Webster Build

[Brian Rupnow]

Thinning the lapping paste with more liquid does not reduce the grain size of the lapping compound. Automotive valve grinding compound is far too course. When I built my first engine I used automotive valve grinding compound, and it feels like a load of rocks compared to a proper lapping compound.

 

[Longboy]

The vinyl tubing around the valve guide to check leakage a great idea John. I use ordinary 5 minute epoxy setting the guides into the heads of my engines with no issues. All are air cooled.

I think using toothpaste for a lap compound is too far fetched reasoning for what one wants to accomplish. Its doesn't cut brass and certainly not a steel valve. If it did, you wouldn't use it for oral care then.

I have tried some diamond lap. It is expensive, probably did some work against a model engine valve and seat in a clean up effort not ordinarily needed. Making a new valve in an hour anyway.... a more economical approach. I think you are on top of it discovering the guide is not totally sealed into the valve block!

 

[simister]

All noted on the toothpaste as a "no go". However, I believe the valve and seat are ok - there appears to be a good seal there.

The interesting point is that initially I put a small amount of oil in the back of the cylinder to lubricate the ring. Now, when I first turned the flywheel by hand, I had good compression, and then after a dozen or more swings on the flywheel, the compression dropped away until there was none at all. When I put the oil in the cylinder again there was compression. Now I know that this is not a lack of compression in the cylinder, because if I remove the valve block there is good compression in the cylinder all the time.

What I think is happening, is the oil is making its way in the valve block and lodging between the valve guide and block and giving a temporary seal. When this is eventually pushed out from between the guide and the block the compression is lost.

I think the proof of this is the carbon that was lodged in that gap between the guide and the block.

Anyway, today I will seal it up and give it another try.

John

 

[10K Pete]

I think your analysis is good, especially in light of the carbon in the crack.

Pete

 

[Cogsy]

I agree that toothpaste isn't for removal of lots of material, it's more of a 'polish' I guess, but it certainly is abrasive with normal toothpaste composed of at least 50% abrasive material including such types as "aluminum hydroxide (Al(OH)3), calcium carbonate (CaCO3), various calcium hydrogen phosphates, various silicas and zeolites, and hydroxyapatite (Ca5(PO4)3OH)" - [from Wikipedia].

I have some diamond lapping pastes in various grades that I use if I need more material removal, but normally to lap valves in I only ever need to use toothpaste YMMV.

 

[Swifty]

Diamond paste in various grits is available in syringes, 13 assorted for $25 Australian on eBay. Will last for years.

Paul.

 

[deverett]

Diamond paste in various grits is available in syringes, 13 assorted for $25 Australian on eBay. Will last for years.

Paul.

Yes. Remember the old advert: Diamonds are forever. If using diamond paste, make sure you wash the surfaces Thoroughly afterwards. Some have even gone so far as to suggest using ultrasonic cleaners.

Dave
The Emerald Isle

 

[bmac2]

Hi Simister th_wwp
I think your spot on with the valve guide. I actually had a similar problem on my Webster. I’d go with Jens (Mechanicboy) suggestion and try the Loctite 638. I have an oscillating engine that I have (once or twice) run on steam and that’s all that’s holding the head on.:hDe:

 

[simister]

Thanks Bob,

Well, I have sealed the valve guide with loctite as suggested. I now have good compression - so hopefully that problem is nailed.

The engine now starts and runs very nicely. However, it runs for a while and then there is a knocking sound. it sounds like the piston is hitting the top of the head. I have checked all the clearances and can't see where it is knocking.

I then thought it may be pre-ignition, as I had the spark advanced to about 5 degrees BTDC. I then advanced it to fire at TDC and the knocking was still there. Then I thought this could be detonation due to the engine getting too hot and causing pre-ignition way before TDC.

Has anyone else experienced this problem with the Webster?

Bob, shortly I am going to do a video and post it up on the forum.

Regards, John

 

[Brian Rupnow]

Check and see that the set screw over the key in the flywheel hasn't come lose. that will give a very definite and hard to track down knock.---Brian

 

[simister]

Brian, I checked the set screw and it is tight.

 

[Mechanicboy]

Check engineparts in case there is sloppery fit between moving parts.

 

[Brian Rupnow]

Check side to side clearance at the big and of your con-rod. If you have exessive free movement there, the rod will move side to side with a strange knock every time the engine fires. Likewise, check for endplay in the crankshaft itself.

 

[simister]

I have pulled it all down again, and gone through all the parts to check for any movement. I found that there is a very small side to side movement on the wrist pin at the piston end of the con-rod. It is very small but as you say Brian, that may be enough to cause the knocking. I will re-make the con-rod bush for a tighter fit.

 

[Longboy]

If you have a few thousandths lateral thrust clearance in the rod ends, you will want to keep that clearance so the rod floats sideways some on the pin. These little engines are sensitive to friction binding of such. This would not be the source of the described pounding or knock in the rod assemble anyway. That would happen if the bearings/ bushings are undersized to the pins.

If the rod slides side to side at a 90deg. relation to the pin that is fine. However if the rod slides and bends/cocks off the 90deg...its a bad bearing or pin fit.

 

[Brian Rupnow]

I don't think side to side clearance at the piston end of the rod will make noise. However, at the journal end of the rod, it definitely will. The most common source I have found for "Knocks" are loose set screws over the key in the flywheel, or undersized key in the flywheel allowing some rotational "slop". It is almost impossible to keep the centerline of the con rod journal perfectly parallel to the centerline of the crankshaft on these small engines. The result is that as the crankshaft turns thru a full 360 degrees, the rod will attempt to slide from side to side to accommodate this misalignment.

 

[Brian Rupnow]

Another source of "knock" on the Webster is that long valve rocker. I found that the torsion spring as shown in the original Webster plans allowed the rocker arm to "float" part of the time, and it would produce a rattling, knocking sound. I solved this issue by using a 1/4" wide piece of rewind spring from an old chainsaw, doubling it over and securing it on one side of the rocker arm pivot. This keeps the rocker arm under spring pressure at all times, and got rid of the noise.

 

[simister]

Thanks guys. I have lateral movement along the wrist pin. However, I think there could be too much slop as the con-rod has some movement off the 90 degrees, as Longboy pointed out.

Brian, I also used chainsaw spring for the rocker arm from your original suggestion and it is very firm.

There is no movement in the flywheel as it is secured with a roll pin and is very tight. I do the timing adjustment from a set screw in the small timing gear.

I will re fit a new bushing in the small end of the con-rod and see how that goes.

It is a process of elimination.

John

 

[simister]

Well it is finally up and running. I found that the knocking was caused by the small end of the con-rod, hitting the inside of the cylinder when the engine fired. The clearance was only a few thou and I couldn't see how close it was. It was just by chance after putting some pressure on the con-rod that I felt it touching the back of the cylinder. I rounded off the radius of the small-end and that fixed the knocking problem.

Then when I started the engine it ran for a few minutes and stopped. On investigation I noticed that the 3 mm roll pin I had used to fix the flywheel to the crankshaft had sheared. I re-bored it and have now increased the size to 4mm. - I hope that is now strong enough.

I have used a rubber band to stop the throttle from advancing to full. I will have to make something to keep it in place.

It now needs a clean up and some final tweaking.

This forum has been an enormous help in getting the engine up and running. This is my first petrol engine and a big learning curve - thanks guys.

Here is a link to the video.


https://drive.google.com/file/d/0B3DfwczDMNaeaUllR1ZxVlpkRHM/view?ts=56724c59

John

 

[GailInNM]

Congratulations John. You have a fine runner.
Thanks for letting us ride along with your build.
Gail in NM