De-Coke

[chrisgriffiths]

Hi Everyone,

I'm in the process of doing a 'de-coke' on 'Mable'. I was wondering ! how do I check if the valve guides are worn too much

Cheers
Chris

[bmcecosse]

Nobody does a 'de-coke' these days!! Don't scrape the 'coke' off the pistons.... The test for guides is simply to release the valves from the springs - and 'wiggle' them in the guides.....the movement should be absolutely minimal - it's a matter of judgement. If you do change the guides - you need a stepped drift, and use the later Mini guides with a little recess to take the much better 'top hat' valve seals - only fit these on the inlet guides.. Since you are in there anyway - check the rocker shaft and rocker arms for wear - often the arms are acceptable and just fitting a new rocker shaft can make a considerable difference. Helps to prevent the guides being flooded with excessive oil. The main reason for lifting the head will be to grind in the valves - don't skimp on that job!!

[bigjim]

pm sent

[chrisgriffiths]

OK cheers
I think I'm of the old school of cleaning of the carbon off but why is the trail of thought yo leave it on. I haven't done anything yet just removed the head cos I'm pretty sure the head gasket has gone. The pistons are covered in coke/carbon and the valve chambers are the samei haven't removed the valves yet but that is the intention while I've got the head off
Cheers

[bmcecosse]

Leave the pistons alone - unless you want to increase the oil consumption, of course.

[MarkyB]

If you think the gasket is gone the chances are that it has
Decoking isn't the regular procedure it used to be, modern petrol and oil is a lot better than it was back then.
To check the state of the guides remove the springs and see if there is any play when the valve is about 1cm from the seat, this is where most wear tends to occur. There shouldn't be any sideways movement.

[philthehill]

You can remove the carbon from the centre of the piston but leave a 3/8" wide ring of carbon round the top edge of the piston. If you remove all the carbon it will as bmc says increase the oil consumption until the carbon has built up again.
Make sure that all loose carbon is removed. I always use a high powered vacuum cleaner for that purpose.

[M25VAN]

Another way to check for guide wear is to pull the valve down the guide and with a finger over the guide hole quickly pull the valve out. You should hear a nice sharp "pop" if there is little wear. Try it on all guides to see how it varies (and the wear).

[bmcecosse]

Hahahahaha - 'pop goes the valve guide? '........ As above the worst wear is with the valve slightly extended - obviously there has to be some movement - it's a matter of judging how much is acceptable. With top hat guide seals fitted - it's not all that important (within reason...) for a standard road engine . ....obviously matters more on a highly modified engine.
Why the 'PM' bigjim?? The idea of the forum is to discuss and share ideas so others may learn from the collective wisdom........not wee secret messages between posters........

[M25VAN]

Hahahahaha - 'pop goes the valve guide?

Eh? Is that not an approved method? Why do I start to worry when you laugh?
Incidentally, I'll post a piccy when I can of my recently removed head (no jokes please ). It was quite badly coked but after a fair few of miles/years so they can need doing. I suppose people don't do the Minor mileage they used to to warrant it being a regular thing anymore.

[philthehill]

The valve guide clearances on a standard engine should be:-

Inlet: .0015 - .0025 in.
exhaust: .0010 - .0019 in.

The inside measurement of the valve guide should be .2813 to .2818 in. (7.145 to 7.157 mm)

Inlet valve stem dia: .2793 - .2798 in. (7.094 - 7.107 mm)
exhaust valve stem dia: .2788 - .312 in (7.081 - 7.094 mm)

All data taken from the BMC Wksp Man General Data Section.

Only by measuring the valve guides and valve stems of your engine and comparing with the above data will you be able to determine correctly if you valves and valve guides are worn beyond limits.

Reference to leaving a ring of carbon around the top of the piston periphery can be found in BMC Wksp Man Section AA.17.

[bmcecosse]

The guides wear to an oval shape - v difficult to measure without specialist equipment...

[philthehill]

If they wear more than the limit in any direction even when worn oval they should be replaced with new.
You can usually see when they are worn oval so no need for specialised equipment.

[chrisgriffiths]

OK cheers guys

I guess its z case of using your own judgement and see what happens

[bmcecosse]

Exactly -but if in doubt - new guides are not expensive...but DO use the Mini type if changing them. Just the cast iron type.... http://www.minispares.com/search/classi ... guide.aspx