Valve-Block clearance

Post by **winger300** » Sat Feb 28, 2004 7:51 pm

I've taken my c-head off today to see if this 12G940 is fit to use.

I first tried it with a blob of bluetak on each valve, loosly bolted the head down, and turned it over. Upon removing it i found valves in cylinder 1 didn't touch the block at all, and it had a good couple of mm of bluetak. Cylinders 2 & 3 exhaust valves had squished the bluetak a fair amount leaving it pretty thin at the edge. Cylinder 4 was not so bad, but still squished.

So i was thinking it was a no-show, but then realised i'd probably put too much bluetak on the valves. Also i hadn't checked the valve clearances.

I tried again, this time setting the clearance to zero, and turning it over. It seemed to turn over smoothly, and this time i had put bluetak on the block itself. I then read about the method of allowing an exhaust valve to fully open with zero rocker clearance, and then screw it down further until the valve hits the block. Doing this i was able to screw it down a couple of full turns.

Obviously when i removed the head this time the bluetak was stuck the the valves, so im still not sure how much clearance there is between the block.

How thick should i spread the bluetak? or would something like covering the valve head in candle wax be a better idea?

All my trying has been done without head gasket, how much does the gasket add? ~1mm?

Post by **Peetee** » Sat Feb 28, 2004 10:15 pm

I would recommend you read the book 'How to power tune the BMC/BL/Rover 998 A series engine for road and track' by Des Hammill. (ISBN 1-901295-26-5).
Because of the offset of the cylinders in relation to the smaller capacity motors, only the exhaust valves in the 12G240 head will foul the block.
it is recommended however that the block is relieved below the intake valves to reduce shrouding and allow the intake mixture to flow freely into the cylinder.

Post by **winger300** » Sun Feb 29, 2004 7:50 pm

I had another attempt at checking the clearance. This time i painted the valves, put it all together, and turned it over before the paint dried.

Taking the head off, the paint on the edges of each exhaust valve was left on the block, the contact can be seen more clearly on the valves themselfs too.

So i guess either the head has been skimmed at some point, or a 12G940 just doesn't offer enough clearance on a 1098 block.

So... whats the next best step???

1) Have the valve seats cut deeper, and replace the valves with race valves that have a thinner face.

2) Have the block pocketed (not really my favoured option since removing the block would be a real pain)

3) Sell the head and find a 12G295

Post by **rayofleamington** » Sun Feb 29, 2004 8:14 pm

It varies from head to head, but this isn't unheard of with the 12G940
I ran one for a very long time on a (non-pocketed block) 1098 and it was great.
The easy way is to seat the valves further into the seats - on a 'soft' seat this can be done DIY.
Grinding the seat back a bit is one part, but you can also cut the contact face of the valve. This makes the valve less durable but if you regrng the valves every 20K or 30K miles you are less likely to develop a blow hole (I think this was a lot more normal practice people used to do this in the 50s and 60's, when they had to do frequent decokes anyway)

If you have hardened valve seats I don't know if it is easy to take a bit off the seat

An engine machinist could do it, but I don't know how much, or whether they would know what the limits are.

Post by **Alec** » Sun Feb 29, 2004 9:24 pm

Hello Winger,
if the valves don't touch the block with zero tappet clearance and no head gasket I would not worry too much.
I have a 12G940 head on my wife's Morris but I pocketed the block in situ. I made up a grindstone to the correct diameter and using the head as a jig I ground the pockets in the block using an air die grinder to spin the stone.
Alec

Post by **Matt** » Sun Feb 29, 2004 9:31 pm

so what happened to the swarf?

Post by **winger300** » Sun Feb 29, 2004 9:50 pm

yeah, i thought about doing the block pockets myself, but i'd probably just go and mess things up. I suppose you can seal up the cylinder bore using tape, and put an oil soaked cloth in there to catch the swarf.

My only tools are a dremel-type rotary tool and an angle grinder, so its not going to be an easy job.

Post by **Alec** » Mon Mar 01, 2004 6:47 am

Hello all,
I brought each piston up in turn and smeared grease on top of it. After the job was finished I cleaned the gease away.

Alec

Post by **winger300** » Mon Mar 01, 2004 12:26 pm

i dont like the sound of that. It would be pretty easy for metallic bits of grit to get down the side of the piston, which would inevitibly score the bores.

From my understanding the only way foreign material can leave the piston chamber is by being burnt up, or becoming airborne and leaving via the exhaust. Unlikely for swarf?

Alec: What did you do about the front water ways on the block? They don't match up with the holes in the 1275 head gasket. I'm read something about tapping the holes, screwing in grub screws, and then drilling new holes. Maybe you can get away without doing anything though? Ray; any comments on this?

Post by **rayofleamington** » Mon Mar 01, 2004 12:57 pm

erm.. I never worried about the front water holes as the other ones allowed plenty of flow,
However if you go for a higher state of tuning then it may be worth thinking about them.

Post by **Alec** » Mon Mar 01, 2004 1:09 pm

Hello all,
Bearing in mind that the block is cast iron (a soft and also slippery material), the particles are small as it was ground out and the grease sealing off the piston I was happy to do it this way. Once complete I lowered the pistons and gave every thing a thorough clean.
I was using a 1098 head gasket and frankly did not check whether there was any water passage misalignment.

Alec

Post by **winger300** » Mon Mar 01, 2004 1:30 pm

yeah, i thought those water ways would be ok left as they are.

Next thing is to find somewhere local that can recess the valves.

Post by **winger300** » Tue Mar 02, 2004 11:13 am

I found a nice motorsport machine shop thats going to recess the exhaust valves by 0.76mm, so with that done it should be fine, with any luck!

Certainly an easier route to take without having to remove the engine block.

I spoke the the chap at DSN classics when i was ordering gaskets and the like... he had done this mod before, and thought that the intake valves also overhang the cylinder. That can't be right though can it? my bluetak test seemed to prove otherwise

Post by **rayofleamington** » Tue Mar 02, 2004 11:38 am

winger300,
Which one are you going to use? I may be doing that myself one day on the unleaded 12G940 that I picked up.

Post by **winger300** » Tue Mar 02, 2004 12:12 pm

Chessman Motorsport
54 edgwick road
Coventry
CV6 5FQ

...down the Stoney Stanton road. Their number is 02476-689665

They're charging me £30, i thought that was pretty reasonable, since they are doing a proper three angle seat.

When i took my A-series head into their workshop, it looked a little punny compared to the V8 monsters they were working on!

In my asking about i've also come across a place in Nuneaton called Jonspeed, they do Mini tuning and racing. So another good source for parts/work and information.