10 replies to this topic

[Brams96]

I see a lot of people talking about replacing the valve guides and was wondering if this is something that can be done at home or do you have to get it professionally done? Also can you change them on a 12A940 or 12A940B cylinder head, because to me it looks like they are part of the casting?
Any help would do.

[dklawson]

The guides are a separate part. They may look like they are from the same parent material because they can be either cast iron or bronze.

I suppose you could press the guides in yourself but that's only the first step in the process. Once inserted you need to ream their inside diameter to size. Then, the valve seats have to be re-cut so they are concentric with the guides. Finally, the valves have to be lapped into the seats so everything seals well.

In short, while you could press the guides in at home, there are so many related tasks that need to be done (and require machine tools) that it just makes more sense to pay a machine shop do the work for you.

[Brams96]

Ah, point taken. I better get searching then I dont suppose you know if I can use an unleaded 1275GT head (12A940) on a 1275 SPi unleaded engine? (old head was a 12A940B)

[dklawson]

I'm afraid I can't help with compatibility questions for any of the injected cars.

I'm also not familiar with a 12A940 head.

[Brams96]

OK Cheers. Anyone else in-the-know?

[Guess-Works.com]

DK's right about the valve seats needing to be re-centred, but should not need to ream the guides if fitted correctly....

Something which will help fitting the new guides is a tool which will set the std depth... http://mra-minis.co....?products_id=46

[dklawson]

I've never know valve guides to have the right clearance when installed but I defer to John. He's worked on a lot more A-series engines than I have.

[Brams96]

Thanks guys. I still dont know if I need to replace them. I read through the FAQ's about unleaded fuel and a few other sites but I am still none the wiser about using the older head on my newer engine. As far as I can see the only difference is the leaded head has bigger exhaust valves.

If anyone knows what I need to do with the head swap please let me know as I'm struggling to find any info.

[dklawson]

The most significant change with an unleaded head is the addition of hardened seats for the valves. On the original heads, the valve seats were cut directly in the casting. When lead was no longer added to gasoline valve seats would recede slowly... sinking into the head. To correct this, hardened steel seats are inserted into the head for the valves to close against. These hardened seats look like simple rings. If you have a clean cylinder head off a car you can typically look closely at the valve seat and see if it's a separate part or not. Sometimes there are clear machined edges to help you tell. Other times there is a color and grain difference in the metal that is noticeable.

Guides are and aren't part of the unleaded conversion. For a while the logic was to go from cast iron guides to bronze when switching to unleaded. I've heard arguments both ways for which holds up better with unleaded gas. There are also material compatibility issues with some valves and the valve guide materials selected. I don't profess to be an expert on this but I'm sure the information is there if you care to Google for it... or perhaps read Vizard.

If you are planning on rebuilding your existing cylinder head and re-using the stock valves, you may or may not need valve guides. With the head off the car, remove the valve springs and wobble the heads of the valves inserted into their guides. If you can feel motion with the valve head an inch or two from the chamber, the valve guides need attention. If in doubt, have a machine shop measure them against the valve stem diameter. They'll let you know what's needed. If you are planning a major rebuild with high-performance valves, be sure and ask the manufacturer what guide material and clearance they want.

[Brams96]

Thanks for the info. I've found an engineering place that will change the seats, guides and machine the face all for £120+vat, does this sound like a good price???

[karlos_junior]

howdy, sorry to butt in on this:
Ive seen a lot latley people needing info on guides (myself included) for guides and seals as well as all the ways to check this and that with the head on and off and what to look for to see if your valves and everything is in order......

There is a welth of info on this forum and Im thinking it would be a good idea for those who know what they are on about to make a complete valve and valve related issues FAQ and basically a more complete guide. (thus weeding out the miss infomation)

just my two cents complete wih bad English

Edited by karlos_junior, 29 September 2008 - 09:23 PM.