731 Head - Route to take

[monkeyboy1979]

I've got a 731 head which is standard and apparantly done 125k.

Should I bother doing a pressure/crack test etc or just go straight for a rebuild?

The head is to be used in a 2.7 conversion.

Going off daimlermans advice I'll probably pop a 323 cam in so the springs etc will need doing anyway.

[m_jermyn]

Always do a pressure test mate, along with a reface, regrind the valve seats and replace the valve seals and guides..

Do it right do it once.

[monkeyboy1979]

Hi, cheers but should I bother before I do the rebuild?
Think people advise that the springs etc need to be replaced at about 130k anyway so I don't want to waste money pressure testing if I'm going to rebuild.

Am I answering my own question here!

[m_jermyn]

Just take it to the machine shop and tell them to make it brand new with new guides seals.

Every shop I know of will always pressure test and crack test a head before rebuilding.

If you like you can buy the seals and guides and supply the shop with them, thats how I did it.

Also put the double valve springs in as they allow more revs.

[daimlerman]

Strip the head completely,clean it properly.

Carefully examine the cam carrier bearings for hairline cracks,check the the waterways around the face of the head.

The head will need skimming before fitting
The head will almost certainly require at least new exhaust valve guides

These last two it is unlikely that you can DIY,so whilst the head is at the machine shop,get the bugger pressure tested,first,before you spend any cash.

Cam choice is dependent on final use of the engine,the more aggressive the cam,the further up the rev range comes the power,323 cam is OK,now I have got used to the idea of using the gearbox again! 325 cam let me use 5th to overtake,but lacks the top end of the 323 item.

Whatever setup you use,it's well worth while swallowing your pride and letting some-one with the right gear and knowhow set the ECU chip up properly,I can reccommend Gunni's services here.

[Jhonno]

You need to pressure test to make sure it is worth rebuilding!

[monkeyboy1979]

Cheers guys. Just having it checked. Got the head from a guy on here and just found out its been welded.... we'll see how she holds I suppose.

Have get the bottom looked at as well, think the rings have a had it so could be digging deep here!

[daimlerman]

As long as the bores are decent,you can keep costs down,if you need a re-bore,find a better s/h block and have it honed.

Set of rings will be about £100,re-bore about £30 a bore,bigger pistons,bend over and lube up......you are into seriously big money...

[Morat]

It seems rarer to find an unwelded head these days

[monkeyboy1979]

Well popped the head off the ETA and 5 of the bores look smooth, showing some light scoring, proper coked. #3 though, thats got some rust in there.....

I saw a post somewhere mentioning what to look for if the thing needs re-boring. Do u

Still waiting for the shop to get back re the head.

[monkeyboy1979]

Well took the head off the ETA and 5 of the bores showing light scoring which I assume is ok, #3 though, thats got rust in it.... light though, would honing sort that? Obviously leaking head.

There was a post somewhere that mentioned what to look for if it needs a rebore. Any one noticed?

Still waiting for the head to be tested.

[Barx325i]

if you do end up using the 731, take to a chap who will correctly port it within an inch of its life & polish it up

If you're brave enough do away with as much of the remaining guide as you feel comfortable with, but smooth it off. Shortening the guide will decrease its life, and also increase ware to the valve stem, but you will notice a good performance increase.

The 731 is a touch wheezy otherwise..

[monkeyboy1979]

I thought the 731 head has the same exhaust ports as the 325 just the inlets smaller thats all I was going to port really as I've got the 325 inlet manifold and tb to go on.

Notice you're local, reccommend anyone?

Not sure whats taking this place so long to test the bleeding head. I want it all doing in 2 weeks!

[Morat]

It's harvest time, lots of combines to fix!

[Barx325i]

The last head I had done was at cms (possibly) but that was back in 90 something. Best asking about but if you want to take a look at the hartge head I have here for reference, you're welcome.

[Frany]

If you are running a 2.7 setup it is pointless fitting a welded 731 head !! rather find a 885 head with bigger intake ports and valves. cuz the 731 narrows to the same valve size as the 200. even if u do have the bigger manifold its going to be useless and ul have a slow airflow through the manifold and fast through the ports as a result almost no air turbulance so less air enters combustion camber as its not swirling. thats the reason the intake ports are left rough to create turbulance. So fitting a 885 head will be your best option as it is already biult that way by bmw in the later 325is moddel here in sa. also u wil have the rough ports for turbulance and bigger valves. all that remains is fitting stronger valves springs and a better cam. no porting needed and will probly work out cheaper cuz u can sell your 731 head to cover costs

[monkeyboy1979]

I see where you are coming from.

I thought if you pop the 885 on the compression drops to not too healthy unless you strap a turbo to it?

Or is that myth. daimlerman runs on a 731, what say he?

[Andyboy]

Scored bores = a rebore, probably. Get the machine shop to look at it - a good old honing might clean it up but if not, throw it away and get a better one. Scored bores means that you'll get blow by.

You can run an 885 head on a standard Eta short engine and have a perfectly acceptable compression ratio. Read though the Zone Wiki on this.

[monkeyboy1979]

Hi, yes I've checked this out but I'm not seeing huge differences between them, apart from top end. The 731 appears to be better for low down power, more useful as a daily imo.

I'll stick with the head, its passed the comp test and the chap says she looks all good.

Bottom end wise, we will see what next week brings.

Thought the scores could be cross hatching I.e good sign.

Cheers for this thread!

[m_jermyn]

The 731 head has the same size ports as the 885, infact the 731 matches the 325i with almost absolute perfection.. The only difference is the between the two heads is the valve sizes and the combustion chambers.

[monkeyboy1979]

Lovely.... valve guides will need doing anyway so might open them a little.
Cheers dude!

[eta]

Also a 731 head on an eta and an 885 head on an eta gives very similar performance.

I am also have a 731 head rebuilt as the 885 head on my eta block has cracked.

[reggid]

The 731 head has the same size ports as the 885, infact the 731 matches the 325i with almost absolute perfection.. The only difference is the between the two heads is the valve sizes and the combustion chambers.

This is not really correct. The B25 manifold suits the 731 head nicely but the B25 manifold is noticeably smaller than the 885 ports.

[monkeyboy1979]

So are we saying that a B25 inlet manifold will go straight in the 731 head? Thought the ports needed enlarging. Well I'll find out this afternoon either way!