M20 Valve Clearance questions....

Post Mon Nov 11, 2013 2:23 pm

Hi guys,

Hoping someone can share a bit of knowledge on this...

I'm wondering how often the valve clearances should be checked/adjusted on the M20??? (Mine is a 2.7 build!)

Also, would like to know what the effects of not adjusting them, or adjusting them incorrectly, could be???

Once my engine was built and run in for a bit, they were adjusted again, but haven't been touched since.
Could they cause any hesitation, etc when revving at wide open throttle??? (a problem I'm getting occasionally - and it just doesn't feel as lively as it did!!!)

After Ant built the engine for me, he told me the clearances should be 0.15mm (cold), as I have a non-standard camshaft and the BTB 6 branch and full stainless system.
Does that sound right, or a bit tight maybe, or would it depend on the exact spec of the cam???
Not doubting Ant, just wondering!!!

I've read all sorts of crap online, mostly saying things like too small a clearance can a loss of performance and overheating valves - and some sites seem to recommend setting a slightly larger clearance on a standard car to gain performance!!!
Obviously, I don't trust most of what I find online, so just looking for some more info & opinions!!!

Post Mon Nov 11, 2013 3:49 pm

Arn't they supposed to be 0.25mm with the engine cold.

If you have a non-standard cam shaft they normally come with a spec sheet which should detail the valve clearances. Most of the fast road type cams have the same clearance as standard. All the info is on the net if you know the cam shaft type.

Have you tried adjusting the clearances to 0.25mm (cold) and taking it for a good run.

Whilst you are there it would do no harm to clean out the intake filter, AFM, TB and pipework incase there is excessive oil in there. A TPS and CPS clean wouldn't harm eigher.

Post Mon Nov 11, 2013 3:51 pm

http://www.pelicanparts.com/bmw/techart ... Adjust.htm

Post Mon Nov 11, 2013 4:01 pm

Just found an old email - it's a 273/273 cam - but I'm sure he commented before that it was different again because of the BTB manifold???

Will all be cleaned and checked when I'm in that area!!! No AFM though - it's running a MAF conversion!!!
Some of the inlet hoses look to have a couple of hairline cracks, so I'll be replacing those, but this problem has been ongoing since before they were like that!!!

Thanks for the link - I'll have a read of that later!!!

Post Mon Nov 11, 2013 4:34 pm

273/273

that's a fast road type profile (in my limited experience) so i'd recommend standard clearances.

The MAF and zaust shouldn't make any difference

http://www.newman-cams.com/pdf/bmw.pdf

here is a 270/270 cam which has (almost) standard clearances

Post Mon Nov 11, 2013 4:36 pm

I'd send simon13 a note on here - he's a guru on these things

Post Mon Nov 11, 2013 5:08 pm

Setting too close could cause loss of compression and/or burnt valves...standard setting is .25mm,I set mine to .30mm as this improves the idle.Mine's a 2.7 as well,BTW.

TBO,.15mm does seem a little tight to me,I would have expected an after-market cam to have needed a bigger than stock clearance.
However,Ant is a well respected guy who knows these engines very well,sure you are not confusing mm with inch,as .25mm=0.010in? So 0.015in would make much more sense to me.

Post Mon Nov 11, 2013 7:24 pm

Hmmmm, I'll have to find my paperwork tomorrow and make sure that is what he wrote down for me!!!

I thought it was a cat cam, but they don't seem to list a 273/273 for the M20.
Also, no idea if I'm looking at duration at 1mm or 0.1mm?!?!?
They do list this cam with a 0.15mm clearance:
http://www.catcams.co.uk/acatalog/1300527.pdf
But most of the others are 0.25mm - including the race/rally cams!!!

Post Mon Nov 11, 2013 11:18 pm

Thinking further on this - what would happen if the valve clearance was set more than it should be??? presumably nothing too major if you're running at .30mm to smooth the idol!?!?! (mine has a rough idol as a characteristic of the cam btw!!!)

For example, if it is meant to be 0.15mm and I had it done to 0.25mm is it going to cause any damage/running problems/etc???

