Webster Engine Build: Piston Questions

[Danuzzo]

Been working on Sheet 5 of the Webster. Got the flywheel, crank (still needs some sanding), crankshaft, crankpin (not finalized yet), and bronze main bearings, pretty much done. Photos below of current state of build. Now on to Sheet 6, the piston and oil tube. In my other threads on this first I.C. engine build, I asked questions on different aspects, and I will continue to do so rather than one build thread that would go on forever with all the questions I have had. I do very much appreciate all the help I have received.

Edit: Decided to go with my original plan of a cast iron piston fit to size in the steel cylinder. Before the edit, I was asking about alternative to a direct fitting piston. Actually, what kind of fit would be best for a cast iron piston to a steel cylinder?

 

[minh-thanh]

what kind of fit would be best for a cast iron piston to a steel cylinder?

If it were me I would choose 0.03 --> 0.04 mm

 

[Jasonb]

I'd lap it and feel the fit as it would be too small a difference to measure with the average home equipment.

 

[Danuzzo]

In one of my previous threads on cylinder lapping, I learned quite a bit about internal lapping thanks to all who responded. Now it is on to another adventure in learning to make an external lap tool. Learning a great deal with this engine build.

 

[BaronJ]

Hi Danuzzo,

There used to be an American company making "Clamp Style" lapping tools. I don't know if they are still about since non of the search engines find anything. Somewhere I have some drawings for the tools that they made.

However they tend to be for relatively fixed sizes (diameters) so realistically I would make my own to suit the diameter that I wanted. Plus it would be cheaper when it comes to changing grits.

Quite some time ago I made a crude lap using a hinge and some softwood drilled and split with abrasive chrome polish to lap a bearing. It did the job at virtually no cost.

 

[Richard Hed]

Hi Danuzzo,

There used to be an American company making "Clamp Style" lapping tools. I don't know if they are still about since non of the search engines find anything. Somewhere I have some drawings for the tools that they made.

However they tend to be for relatively fixed sizes (diameters) so realistically I would make my own to suit the diameter that I wanted. Plus it would be cheaper when it comes to changing grits.

Quite some time ago I made a crude lap using a hinge and some softwood drilled and split with abrasive chrome polish to lap a bearing. It did the job at virtually no cost.

A hinge and some softwood? I can't imagine what that looks like. Have a drawing or foto?

 

[awake]

It is looking great! It won't be too long before it makes some noise.

 

[BaronJ]

Sorry Richard, But no, that would have been thrown away after I finished with it.

Simple to make though, two long pieces of wood, wide enough to cover the distance that you you want to lap nipped together in a vise whilst you screw a hinge to the end. At this point drill a hole through the wood where it meets slightly smaller than the diameter of the piece that you want to lap. Smear the inside of the drilling with chrome polish. Place the piece that you are going to lap in the hole and squeeze the open end of the wood whilst rotating the workpiece.

Simples

 

[Danuzzo]

Jason, on your "Firefly .46 Build" in a UK forum that you linked in another thread, concerning the piston, you stated "[t]he wrist pin hole was then done making sure not to break through and allowing for the dia being larger than finish size." What was the reason for not to "break through"?

 

[werowance]

one thing i dont know if it has ben mentioned or not but the webster doesnt have a long run duration as it doesnt disapate heat all that well. yes its a solid little engine and i love the valve body setup on it. but some told me and has been my own experience is about 10 mins run time or less and a cool off re-oil and maybe a wipe down. which is the most fun i can think of after running your first or maybe even fourth or fifth engine. and if it helps, mine kept rocking the head block screw lose. i finally lock tighted it which helped alot and probably my own poor quality machining but i still love my little webby.

edit - point being to my post is it may not be so critical on fit of that piston since it will over heat in a short amount of time. some i lap in with 600 or finer grit paste. just be careful if you do that because they will grab. be prepared to let go quickly. no rings or jewelry, no long baggy sleves around and be prepared to let go and say "oh my gosh" before realizing its all ok, just turn off and remove, wipe out and reapply some compound or as some call clover compoud. grease and some sort of abrasive.

 

[Danuzzo]

If it were me I would choose 0.03 --> 0.04 mm

Hi Minh-Thanh. Just to clarify. What I should attempt to achieve is .03mm (.00118") to no more than .04mm (.00157") under the size of the bore after lapping, is that correct??

 

[minh-thanh]

Correct !
Because we work with different types of steel, cast iron so with that tolerance will be ok

 

[Danuzzo]

Thanks Minh Thanh. I am just trying to get an idea of how the fit should feel.

 

[minh-thanh]

Hi !
Another way, because your cylinder is a little taper at TDC, if the piston has a slight stick at TDC that's good enough - it's not necessary for a 4 stroke engine - but it will give you the feel and a little lapping for the piston to slide slightly past TDC. (Or simply no lapping needed, it will be fine )