Help with valve guide - non marine

[davidej]

This is not a marine engine problem but I know a lot of knowledgeable engineer look at this forum so I hope some of you can help.

I have a 38cc air cooled four stroke model aircraft engine.

http://www.justengines.co.uk/acatalog/NGH_Petrol_Engines.html

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 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.

 

[superheat6k]

I would consider use a high temperature chemical metal epoxy, although Loctite 601 might work - google Loctite and look at the 600 series of bearing lock compounds. The guide temperature is unlikely to rise about 150oC, otherwise the lubricating oil would burn. 601 is rated to 150oC.

 

[vyv_cox]

The guide should be a hard press fit in the head. One way of expanding it to improve the fit is to knurl it, which can add a significant amount to the diameter. I agree that Loctite will help but I come from the school that says adhesives and sealants are an extra to a sound mechanical method and should not be relied upon totally.

 

[rotrax]

Perhaps another engineer can make a tighter fitting guide with a circlip groove at the correct installed hight. A shallow chamfer can then be made at the top of the cylinder heads bore for the guide to allow the circlip to "seat". Twisting a suitable sized twist drill with your fingers may be enough to do this.

Circlip positioned valve guides are not uncommon. Note I said positioned-as Vyv says, the guide should be a hard press fit in the head.

At the tempretures found in an internal combustion cylinder head I dont believe Locktite will be successful.

After all, to remove locktited fastnings or components, you heat them up............................

 

[Heckler]

This is not a marine engine problem but I know a lot of knowledgeable engineer look at this forum so I hope some of you can help.

I have a 38cc air cooled four stroke model aircraft engine.

http://www.justengines.co.uk/acatalog/NGH_Petrol_Engines.html

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 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.

I agree with the others, it should be a tight interference fit. Any competent motor refurb shop should be able to make one for you. The one in Rhostyllen near Wrexham, made some for a petrol Frontera of mine that bust a cam belt a few years ago.
Stu
here http://erwilliams-engineservices.co.uk

 

[davidej]

How could an amateur with only hand tools knurl it?

 

[RichardS]

How could an amateur with only hand tools knurl it?

I doubt whether you will be successful as any hand formed ridges will simply flatten out when you press it back in.

You really need more metal on the guide. I wonder if you buy some copper sulphate https://www.ebay.co.uk/itm/COPPER-S...910090?hash=item43e497004a:g:alsAAOSw0ndZ5etO and make up a solution and use this with 12 power source (car battery or DC PSU) to copper plate the ouside of the guide (plug the lower end and connect the battery -ve to the inside of the top end).

Measure the diameter with a micrometer at intervals (or just wing it!) and leave it plating until you have sufficient thickness. It might take a few days but it should build up.

Richard

 

[Poignard]

How could an amateur with only hand tools knurl it?

You can't. Because you would need to mount it on a mandrel held in a lathe and apply a knurling tool. It's also likely to be hardened in which case the knurling tool won't make an impression.

If it were not hardened, you could knurl to some extent by rolling it under a bastard file. Or you could make many indentations with a centre punch.

If I were doing the job without access to a lathe I might try making the aluminium head grip the guide better by putting a steel ball over each end of the hole and pressing them towards each other in a vice.

But I think your best bet is getting a machine shop to do the repair. With the facilities and skills they have the job will completed while you're still hunting around for advice, steel balls, bastard files, high temperature locking compound or whstever.

 

[Laysula]

I remember many years ago as an apprentice, a fellow apprentice had made a bush undersize, so he pinged a few centerpunch marks into it to make an interference fit. The instructor on seeing it hit the roof with the words of "That's a bloody garage job, get out of my workshop". The air remained blue for some time afterwards.
It shouldn't be difficult to find someone in a machine shop to turn up a new guide, and possibly ream out the housing to give a good fit. If it is a push/ sliding fit you may get away with plating it, but if its loose you will have alignment problems and have difficulty sealing the valve.

 

[earlybird]

You really need more metal on the guide. I wonder if you buy some copper sulphate https://www.ebay.co.uk/itm/COPPER-S...910090?hash=item43e497004a:g:alsAAOSw0ndZ5etO and make up a solution and use this with 12 power source (car battery or DC PSU) to copper plate the ouside of the guide (plug the lower end and connect the battery -ve to the inside of the top end).

Measure the diameter with a micrometer at intervals (or just wing it!) and leave it plating until you have sufficient thickness. It might take a few days but it should build up.

Richard

I tried exactly that many years ago on a motor-cycle valve guide. It didn't work! The guide diameter actually reduced.
Afterwards I found out that the school lab experiment of copper plating by immersion doesn't give a useful plating effect. For that you need cyanide solutions etc.
The fortunate aspect of my efforts was that it wasn't my 'bike! My pal wasn't impressed.
The OP needs a new guide. I'd have thought that making a new one could be quicker than knurling the old one.

 

[RichardS]

I tried exactly that many years ago on a motor-cycle valve guide. It didn't work! The guide diameter actually reduced.

That doesn't make sense I'm afraid. If you use a piece of copper as the anode and the guide as the cathode in a strong solution of copper sulphate a thickness of copper must build up on the cathode. The OP is probably only looking for a few microns as I read it.

