8 replies to this topic

[mini670]

I recently bought a spare very good condition and low use 12G940B head (believe the B, makes it a later SPI or MPI head). I will be mildly porting my existing heads as per Vizard. This `new` head was bought as a spare should I make a mess of the porting..... However, this new head has hardened valve seats on the exhaust so perhaps it would be better to fit this...... This said it does have smaller inlets (33.3mm as opposed to 35.6mm on my other two heads)... How much power and/or torque could be potentially lost by using the smaller valved head?

 

The other two 12G940 heads and engines I have are from a MG metro and a HLE Metro, both with the bigger valves, but no hardened seats.

 

 

Aside from that question i have noticed a couple of things that i think are rather poor on this new head compared to the earlier versions:

• Firstly the valve guides are shocking, there is a lot of slop compared to the other heads that have done many thousand miles. It almost looks like they were rough turned massively off centre then ground. As such the grooves for the seals are way off centre, along with the chamfers. One of the guides doesn`t even have the grooves!
• Looking down the bore of the guide can also see what appears to be off centered drilling, which perhaps didn't clean up when finishing to making the bore concentric to the O/D. See photos.
• Exhaust valve spring seats seem to have an extra material removed around the guide. Looked like they had been bushed out at first, but they haven`t. Not sure why this would be?
• Rocker alignment to valve stem is way off on some rockers. Comparing this to the rockers from the HLE metro is night and day. How can they be this far off. Surely would have been fairly easy to get this within at least 1mm. Do you guys have a fix for this? I can shim certain rockers to improve alignment, but others need material off, is it acceptable to surface grind rockers to bring alignment back somewhere closer to centre of rocker....... However doing this effectively moves the vertical angle of the push rods.... how much angle can they take before sidle loading the followers?

Cheers

 IMG_20210102_181310.jpg   41.69K   2 downloads

 IMG_20210102_181320.jpg   40.6K   6 downloads

 

Wear marks on rockers. The HLE in the middle has the best alignment, the rockers from the new head are at the bottom, while the wear is hardly noticeable it is definitely off centre more then normal

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Some of those rockers just hanging on edge of valve stem

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[nicklouse]

Yes the valve and rockers are in different positions for the 998 and 1275 heads. You can tell with rockers came from what head.

 

if you are going to do some operating you will be pushing the guides out won’t you? And then fitting new. Having them reamed out a d then new seats cut. So current condition does not matter.

[nicklouse]

There are left and right handed rockers

 http://www.minispare...|Back to search

http://www.minispare...|Back to search

and sets for 998s and 1275s as they are different.

[mini670]

Cheers,

Will look at the rockers closer. The rockers in the photo (well the bottom two sets) are both from 1275`s.

The valve guide thing is no issue really, but was really surprised at how crappy they were pretty much out the factory... Really really bad workmanship IMO, considering the actual cost... Did rover perhaps have these subcontracted in maybe to save a couple of quid?

Edited by mini670, 04 January 2021 - 10:05 PM.

[Cooperman]

The 35.6 mm inlet valves are really necessary for meaningful improvement.

Since a Mini is usually just a classic car and does little mileage, add a bit of lead replacement liquid to each tank of petrol.

When gas flowing, remove virtually all of the valve guide bosses where they intrude into the inlet & ex. tracts. New valve guides and re-cut valve seats will finish the job (don't forget to ream the valve guides after fitting).

After doing the gas-flowing, you need to do a trial build of the head and measure the combustion chambers so that the necesary calculations for comp. ratio can be calculated and the head finally skimmed to achieve the required final volumes.

[Spider]

There's been about 6 or 7 different versions of the 12G940 Castings that I've come across so far..

 

The valve guide thing is no issue really, but was really surprised at how crappy they were pretty much out the factory... Really really bad workmanship IMO, considering the actual cost... Did rover perhaps have these subcontracted in maybe to save a couple of quid?

 

Rarely is any Cast Valve Guide bore on centre. It's why the seats need re-cutting after replacing them.

[Turbo Phil]

You’ll need to remove the guides if you’re going to port the head, and as you’re replacing the guides you’ll need the seats recutting, so cut the inlet seat for the larger 35.7mm inlet and port it to suit. Job done!
The factory fitted inserts are extremely good so it’s worth using the SPI head.
There are some videos on my YouTube channel you might find useful if you’re going to do your own head work.
 

 

 

Phil. 

Edited by Turbo Phil, 05 January 2021 - 07:59 PM.

[mini670]

Thanks for replies.

 

Yep the guides are out of two of the heads. The SPI head will be guide-less too shortly.

Was just amazed at how poor they are. Yeah can understand that the I/D and O/D has a concentricity tolerance, which as such requires a seat recut when they are replaced.... But been that far out that the stem seal groves are waaay off centre and not present on some is crackers. 

 

Checked the sintered rockers, they don't seem to be handed left or right. Seem quite symmetrical. Just contact area is off. Think ill leave them alone, as don`t want pushrods running at and angle with the risk of catching the edges of the bore at top of cylinder head.

 

What is peoples opinion on who to use for valve seat cutting? Also require a mains bearing grind so preferably someone who cant do that also.

 

Thanks for the link Phil, will be having a look at that shortly.

[GraemeC]

Knowing where you are would help people recommend someone for valve seat and crank machining work