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-   -   Help with valve guide (https://www.rcuniverse.com/forum/gas-engines-142/11643082-help-valve-guide.html)

davidej 11-19-2017 05:59 AM

Help with valve guide
I have a 38cc air cooled four stroke model aircraft engine.


The engine stopped in flight and made expensive noises when I tried to restart it. I removed the head and found it had dropped a valve and its guide. Instead of conventional collets, the valves are retained by an 'E' (or is it 'C') clip. I can't quite understand why, but the guide is an easy sliding fit in its location - I thought it should be an interference fit and thus hard to press back in but this is not the case. I wonder if the guide was not properly fitted from new and it dropped, forcing the valve down and shattering the clip. but that is all conjecture.

My local engineer was going to make a new guide which would be an interference fit, but then decided it was too difficult. The guide is about 6mm dia with a 3mm hole for the valve stem and about 15 mm long.

I can buy a complete new head but this is barely economic on a second hand engine, so I wondered if there is way to fix it. Anything is worth trying as it can't make things worse.

Is there any grade of Loctite or epoxy which would work at these sorts of temperatures? Alternatively, could the guide be knurled in a way to raise high points which would hold it in its location? How would I go about this? The guide does not really have axial force acting on it so it should not be too hard to secure it.

Any suggestions would be welcome.

w8ye 11-19-2017 10:22 AM

Typically when a model four stroke drops a valve, It also bends all the mechanism. In the long run, it requires a new head assembly.

Cyberwolf 11-20-2017 10:21 AM

Hi davidej
Your delema is not uncommon in the automotive world, however it can be fixed providing the valve has not hit the piston and bent it.
I suspect the interference fit was not what it should have been from the factory and the heat made it come loose and inabled it to move up and down, it takes very little movement to wear any small amout of press fit it had to start with off and you get whats left a loose fitting valve guide.
I would not reccemend any form of a locktite unless an eclip has been added to insure there is no movement in the future.
That requires an eclip and someone to machine the old guide to accept it. This is providing the old guide still fits the hole fairly snug, Zero Slop.. Then you can apply green thread locker and push it in place and all should be well , but a valve job will need to be done on that valve if not both to insure a good seal.
Or a new guide can be made with the proper press fit ,either way and valve job will need to be done to insure a good valve seal.
Valve guide material is usually made of a tight grain cast iron or bronze, either work well.
It is to bad you don't live closer I could fix it but the cost of shipment would be more than the cost of a new head im sure.
I didn't catch what brand of engine it is.
Oh and the valve retainer.the small C looking lock is common in the smaller 4 stroke engines. My opinion is NO they dont work as well as the tapered keepers. But if a person keeps theres valves in adjustment they will work for years WO issues.

w8ye 11-20-2017 03:21 PM

1 Attachment(s)
The engine is a NGH 38

Attachment 2245835

Propworn 11-21-2017 06:49 AM

The repairs will cost more than a new head.
New guide needed for press fit on the heavy side. Head heated to 400 degrees and guide left in freezer or better yet in dry ice. Press into head quickly.

Now you need to recut the valve seat to match the new guide.

Valve is likely bent so its best to get a new valve then seat/lap the valve in with valve grinding compound. The best way even with a new valve is to just touch the face with a valve grinder to insure the face is concentric with the valve stem before lapping.

Used to do Harly heads all the time and it was quite common to replace valve guides. Luckily though the guides came in several different over sizes from the dealership. Most made the mistake of attempting to press them in and out without heating the head and the result was removing some aluminum requiring extra oversize guides or guides working loose after a short period of running. Then of course there was the odd guy who hayrubed it together and got lucky.


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