inspected, etc..

Just had a thought.. what if I just went ahead and put it all back together without getting a machine shop involved? Is that a terrible idea or is this a necessity?

I've never rebuilt a motor I'm not really sure what to do here

[gonomo]

Rebuilding an M20.. do I REALLY need to get a machine shop involved?

I’ve only built one M20, but I decided to save some time and money and skip the head shaving I was originally planning on doing. I did lap the valves myself which was pretty easy. The engine is coming up on 8k miles without issues and with great compression in all cylinders.
I do want to hear if any of the experts are really set on machining.


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[the_eskimo_wonder]

I just finished up my first M20 build a few month ago. It’s got 500 miles on it now with no mechanical issues and perfect compression.

I had a machine shop deck the block, and I bought a head from Bimmerheads but everything else I did myself. Checked and honed bores, sized bearings, and assembled all by myself in my garage. In my opinion most things can be done at home, save for skimming block/head.

Now watch, I just jinxed myself and it’s gonna go boom

[kronus]

there is a variety of things a typical machine shop will do while rebuilding a motor. namely,
- dunk the block to clean residue out of coolant channels and de-oil the thing
- dunk or blast the head to clean combustion residue
- measure the piston bores for barreling or other deformation
- hone the bores to remove glazing and provide a new surface for new rings to seat on (and take out the typical lip at the top of the bore, which can break new rings)
- check the head for casting cracks around the valve seats
- check deflection of valves in the valve guides - if it is out of spec you will burn oil badly
- lap in the valves
- flatten the head surface - if the head is warped you will get less time out of your headgasket
- pull the oil galley plugs and wire brush out the galleys
- gap piston rings
- measure the crank journals and figure out bearing oversize/undersize necessary for proper gap

all contribute to engine life. you can have them do any subset of them.

at minimum, you should have the head decked, and since you are changing the crank, you should measure it to understand which bearings you need to buy for it.

[Stanley Rockafella]

there is a variety of things a typical machine shop will do while rebuilding a motor. namely,
- dunk the block to clean residue out of coolant channels and de-oil the thing
- dunk or blast the head to clean combustion residue
- measure the piston bores for barreling or other deformation
- hone the bores to remove glazing and provide a new surface for new rings to seat on (and take out the typical lip at the top of the bore, which can break new rings)
- check the head for casting cracks around the valve seats
- check deflection of valves in the valve guides - if it is out of spec you will burn oil badly
- lap in the valves
- flatten the head surface - if the head is warped you will get less time out of your headgasket
- pull the oil galley plugs and wire brush out the galleys
- gap piston rings
- measure the crank journals and figure out bearing oversize/undersize necessary for proper gap

all contribute to engine life. you can have them do any subset of them.

at minimum, you should have the head decked, and since you are changing the crank, you should measure it to understand which bearings you need to buy for it.

I agree.

There's many things which a shop will check that will ensure a problem-free rebuild once everything is buttoned-up.
You have no idea how many times I've read threads of owners attempting a rebuild just to have to tear it all apart again because they overlooked critical details.

[digger]

So everything is running fine yet you decide to pull it apart anyway but once it's apart you don't want to wait a few weeks so you are going to put it together again?

[varg]

If you're going to rebuild an engine you may as well do it right, because if you have to do it again because of something you neglected to do it's going to cost you more in the end.

[ForcedFirebird]

At minimum I would suggest at least have everything measured and inspected by a machine shop, or qualified machinist/builder. As mentioned, do you want to have to do it again?

[tinoe30]

Hmmmm.....

You said.....I've never rebuilt a motor I'm not really sure what to do here.

Not saying it can't be done DIY but you really need to have a plan and do it right. As others have said this is not a job you want to do twice.

[IRON-E]

At minimum I would suggest at least have everything measured and inspected by a machine shop, or qualified machinist/builder. As mentioned, do you want to have to do it again?

+1 Agree.



I work for a domestic eng/trans remanufacturer, the amount of cleaning, decontamination, machining, measuring and clearance specs that go with any build, are unbelievable.
Can you do it yourself? Sure, but do you want to run the risk of granading your rebuild, as others have mentioned previously?

[hotsaucewizard]

So everything is running fine yet you decide to pull it apart anyway but once it's apart you don't want to wait a few weeks so you are going to put it together again?

Sorry, should've been more clear. The timing jumped a tooth, exhaust valves were making friends with the pistons. Those items aside it was running fine lol. Obviously the necessary remedies will be in place for all of that. Additionally it's being stroked to 2.8 so the motor is not just torn apart for fun haha

[hotsaucewizard]

there is a variety of things a typical machine shop will do while rebuilding a motor. namely,
- dunk the block to clean residue out of coolant channels and de-oil the thing
- dunk or blast the head to clean combustion residue
- measure the piston bores for barreling or other deformation
- hone the bores to remove glazing and provide a new surface for new rings to seat on (and take out the typical lip at the top of the bore, which can break new rings)
- check the head for casting cracks around the valve seats
- check deflection of valves in the valve guides - if it is out of spec you will burn oil badly
- lap in the valves
- flatten the head surface - if the head is warped you will get less time out of your headgasket
- pull the oil galley plugs and wire brush out the galleys
- gap piston rings
- measure the crank journals and figure out bearing oversize/undersize necessary for proper gap

all contribute to engine life. you can have them do any subset of them.

at minimum, you should have the head decked, and since you are changing the crank, you should measure it to understand which bearings you need to buy for it.

Yeah getting the block cleaned up was very appealing. I figured although I haven't done it that I could probably fumble my way through a decent enough hone for the cylinder walls.. as well as lapping the valves and gapping rings and basically everything else I could do at home.



I'm generally of the belief that if it can be done at home it should be done at home, I'm pretty hands on DIY. For example, rather than just trusting a pro-built Megasquirt unit I opted to build my own. Would I do that again to save $500? .....debatable. But am I that much more proud every time I turn the key? Absolutely! I kinda have the same train of thought for the motor but I understand there are certain things that need to be left up to the pros.


Sounds like I'll go to the shop anyway, I may just meet in the middle and take care of the things that I can get away with at home. Hot tank and decking I'll for sure pay out for but I'll have to do more research on the rest as far as whether it's really worth my while to pay someone else.

[digger]

Sorry, should've been more clear. The timing jumped a tooth, exhaust valves were making friends with the pistons. Those items aside it was running fine lol. Obviously the necessary remedies will be in place for all of that. Additionally it's being stroked to 2.8 so the motor is not just torn apart for fun haha

even more reason to get the machine work done.

changing the stroke without approriate measures taken regarding the bores is not a good idea as the rings need good fresh bores to run on, plus the more invesited in the engine the more it makes sense not to cut corners

[zaq123]

do you absolutely have to involve the machine shop? no
should you? absolutely yes.

look it this way.. would you install used water pump, wheel bearings, used brake rotors etc etc? you could but how long will they last? same concept here