2.0 and 2.5 heads

[tonyppe]

howdy...
apparently the 2Litre head gives more compression than a 2.5 head when put on a 2.5 block.
is this because of the bowl where the valves are?

also if i were to swap heads, i suppose i would need to swap cams and have the 2.0 head wit the 2.5 cam yes? standard cam..

also will the valves on the 2.0 be smaller than the valves on the 2.5 ?

and with a standard bottom end on a 89' 325, with 2.0 head with 2.5 standard cam, the valves wouldnt touch the pistons even at high revs is that right?
cheers

[Jhonno]

2.0 head on a 2.5 is a no no - it wont work, it'll all go a bit wrong..

valves are smaller on the 320 head also

[tonyppe]

that was quick..
so how comes ive seen people saying they put the 320 head on the 325 then??? im well confused now.

[Jhonno]

they havent basically.. combustion chambers are too small, valves too small

[tonyppe]

so what are they going on about then?

[Jhonno]

no idea.. 320i head on a 325e bottom end maybe

[--alpina--]

I was told the when building a 2.7 engine to use a 320/323 head cause the chamber is small BUT somewhat reason i dont feel right doing that cos the valves are bigger to let more air in and fumes out quicker. however just stick a 325 head on gasflow and get it all ported right and you shoud be ok... ive built my 2.7 using pistons from the eta on the 1983 model as the pistons are highr comp ratio then 1986 onwards (super eta) you definetly get that grunt.. well i do... cheerio


jimi

[SGP]

I'd be interested to know if you could put a 325 head on a 320 and reap the benefits of those bigger valves and throttle body etc.Presumably not?

[Quaser]

Why go to all that trouble - leave the 325i engine complete and just put that in!

Cost of 325i head is around Ԛ£150
Head gasket kit Ԛ£50

or

complete 325i from Ԛ£200 + ??

[Jhonno]

I'd be interested to know if you could put a 325 head on a 320 and reap the benefits of those bigger valves and throttle body etc.Presumably not?

325i head will not work on a 320i

[tonyppe]

ive got a 325 :p
i was asking all this cos a stud has snapped in the head, and i want it done all propperly. so if its more beneficial swappign heads then i was going to port / gas flow a 320 head then take a few hours swapping them one day.
but now u lot are telling me this, ill just get the stud extracted from mine, have it ported out, all reconditioned, skimmed and ill hunt around for a thinner head gasket to get higher compression.
im guessing (i might be wrong) that there is PLENTY of valve clearance to skim 2mm off the head, and put a ultra thin head gasket on?

[Jhonno]

dont think there is.. 325i's hav valve cut outs to get clearance as it is..

[Brianmoooore]

I was supplied with a 2.5 head once, by a "professional" head rebuilding company, that had been skimmed just past the official BMW limit (124.7mm). (This is the same head I've mentioned before, where the rocker shafts had been hammered so hard that they would no longer pass through the cutouts in the end of the head.)
When fitted, this head was OK when cold, but as it warmed up, the pistons began lightly touching the head.

[Morat]

Could someone suggest a decent place to get a recon head? My existing head is working just fine atm, but it has 2 knackered exhaust manifold studs and the valve stem seals need redoing.
Its not urgent yet but I was thinking I'd be better off building up a recon head with good stainless manifold studs rather than leaving myself carless when I knacker the existing head with a helicoil kit and my banana fingers. I could probably fix the existing head in slowtime if its feasible....

Seeing as I'm taking the valves out, would new springs be a good idea or not really worth it?

[Brianmoooore]

There are loads of small engineering shops around who are members of the FER. Book it in in advance, and they should be able to turn it around for you in a couple of days, sending it back all ready to fit with everything sorted.
Expect to pay around Ԛ£300 in total for a properly done job, including all gaskets.
BMW valve springs are as long lasting as other BMW parts.

[Morat]

Cheers Brian!

[tonyppe]

i got a quote to do work on my head to bench flow it and rebuild it for Ԛ£450 - Ԛ£500

brian u mean the pistons were touching the valves? i bet this was a serious problem??!! was this at high revs only though? because the valves stretch dont they. (i dont mean like elastic though lol) ive not been taught about if the con rods stretch, thinking about it i doubt they do hardly at all. i mean the exhaust valve gets scortching..
johno-> reason i ask is last time i was had chosen a cam for my engine i built a couple of years ago my pistons already had valve cut outs. kent cams had advised them to be cut out further, and their 'absolute minimum' i was told to ignore. reason being that compression is lost when the pistons are milled. the ammount that we got milled was just a little more than half their recomendation
i had no problems with the pistons kissing the valves. even at 8500rpm lol

brian whats the official limit for skimming off the bottom of the head? cheers :p

[Brianmoooore]

i

brian u mean the pistons were touching the valves?

brian whats the official limit for skimming off the bottom of the head? cheers :p

Skimming limit for the M20 head is 124.7mm minimum total thickness of the head, from the face that goes on the block to the machined surface where the rocker cover gasket sits.
The engine in question would begin to tick as it warmed up. I don't know what happened at high revs, because it was never revved over about 3000 RPM before it was pulled apart again.
From the marks on the head, it looked like the pistons were just kissing parts of the combustion chamber. Don't think the valves were hitting at all.

[tonyppe]

o right! i would of been interested to see that. so the pistons must protrude from the block slightly. the engine i built, the pistons came dead level with the block on TDC. and the head was completely flat aswel, there was no combustion bowl and all the valves were smooth