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Duckmang
04-29-2013, 11:02 PM
I've got a Yanmar clone 10hp air cooled diesel engine. It recently seized and bent the exhaust valve and subsequently the push rod. I've managed to make another valve from a slightly larger automotive valve with the same size valve stem. My next task is to drive out the old guide and install a new one. The question at hand is what material is suitable for a guide in this application. It has a ferrous guide but I'm not convinced its cast iron due to the way it galled. I played the dickens trying to get the valve out of the guide, due to a piece of guide welded to the valve stem.

I have materials on hand to make a cast iron guide, or a brass guide. Would one be preferable over the other? Brass seems less likely to gall in service but cast iron seems more durable. The head is aluminum, and there are no valve guide seals. There is no oil pressure to the head, it only gets what splashes up through the pushrod box.

My concerns are keeping the guide in the head despite the thermal cycling normal to an air cooled engine, and of course not galling and seizing the exhaust valve in severe duty situations. Any input is appreciated.

wierdscience
04-29-2013, 11:23 PM
Cast would be good due to it's self lubricating quality,so would Silicon Bronze and Leaded steel such as 12L14.On a lot of older heads the guide was simply the head casting.

Willy
04-30-2013, 01:27 AM
WS pointed out some suitable valve guide materials. My personal first choice for air cooled engines has always been AMPCO 45. It's a nickel aluminum bronze alloy. It does like a little lubrication but the splash oiling system for the valves on your engine should give it all it needs especially with no valve guide oil seals.
I usually shoot for about .0015-.002 interference fit, heat the head up to about 250F before I install the room temp valve. Oh and about .0025 clearance for an exhaust valve.
http://www.ampcometal.com/en/index.php?page=a45ext

I forget what I paid for a 4 ft. stick of it a few years ago but it wasn't cheap. Do a quick search of of "Ampco 45 valve guide material" and you'll see it is quite popular. Although it is certainly not the only choice.

quadrod
04-30-2013, 08:08 AM
Call these guys, they can get you a valve guide blank and you can turn to size. http://www.sivalves.com/

EVguru
04-30-2013, 04:01 PM
Colsibro is a very common choice for performance applications in car or motorcycle engines with Trojan sometimes being preferred in cars. Both available from Columbia Metals.

MrFluffy
04-30-2013, 06:31 PM
Another vote for nickel aluminum bronze ampco45 aka AMS4640, we upgrade valve guides to that when reworking a head for dragbike use. If you cant get hold of the bronze, cast iron would be my second preference.
Had a look at the si valves link, sinstered to get the wear properties. Interesting, have to read up a bit more.

Another option, get hold of a replacement valve guide to suit the engine and press it in rather than going to the bother of hunting down raw stock and making your own. Depends on your motives, but there are times I value my time more than a few bucks on a replacement part for a stationary engine.

Duckmang
04-30-2013, 09:02 PM
Thanks for the replies. I'm leaning towards the cast iron. The cast I've got has a lot of graphite in it and cuts like butter. I'd like to use a specialized bronze alloy but I'm shooting to have it back running for the weekend after next and I'm not sitting on any spare cash. The motor only turns 3600 +- RPM so the fancy alloys may be a bit overkill.

Any suggestions on how much interference I should have between the guide and the head? The valve is 7mm and the guide OD isn't much over 10mm.

FWIW, Yanmar dealers are very few and far between in the US, and one helpful with ordering parts for this series engine doesn't exist. When the hard injection line cracked, I eventually gave up ordering a new one and tig welded a repair.

quadrod
05-01-2013, 12:31 AM
For the bronze guide i got from SI valve they said .002" press fit to head, and about .0015" clearance on the guide to valve. Don't know if cast would be any different. I heated the head to 300 degrees and chilled the guides in dry ice and acetone. Also us a small amount of lube when pressing/driving in the guide.

TRX
05-01-2013, 09:09 AM
Cast iron works just fine. Or you can just go to your nearest auto machine shop and buy a guide for that size valve for a few dollars.

I've made guides before, for conversions and antiques. Given the turning, drilling, and reaming needed, I'd buy one every time, were one available in the size I needed...