To machine or not to machine...

[Zeuk in Oz]

BillR, Winfred, Shogun et al . Advice needed.

Have removed the head from my M20 and it would appear that the head has never been off before given that hexagonal bolts (apparently seen in early e34s with M20) have not been replaced. It is a 12/88 car.

Speaking to a couple of machine shops the advice seems to be to mill, pressure test and vacuum test no matter what state the head is in. They aren't interested in checking to see if it needs machining - they will machine it regardless.

Is this logical ? It has done 240,000 km (approx 150,000 miles) and it did not overheat when the head gasket went.

They seem to argue that all alloy heads will be a little warped and thus will need machining.

I have no problem with this except the logic - cost is not an issue.

Any advice ?

[pundit]

It's worthwhile having the head properly tested and machined.
It's rare that any head will be perfectly true, and even if the head is true a very light surface skim will clean up the gasket mating surface ensuring a better seal.

Often just cleaning off the old gasket can lead to some scoring or damage to the surface and this could lead to some problems later on.

If you are intending to keep your car for sometime (at 240.000kms) I would consider a full reco ie. machine valves and seats, stem seals etc. M20's are renown for cracking heads so any reputable shop should suggest a pressure test.

It's case of doing the job right once. You need to decide if you want to risk pulling it apart again in six months or not. Also make sure your cooling system is perfect while you're at it.

[Bill R.]

any shop is to clean the head up first, then clean the head gasket seating surface. Don't let them use surface conditioning disc's! Then you take a mill smooth file and carefully run it accross the flat surface of the head locating any nicks or high spots, take them down carefully with the file. Then last you take a long precision ground straight edge steel thats made just for checking head flatness.They lay this across the head from different directions lengthwise and across the head. X pattern etc.. With the straightedge laying across the length you take a .002 inch feeler gauge and see if there are any gaps under the straightedge big enough to allow you to slip the feeler gauge in between the straightedge and the surface of the head. You check it from multiple directions with a light held to the backside of the straightedge so you can see and gaps that occur. If its greater than .002 then the head should be resurfaced. If less then there's no reason to do so. But this process takes time to do it correctly and many shops don't want to spend that time. They want quick and dirty. With a maximum of only .019 allowed to be cut from the head anyway , compared to a number of engines i work on thats a very small amount. The machining process can also be pretty easy to screw up if they don't care or are sloppy. For the head surfacing machine that i showed you earlier the greatest amount of time is spent mounting the head and getting it shimmed up and leveled so that your cut is equal across the length of the head... This can take a lot of time to do it correctly and then again alot of shops won't because you as the end user really can't tell if it was level before the cut was taken.

BillR, Winfred, Shogun et al . Advice needed.

Have removed the head from my M20 and it would appear that the head has never been off before given that hexagonal bolts (apparently seen in early e34s with M20) have not been replaced. It is a 12/88 car.

Speaking to a couple of machine shops the advice seems to be to mill, pressure test and vacuum test no matter what state the head is in. They aren't interested in checking to see if it needs machining - they will machine it regardless.

Is this logical ? It has done 240,000 km (approx 150,000 miles) and it did not overheat when the head gasket went.

They seem to argue that all alloy heads will be a little warped and thus will need machining.

I have no problem with this except the logic - cost is not an issue.

Any advice ?

[winfred]

i haven't had any luck with getting heads pressure tested successfully, but definatly straight edge it i've gotten some that were 20+k warped

[Zeuk in Oz]

It's worthwhile having the head properly tested and machined.
It's rare that any head will be perfectly true, and even if the head is true a very light surface skim will clean up the gasket mating surface ensuring a better seal.

Often just cleaning off the old gasket can lead to some scoring or damage to the surface and this could lead to some problems later on.

If you are intending to keep your car for sometime (at 240.000kms) I would consider a full reco ie. machine valves and seats, stem seals etc. M20's are renown for cracking heads so any reputable shop should suggest a pressure test.

It's case of doing the job right once. You need to decide if you want to risk pulling it apart again in six months or not. Also make sure your cooling system is perfect while you're at it.

Thanks pundit.

I will be having the head reconditioned but my concern was that these heads can only lose such a small amount before you have to throw them away.

I am replacing the radiator and all hoses plus I changed the waterpump and thermostat 4,000 km ago.

[Zeuk in Oz]

Thanks again Bill - what a fountain of knowledge and experience.

I will keep talking with head reconditioners until I find one that will do what I want - a check first.

[Zeuk in Oz]

Thanks Winfred - your advice is noted and really appreciated.