M20 Rebuilders guide

[xikyle]

Want to rebuild your m20, but arent sure what to use or how to do it?

This thread is to be an exhaustive list of parts, tools, tips and tricks for rebuilding all m20's (b25, b27, for forced induction or NA).

->Please specify which motor and for which application you are posting

if it goes well then maybe itll become a sticky!

[Blaine0002]

or this.

http://www.e30tech.com/forum/showthread.php?t=69492

[hotballs]

How about the poor man's route? Just the essentials. Assume you have all the used parts, what is MANDATORY to replace? What machine work also MUST be done to the bottom end?

[jlevie]

How about the poor man's route? Just the essentials. Assume you have all the used parts, what is MANDATORY to replace? What machine work also MUST be done to the bottom end?

What is mandatory to replace (other than the TTY head and rod bolts) are the parts that are worn and all gaskets and seals.

Nota Bene! Do not have the block hot tanked. Doing so will destroy the intermediate shaft bearings. Replacing them is a real pain and for most that will be a job for a machine shop. That may be necessary for wear, but there's no point in doing the job if it isn't required.

The first step in an engine rebuild, after tearing down and cleaning the block, is precise measurement of wear points. That means measuring each cylinder bore and bearing journal at several points with gages that can resolve 1 ten thousandth of an inch. For the cylinders at least three measurements should be taken down the bore and at each vertical cut three measurements around the bore. For the journals three across the width and three around the circumference at each width cut. Since most folks won't have the measuring tools needed, have this done by your machine shop.

If the cylinders are out of round or have excessive taper a rebore and new pistons will be needed. If the cylinders are okay, the block will need to be cross-hatch honed. The pistons might still require replacement if the ring grooves are worn. And it can be necessary to have the rods rebuilt to remedy worn wrist pin bearings. If the journals are round, you have to pick the bearing (by size color) for the journal diameter. In some cases more than one color will be needed to get the bearing clearances correct.

The head should be decked (a minimum cut will suffice if the engine hasn't overheated in the past) the valves ground and new seals installed. Measurement of the cam may show that it needs to be ground back to spec or replaced. It may also be necessary to replace valves are valve guides.

During reassembly the piston ring gaps must be checked and adjusted if the gap is too small.

There are a number of other things that may need attention, depending on the previous history of the engine. It may be necessary to replace the oil pump and PRV, rod out the oil galleries, replace the intermediate shaft bearings, deck & line bore the block, etc.

Once rebuilt and ready to fire it is best to prime the oil system by spinning the oil pump drive shaft with a drill. Then fire the engine, allow it to reach normal temperature and then hold it at 2500rpm for 30 minutes. That will fully break in the engine.

[riceh8r]

^^you da man...sounds about like my rebuild. german blocks are very tough, mine was still very well cross hatched. machine shop said cylinders were very well toleranced. so a good hone job was all i needed. plastigauge is your best friend during a re-build. make sure to check all journals as jlevie mentioned. i am now approaching 500 miles since re-build, and an oil and filter change is on the books, as well as a valve adjustment ( although they are quite as a mouse...surprise). do yourself a favor and do all valve adjustments after the car has set for the night...cold, cold, cold. best of luck to all who take on this rewarding, and learning experience...

[xikyle]

How about the poor man's route? Just the essentials. Assume you have all the used parts, what is MANDATORY to replace? What machine work also MUST be done to the bottom end?

This would be nice to know.

[jlevie]

There is no fixed answer. One tears down the engine, inspects and measures everything and then replaces parts or has machine work done as indicated.

[hotballs]

^^you da man...sounds about like my rebuild. german blocks are very tough, mine was still very well cross hatched. machine shop said cylinders were very well toleranced. so a good hone job was all i needed. plastigauge is your best friend during a re-build. make sure to check all journals as jlevie mentioned. i am now approaching 500 miles since re-build, and an oil and filter change is on the books, as well as a valve adjustment ( although they are quite as a mouse...surprise). do yourself a favor and do all valve adjustments after the car has set for the night...cold, cold, cold. best of luck to all who take on this rewarding, and learning experience...

I guess I was just going on a build like this one here. So all your stock used internals were fine like pistons, rods and crank. And your block was fine like bore and journals. And of course this was done by a machinist. I'm mainly referring to the bottom end.

I'm familiar with a head rebuild and from my experience the MUST category generally for new parts is new valve guides. For the MUST on machining it's a valve job and minimum decking. Sometimes the decking isn't even necessary, but others will argue. I've pretty much heard all used m20 heads will need new valve guides at a minimum. To go even further in a head rebuild new parts to replace are the rockers, eccentrics and the cam, rocker shafts and valves. All can be re-used though. We could rate these on a level of should and should not be reused. Like cams, and rocker shafts are generally always reused when in spec, but alot of people like to use new rockers not matter what for the added insurance.

Of course the other MUSTs are head bolts and a gasket set.

So back to the block. In example above, all stock internals are within spec and main journals and bore are good. So technically the MUST category for new parts are main bearings, rod bearings and piston rings? connecting rod bolts? The only machining is a minimum hone?

Feel free to add whatever, say I'm wrong, waste of time, why would you do it that way etc. or suggest more ways to go about it. Just looking for the most affordable yet useful rebuild do and do nots.

[xikyle]

Doesnt anybody have a chart of what MUST be done, What SHOULD be done, What COULD be done, and what DOESNT need to be done or something of that nature? in a generalized manner of course, all motors wear slightly differently but a m20 is and m20...

[Kev]

Doesnt anybody have a chart of what MUST be done, What SHOULD be done, What COULD be done, and what DOESNT need to be done or something of that nature? in a generalized manner of course, all motors wear slightly differently but a m20 is and m20...

There is no fixed answer. One tears down the engine, inspects and measures everything and then replaces parts or has machine work done as indicated.

[Northern]

http://www.e30performance.info/viewtopic.php?f=21&t=269