"Original guides have a very small taper and the top is slightly bigger in diameter than the bottom"

Michiel, I expect any taper you've found on a valve guide is a manufacturing error.

Mark

email (option): pes.sales@btconnect.com

"I expect any taper you've found on a valve guide is a manufacturing error."

Don't think so, Mark...they are all tapered in the same direction, the bottom, (the side with the larger chamfer) is a fraction smaller, average approx. 0.1 mm.

So the guides have a top and bottom, the exhaust is a bit longer and on top has a small chamber bored in to protect the valve from the heat.

Cannot measure the holes in the barrel but think they are tapered also.

When fitted from the top the guide goes in much further than from the bottom, so fitting from top and following the manual is very critical..., sure others who have dealt with this will confirm.

No press needed, I found they fit quite easily from top with a reduced brass punch and hammer.

Please look into it, you warn for a ruined barrel, but actually, I think your advice increases the risk.

BR Michiel

Yes Micheal you are correct about the taper on the guides, but the bore is Parallel.
A serious mistake in my opinion.
When new I can see the ease of assembly, unfortunately once used it would be very hard to clean the relief in port side of the bore.
Funnily enough I'm quoting on the manufacture of valve guides for a much older engine and they have parallel guides.

You live and learn.

BTW, my guides pressed in with easy.

Mark

email (option): pes.sales@btconnect.com

['A serious mistake in my opinion']....Perhaps not theoretically perfect but not a serious mistake...I bought a one owner M21 with 112,00 miles on it years back and it had had three exhaust and two inlet guides guides in that time...One I built has now completed around 60,000 miles with no replacement guides to date and it's still running...
Ditto Deans comments about powder coating...Been there, done that and burnt off the results...It doesn't last long if the bike is used and will also burn off the cylinder head. Usefull if you want to know where the hot spots are though:laughing: ....Saying that there are now some high temperature powders but I've yet to look at their effective temperature ranges after coating..If you could get the barrel clean enough ceramic coating would be the answer I'd have thought....Powder coating will last fairly well on the much cooler running OHV B31/33 motors and I would say it's a practical finish for them which I routinely use...
Details of the valve guide dimensions are in the M20 standards book... (.624"/.625" tapering to .626"/.627")...So, a .001"-.003" taper along the length which equates to a very shallow angle....Ian

email (option): ian@wright52.plus.com

To find such a large taper on the outside of the guide was a shock to me Ian, you live and learn every day.
I've still not got a registration number for the bike, it seems they want to inspect the "running" bike.
That should be taking place at the end of the month.
I think I will try painting the head with epoxy primer and topped with 2k acrylic paint.
If it doesn't work, I guess it can be removed again.

Mark

email (option): pes.sales@btconnect.com

Mark I've just been through the DVLA age related process for this bike which also involved an inspection. (One of many over 37 years) I've never had to run an engine before and in several cases the bike wasn't at the running stage. The bike should be fairly complete and "original"!!!???. The inspector is only there to check the numbers and make sure there is nothing suspicious.

I only ever spray my barrels and heads with VHT rattle cans which is easy to touch up with "Pot Black" when it burns off. Stove enameling is an option, but I don't think they used it on WM20's

This bike is running fine now, but was not running 3 weeks ago for the inspection. Ron

email (option): ronpier@talk21.com

Yes, my T120R was checked long before it was running (or fully completed)...Ian

email (option): ian@wright52.plus.com

Strangely they must think me untrustworthy as it clearly states it must be running.

Mark

email (option): pes.sales@btconnect.com

Interesting that they state it must be a runner. I've recently registered two 1970's TY250 Yamaha trials bikes, the chap who inspected them really wasn't bothered about me running them, he just took a few pictures of the whole bikes and frame / engine numbers. Seems to me there is absolutely nothing to worry about as long as the bike is what you say it is. I had both log books and reg numbers back in less than 10 days for both bikes.
Some people clearly take the mickey, I got talking to the inspector and he showed me several pictures of a Triumph Cub trials bike which he had just inspected. It was quite obvious - And he knew it, that the bike had no original parts except the crankcases, frame,forks,wheels and tank were all brand new. He just shrugged his shoulders when I asked him what the DVLA would make of it when he submitted the pictures.Your M20 will be no problem !