Head Questions

Post by **zippy500** » Fri Aug 25, 2006 9:54 pm

Well thanks to good ol ebay I now have a 940 Head, which obviously looks like it has been 'sitting around'.

So I'd like to give it a going over before fitting. I'd like to replace the valve stem seals, did I read BMC say that only the inlets are sealed, so doesn't oil seep down the exhaust into head?

Do valve springs need replacing.

Are the valve guides a diy job/worth doing.

Am going to relap the valve seats.

Dont think i'll bother with new unleaded valve seats, cost will start to rise then. Is it true that really only the exhausts seats need doing not the inlet?

Thanks
Andy

Post by **les** » Fri Aug 25, 2006 10:08 pm

The theory seems to be that the exhausts need a little oil as they run hotter. I'm sure you will be getting a post to elaborate on this!
You do get 8 in a set though!

Post by **Peetee** » Sat Aug 26, 2006 6:34 am

In a sound engine the clearances between valve stem and valve guide should be so close that oil won't 'run' down them. However if the seals weren't there the action of the engine drawing on the induction system when the inlet valve is open would draw oil down the valve stem too.

When the valve guides are worn the oil runs down the stems pretty much all of the time. While the engine is running it is burned in the combustion process (usually unnoticed, until it gets really bad) but after the engine is switched off some residual oil will run down and sit on the back of the valves. Then when you start up again there's a larger quantity to burn and you get the obvious signs of blue smoke from the exhaust.

Post by **bmcecosse** » Sat Aug 26, 2006 2:36 pm

I usually only fit the stem seals on the inlets - any gasket sets I have come across lately only have 4 seals included.
Gring in ALL the valves - and it's your descision whether or not to fit new guides. You need a stepped punch to do so. Aim for perfect grey seats all round - no little holes or 'cracks'.
If this 940 head is going on a small bore engine (948/998/1098) then you MUST sink the exhaust valves into the head by 40 thou, or of course you can grind little clearance pockets in the block. The 40 thou will take you for ever by hand grinding - mine were done with a 45 degree countersink held in a pillar drill - took about 10 mins to do all 4.
You also need a 1275 head gasket - and there is a waterway in the block that is not matched by the gasket or head. I put a blob of silicon sealer in this just to make sure it doesn't start to seep across the gasket.
Finally - you need rocker gear from a 1275 - or you can re-align the old rockers on the shaft (with washers and some filing) so they act directly on the valve tips.

Post by **zippy500** » Sat Aug 26, 2006 3:23 pm

this water way that needs to be blocked. Does a new hole need to be drilled to allow water throu?

How about the valve springs shall I just keep what I got

Thanks for your answers

Post by **bmcecosse** » Sat Aug 26, 2006 3:30 pm

You could drill a hole in the head and the gasket - but the gasket will have no 'curb' around the new hole and may leak. I just plugged it with silicone. In the past - I did nothing - and never had any problems. The valve springs will be fine - unless they are obviously damaged/leaning to one side that sort of thing.

Post by **Alec** » Sat Aug 26, 2006 6:20 pm

Hello Andy,

valve guides are not a DIY job unless you are equipped with seat cutters as the new guides will not necessarily be concentric to the existing valve seats. Removing and replacing them is quite straight forward (ensure they are fitted to the correct height) but as I say the seats need to be re-cut, or you may find yourself lapping the valces forever and a day. New valves, guides and re-cut seats will need hardly any lapping.

Alec

Post by **RogerRust** » Sat Aug 26, 2006 7:26 pm

When using the 12G940 head on a 948 block do I need to pocket all the valves or just the exhausts? If so why??

Post by **RogerRust** » Sat Aug 26, 2006 7:28 pm

I found an excellent old thread on the subject;

http://www.morrisminoroc.co.uk/index.ph ... sc&start=0

Post by **zippy500** » Sun Aug 27, 2006 6:07 pm

Well I have removed the valves from the head, some were rusted in some have been replaced in the past.

What can I use to remove/clean the gundge/rust from the head/valves.

I read somewhere on past threads that coka cola can be used. If I got a few litres in a bucket and submersed the head will it clean it.

Will it eat into the metal.

Post by **bmcecosse** » Mon Aug 28, 2006 9:49 am

It' s just the exhausts - the inlets can go down the bores ok. Wire brushes in leccy drill do a good job. Coke contains some phosphoric acid - this is waht does the job. I wouldn't immerse it - just brush on.