Immersion plating is no good. It has to be electroplating onto a very clean cathode.

Richard

 

[earlybird]

It wasn't what I expected at the time, with my school chemistry knowledge, but it's what happened. Applies to ferrous materials AIUI.
Add, I think the explanation was that hydrogen was released at the cathode rather than copper, hence the need for different solutions, but it was over 50 yrs. ago. Perhaps a plating expert will know.

 

[rotrax]

I have 55 years experience of repairing small high speed engines.

I cant imagine ANY decent machinist with a lathe and some suitable material not being able to knock up a valve guide in 15 minutes.

I could, and I am just a mechanic, not an engineer or machinist. I have one of those cheapo Clarke Lathe/Mill jobs and a few lathe tools and a copy of Leonard Sparey's " The Amateurs Lathe "

Once installed, the tiny valve seat may require recutting to achieve a good seal.

By the time you have got the gear together to plate it, you could have had ten made................................

 

[Heckler]

I have 55 years experience of repairing small high speed engines.

I cant imagine ANY decent machinist with a lathe and some suitable material not being able to knock up a valve guide in 15 minutes.

I could, and I am just a mechanic, not an engineer or machinist. I have one of those cheapo Clarke Lathe/Mill jobs and a few lathe tools and a copy of Leonard Sparey's " The Amateurs Lathe "

Once installed, the tiny valve seat may require recutting to achieve a good seal.

By the time you have got the gear together to plate it, you could have had ten made................................

Back of the net! Me as well!
Stu

 

[RichardS]

It wasn't what I expected at the time, with my school chemistry knowledge, but it's what happened. Applies to ferrous materials AIUI.

I don't usually trust the web for anything but Wikipedia https://en.wikipedia.org/wiki/Electroplating says that electoplating as I've described it is "also used to build up thickness on undersized parts" ...... so I'm officially confused.

Richard

 

[Poignard]

The problem is that the valve guide is not an interference fit in its hole.

The way to approach this is first of all to find out which of the two components is at fault .

Is the valve guide undersize or is the hole into which it fits oversize?

The guide can't have shrunk, or worn, it can only be under size if it was made under size. So the likely culprit is the aluminium cylinder head.

Generally speaking, in engineering it is bad practice to alter one component to correct a fault in another. It should only be done as a last resort; when there is no practical or economic alternative.

A competent machine shop will find out what is wrong and put it right. Their charges are usually very reasonable because they know what they're doing and they know how to do it quickly.

 

[rotrax]

I don't usually trust the web for anything but Wikipedia https://en.wikipedia.org/wiki/Electroplating says that electoplating as I've described it is "also used to build up thickness on undersized parts" ...... so I'm officially confused.

Richard

To do what is reqired and build up thickness that will stick, the gear is fairly sophisticated. First mate, who has a very logical and disiplined mind, put her hand up to be I/C nickel plating in our Classic, Vintage and Veteran motorcycle business. We offered the old fashioned soft nickel plating, as used on pre 1928 cars, motorbikes and bicycles as well as domestic items-wind up gramaphone arms for example-we did one of those for a local antique shop.

My results were OK, but by adding some kit, using a larger vessel and VERY carefully controlling tempreture and cleanliness her results were outstanding.

The corner of the workshop where this was carried out looked like Frankensteins Lab!

My experience is that it cannot successfuly be caried out without a bit of investment and a lot of practice at home.

No need to be confused, it can be done. But hooking up a bit of copper pipe on one battery lead, the work on the other and dropping it into a copper sulphate solution probably wont work in practice.

We had to use a very special powder type soap and carefully clean EVERY surface. Then flush several times with running water and dry carefully.

The electrolyte tempreture was critical and a heavy duty aquarium pump circulated the mixture at exactly the recomended GPH.

We used a battery charger permanantly connected to a battery to supply current. This kept voltage variation to a minimum.

As I said, not worth the candle when making a new guide is the proverbial piece of piss.

 

[rotrax]

The problem is that the valve guide is not an interference fit in its hole.

The way to approach this is first of all to find out which of the two components is at fault .

Is the valve guide undersize or is the hole into which it fits oversize?

The guide can't have shrunk, or worn, it can only be under size if it was made under size. So the likely culprit is the aluminium cylinder head.

Generally speaking, in engineering it is bad practice to alter one component to correct a fault in another. It should only be done as a last resort; when there is no practical or economic alternative.

A competent machine shop will find out what is wrong and put it right. Their charges are usually very reasonable because they know what they're doing and they know how to do it quickly.

Good reply-if the guide HAS been going up and down with the valve, the bore in the head will possibly be out of round and oversize.

That will require attention if that IS the case.

As you suggest, a competent engine specialist should be able to inspect, quote and repair.

 

[steve66]

Fit a honda gx35 engine instead. Cracking motors , not expensive and cheaper readily available spares

 

[lw395]

The aluminium head will expand as it heats up. That will vary according to the alloy.
But the new guide should be a press fit at 200degC or whatever SWMBO's oven goes up to.
The guide can go in cold.
A circlip as backup would be good.
There may be other ways of mechanically fixing the guide, but a circlip seems best, so long as the groove does not weaken the guide too much. Unlikely, as the new guide will be bigger than the old?