M20 refresh questions

[knapmann]

Hi Guys this is my first post!

I'm beginning a refresh of my 2.5 M20. I have some questions though if I can get answers to any I'd be grateful indeed.

My refresh is on a budget, I dont want to be spending £££ where not needed right now. My thoughts are to put the money into the bottom end, the thinking being that I can pull the head in the future if I need to do more work, doing so with the bottom end would be a PITA. My engine ran ok having done 120k. The car will be a bit of a weekend toy, so will probably do <5k miles per year.

With this in mind I'm wondering about the requirement for valves, guides and springs. A full new set of each of these along with a machine shop fitting them would be nearly £500 probably.

1) Can I get away with using my current springs if I fit a 272 cam?

2) Am I realistically likely to need new valves and guides? or will a clean up, relapping and new valve guide seals suffice? Any idea of acceptable tolerances?

3) Bottom end i'm thinking about pistons and bore, what is the sensible limit of piston to wall gap that is acceptable?

4) What about crank bearing shells, I have read that the as-new clearance should be 0.03-0.07mm? At what point should I change the shells? (I'm 90% sure I will change them anyway but what if my clearance is 0.08mm for example, is this engine threateningly bad or will it run fine up to 0.2mm? I have no idea.)

5)Im going to get my flywheel lightened, how light is sensible for a mildly tuned M20?

6) As im probably going to get a bigger cam, I will test the piston - valve clearance, what is an acceptable gap for this? what is too close?

7) I have a 1988 1.3 motronic 325i engine it appears to have a cable for a lambda that is blanked off. Is fitting one simply a case of plug and play? or is it more complicated than that? will I see much benefit in fitting one?

Thanks in advance guys!

[reggid]

personally id be saving up to do it all at one go if possible but yes change only what needs to be

get the valve springs checked. you can indeed run the 272 on stock springs provided they are in good nick and haven't lost their stiffness.

valves could be ok, exhaust ones cop it more so depends, likely need guides. the key to a good valve job and seal is a firstly a good guide. seats probably need to be recut.

Bentley manual has piston to bore clearances etc etc

single mass flywheel around 12 to 13lb is the limit from what i am told perfectly streetable.

1.5 mm intake valve and 2.0 mm exhaust piston to valve clearance is ideal. Some have run less without issues