Post Mon Nov 11, 2013 11:28 pm

mine has a rough idol as a characteristic of the cam btw

i'm surprised by that with a 273/273 cam, should be fine at idle

Post Tue Nov 12, 2013 10:23 am

I have wondered if that was a little typo from Ant and it's not a 273/273???
It was always meant to have a rough idle though, that wasn't just realised after!!!

The engine was meant to be the same spec as Oguz on here and the wiki says he has a 285/285!!!
http://www.e30zone.net/e30zonewiki/inde ... phs#M20B27

285/285 makes more sense... cat cams do list one of those, with 0.15mm clearances!!!
http://www.catcams.co.uk/acatalog/1300525.pdf

Might have to try and email Ant again and see if he can remember!!!

Post Tue Nov 12, 2013 7:20 pm

Found the note from Ant stating:
0.15mm cold on inlet/exhaust (BTB = 0.20mm exhaust)

Should've dug that out first I guess!!!
So 0.15mm inlet & 0.20mm exhaust.

So, how often should these clearances be checked then???
Are they likely to go out (or in) with time/use???

Just trying to gauge whether to spend time/money getting this checked!!!

Post Tue Nov 12, 2013 10:37 pm

Depends what wears faster, the valves into the valve seats or the rocker arms and tops of the valves. With an engine such as these, it would be fairly logical to conclude that the rocker arms and tops of the valves will wear quickest, resulting in the clearance getting larger.

I personally check mine every time my service or inspection light shows, seems logical.

Theoretically, a smaller clearance increases the lift and duration, although only by small amounts, leading to an increase in top end performance, whereas a larger clearance will do the opposite, leading to better low end performance, especially in regards to idling.

However, in the interests of your engine, I would stick to the clearances given.

And yes, I would say it is worth getting the valve clearances checked, mainly because it doesn't take very long, and can make a big difference to the running of the engine.

Post Wed Nov 13, 2013 1:54 am

I shall get them checked/adjusted and see what happens then!!!

Many thanks!!!

Post Wed Nov 13, 2013 9:42 pm

some of the catcams call up 0.15mm clearance....i pressume so that slack is taken up on the clearance ramp part (they have shorter ramps) rather than flank which would be harder on the valvetrain.

Post Thu Nov 14, 2013 1:16 am

i've used that cam before although the duration is as long as some cams, it has much higher valve lift than most other cams. They idle a but piggy

The old trick is they run hotter with a 6 branch on the exhuast side than with the stock log and to help is to set them 0.30 and keep the inlets at 0.25 from cold

Post Fri Nov 15, 2013 7:42 pm

So you reckon 0.25 inlet & 0.30 exhaust???
0.1mm more than Ant originally said - 0.15 & 0.20 (presumably allowing that extra bit for the BTB temps)!!!

Maybe I should go in the middle with 0.20 & 0.25, just to be a bit nearer what it was?!?!?

My 6 branch is ceramic coated, but I don't know how much it actually lowers the temps around the manifold??? I know they say it makes a big difference, but....

So if setting them too close causes problems (as I've been informed above), what would happen if it was set a bit too wide???
If it was meant to be 0.15 and i set it at 0.20/0.25 what symptoms could it give??? And would it cause any damage???

Post Sat Nov 16, 2013 3:42 am

i would run at most 0.25mm just because you are deviating a fair bit from what catcam say and at some point you will run of the lash ramp of the cam but i dont know at what clearance that would occur.

i run a 298/285 catcams cam and use 0.20mm sometimes 0.25 but it is much of a muchness in reality. either works as will 0.15 per manufacturers specs. My cam looks like new after 5years which cant be said for many m20 cams,

on a stock cam you can go looser than the catcams, its ground differently with slower longer lash ramps especially the b23 cam.

looser on the exhaust helps keep the idle smoother since you have slightly less overlap area and less chance for reversion.

on the inlet side a looser clearance slightly reduces overlap area to,

looser means less seat to seat cam duration which usually is slightly more driveable.

pro racers play around with lash as it can make a small difference when every single hp counts against the clock or an opponent.

the effect is small but detectable

if you totally stuff and end up making it too tight so the valve doesnt actually seat properly you likely hurt something but setting them a bit tighter than spec wont hurt squat but will make the engine act like it has a bigger cam