Exhaust Valve guide replacement

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alan s
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Exhaust Valve guide replacement

Post by alan s »

This has been taken (on the authors authority) from another board.
It related to a job done by this guy & using Gerry, a Morgan rebuilder as a mentor & assistant when doing the job.
It was done on the head of an 8 valve BX Gti, however, the principles used would be applicable to other engines in the range.
Some time ago I mentioned in this Forum about the possibility of valve guide replacement in the cylinder head. I thought I would share my experience with you.
Generally wear occurs more in the exhaust valve guides as they are exposed to hot gases and inlet valve guides are less likely to deteriorate. In my crazy enthusiasm to rebuild the engine, a mate of mine and I decided to replace the exhaust guides. We got the new guides made by Federal Marshall. The dimensions mentioned below are appropriate for the BX GTi 8v and might differ for the other models. The guides are 40 mm long and have an OD of 13.07 mm and a bore of about 8 mm. The trick in pressing out the old guides and replacing with the new ones is to make use of the difference in thermal expansion between brass or bronze guide and the aluminium head. Aluminium expands by about 24 µm /ºC while brass expands by about 18 µm /ºC. So we decided to heat the head and cool the guides in liquid nitrogen. If the guides are cooled in liquid nitrogen the OD of the guide reduces from 13.07 mm to 13.03 mm
It is essential that the guides must project by 2 mm from the bottom of the ports (i.e. towards the liners). So we decided to put wooden blocks of appropriate length (about 38 mm long) at the bottom of each port so that when the guides are pushed from the cam side they stop at the correct depth. Guides must be pushed out from the cam side towards the liners and inserted in the same way to prevent any damage by the carbon deposit in the ports on the liners side.
Pressing out the guides is preferred over the use of a punch and hammer. If the later method is resorted to make sure that a stepped punch is available with 8 mm diameter and about 25 mm long and then a step up in diameter to 12 to 13 mm and enough length to project well out of the head to hammer it. If a lathe is available this is a simple turning job on a steel rod. We chose an alternative method. We drilled and tapped the old guides from the liners side to fit a 3/8 inch 16 TPi UNC thread (to match the 8mm dia bore) of about 30 mm length in each one. Then we decided to use a punch of 8 mm diameter to press out the guides from the top while the head was hot. The advantage of this method is that even if the press is not available, use of hammer does not damage the top of the guide. Brass could easily deform while it is hot and may damage the cylinder head bore while it is being pushed out or worse still may even get jammed in the head if the top of the guide is slightly bent.
Initially the whole head was stripped bare of all bolts, studs and any other ancillary parts. The head was thoroughly cleaned to get rid of any oil (otherwise expect plenty of smoke while heating the head) and most of the carbon deposit was carefully removed. Heat the head to about 170ºC to 180ºC. Too high a temperature or too rapid a heating could distort the head. We heated the head to the required temperature in about 45 minutes and then let it soak at that temperature for about half an hour. We used a thermocouple to monitor the temperature. An oven in the kitchen can be used but make sure that your oven can heat as heavy a bird as the cylinder head! Also expect your dinners to smell of burnt oil for the next few weeks. I would not be surprised if the lady of the house puts you in the garage for the next few weeks. Also make sure that you heat the head from cold and do not put the head in pre-heated oven! You are not re-heating a frozen dinner! We then quickly pressed out the guides which did not require too much of a force although it could not be pushed out manually. If the temperature drops down to about 120ºC reheat the head and soak for a shorter time about 10 minutes.
After the guides were pushed out reheat the head to 180ºC to 200ºC and while the head is being heated, soak the guides in liquid nitrogen for about 5 to 10 minutes. If liquid nitrogen or liquid oxygen is not available just leave the new guides in the coldest part of the freezer for a few hours. Remove the head from the oven and place the wooden blocks under each port. Push each new guide into the bore of the head. This should be done as quickly as possible preferably under one second. I was a bit hesitant on the first guide and it got jammed half way. But the others went through without a hitch. For the jammed guide I reheated the head at the end and pressed it down to the required length.
After the head has cooled down, the guide projections were checked and they were within 0.3 mm of the required 2mm which has surprised us pleasantly. All the guides were reamed with an 8 mm reamer to make sure that the clearance between the valve stem and guide is adequate. The top and bottom of the guides were gently reamed with a tapered reamer to remove any burrs.
Whenever new guides are placed it is advisable to cut the valve seats as the centre of the old and new guides do not match precisely. Neway seat cutters seem to be referred to most often, but these could set you back by about £200/-. I went to an engineering shop which cut them for a fiver. I was advised to get the head skimmed because the head was heated a couple of times. Head skimming set me back another £25/- but the engineering shop person later told me the head was in pretty good condition. So perhaps you could save on that cost if you are careful with the heating. Lap the valve seats with the ususal fine grade grinding paste to get a good seal.
All in all we were quite pleased with our first attempt at replacing the valve guides in the cylinder head.
Dinakar
Posted by Alan S on behalf of Dinakar [:D]
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