How I build my Model Engines


Active User
H-M Supporter Gold Member
Feb 2, 2014
This is from RogerL and others

I was wondering if you could do a build tread on just how you make the piston, ring, and cylinder liner. Go over in detail on the process you use to make all the critical parts, Surface finishes, lapping, etc. The theories you use for the design. Also do the same thing for the valves and valve seats. What valve seat angle do you use, how do you cut the valves and valve seats. How do you lap the valves? How do you test to see the valves are sealing properly. I know that I could learn a great deal from your experience on these two key areas.

First the disclaimer I am not a professional machinist I will show my methods of building a model engine. These are not necessarily the right way and surly not the only way to do it but my way.
These pictures are from several different model engine builds I have done and don't show all the operation.

This shows turning a 5/16" valve using some 5/16" drill rod. Many of my engines have valves made from mystery steel or steel from the local hardware store. This may not be the best selection of materials especially if your completed engine doesn't get run very often or if you don't have a lubricant in the fuel mix to ward off rust. I use a 10 to 1 solution of Colman fuel to 1 part of WD40 in all my engines which is quite high considering that many two cycle engines run a 50 to1 ratio or more.
In this case I am shooting for a valve stem size that is around 0.125 in diameter and some longer in length than I think I will need. I am not concerned if the final size is 0.130 or 0.120 just that it has a nice finish and is consistent along the length. I will try to get close to my 0.125 by filing and sanding the stem but I am not anal about it.
The next operation is to cut the seat angle but remember never remove the part from the lathe between operations. The valve face and stem must be concentric to each other.
I move the tail post out of the way and set the compound slide to 45 degrees. When I first started making valves I would spend a lot of time trying to get exactly 45 degrees I even used a sine bar and shims to get as close as possible. I have since found a small deviation of degrees either side of 45 works just as well with a lot less hassle.
Using the same tool shown and using the compound slide I cut the face of the valve from the stem to the outer lip.
I touch up the face with a small fine file and sand paper.

This shows 4 completed valves of varying lengths after the live center end was cut off and the face end ground smooth.

Valve seats
Valve seats are made from 5/16" brass and again all operations are preformed except the last one with out the removal of the stock.
I start by measuring and finding the smallest diameter of my valve stems and drill the appropriate size center hole deeper into the brass than I will need. Next I use a 0.200 drill to bore in a distance past where the intake and exhaust ports will intersect the guides when installed in the head.
The picture shows 5/8 brass rod that I have turned down to 5/16".

Here is a very expensive and precise 45 degree deburring tool (one of a cheap set from China) is mounted in the tail post for cutting the guide face.
Cut it deep enough to raise a small burr on the end of the brass guide then use a file to smooth over and clean up the end.


Part off the guide, flip it around and make a step for the valve spring to set on.

The valve guide should have a light press into the cylinder head with some Loctite added for good measure but only apply pressure to the valve spring end not the guide's face. The valve seat end of the guide is very thin and easily damaged. Don't get too hung up here if the guide is just a finger press or even some looser as Loctite is your friend.
When the Loctite has cured drill the intersecting holes in the head for the intake and exhaust ports into the valve guides.

Theses pictures show the head from my Two Timer build.

Lapping in the valves

Test fit the valves into the valve guides and choose the best fit for each. Each valve with it's the chosen guide will from now on be married together.
With the cylinder head in hand and a short piece of rubber hose stuck on the valve stem a small amount of valve grinding compound is applied to the valve face.
With a little pull and a back and forth rotating motion, the hose is spun between your fingers for about 50 cycles. Stop and turn the valve about a quarter of a turn and repeat this sequence at least 4 times. Remove the valve and clean all the surfaces of grinding compound. Valve grinding compound is quite course but as it is worked it gets broken down into finer and finer particles but not fine enough.
I now apply a coating of a copper cleaner paste to the valve face and complete the above process again. I understand that tooth paste works just as well but I never have tried it. I did try using an electric drill to spin the valves instead of twisting them by hand but I find I still have to revert back to this process to get a good seal.

The next step requires making a test plate that has an air fitting on the back and the cylinder head bolted to the front. A recess has been milled in the front of the plate to allow for movement the valves. Attach an air hose with about 60 lbs pressure and cycle the valves to clear them of any debris. Any leaks will be easy to hear and find as you rotate and cycle the valves. If you are really into perfection you can dunk this whole assembly into a bucket of water and any leaks are quite easy to see. Remove the head and do some more lapping with the copper paste and recheck. I have found that a reasonable amount of care and sealing is fine and 100% is not necessary. I have also found that valves tend seat in some after running the engine and usually get better with age.
I have tried in the past to just skip making a test plate and install the heads after lapping. I usually end up making a test plate anyway after disassembling the engine.

This picture is from one of my single cylinder builds.

At this stage you know how well your valves will seal so if you have a low compression problem it is probably somewhere else.

Hope this helps

Thanks for looking
Thank you for taking the time to do this. I think that this type of information will go a long way to making a successful engine build. When I built the hit miss engine I spent a lot of time trying to get the valves to seat properly. In the end they were not perfect but I think they were close enough. The engine would start out with good compression and the compression would go away. I think the problem was the piston / cylinder interface.

Roger L
Wow, thanks Ray for this great write up.
with the heat of the engine, won't loctite loosen on the valve guides?
I was concerned about that in the past woodchucker so I took temperature readings around different places on several of my engines.
The highest temp I could find was 184 degrees on the cylinder head on one of the engines. Most temps were in the 150s with some around 160. Checking the loctite spec sheet their lowest rated retaining product is rated at 300 degrees and #620 that I use is rated at 450degrees.

You are correct that if the engine is run at high speed for extended periods the loctite could let go but I try to do just the opposite on my builds and see how slow I can get them to run.

Thanks for the question
This post will cover piston liners and pistons.

I am afraid that many new builders are intimidated by hitting an exact size of a part or dimension and become frustrated. There are a few times when accuracy does matter like turning a shaft to fit a bearing or boring a pulley to run true but many times a couple of thousands of deviation does not matter. I will show you what I mean.

Piston Liners
I have used cast iron liners on all of my engines except for the Ford and the Horizontal Twin engines. I have come to the conclusion that if the cylinder is air cooled a liner may not be needed. If it is water cooled a liner of some type is required.
I don't like cast iron because of the mess it makes and the possibility of it causing wear problems in the machine if not cleaned up properly.
I do like the way it cuts to a fine finish that requires a minimum of secondary work. In this example I will be using a 1" piston and I want the finished liner wall to be 0.125 thick so I am shooting for an outside diameter 1.250 along most of it's length. There will also be a raised lip on one end of the liner that will fit into a groove in the cylinder.
Turning the outside diameter for a hit miss engine. The internal hole was drilled to close to 1" diameter.

The final inside diameter is turned using a boring bar tool. When you hit your size run the tool back and forth for several passes on a fine feed. You may find that lapping the liner is not needed or just a little to make a nice finish.

It may be hard to see from the picture but the liners have a larger diameter lip on one end to fit into the slot of the cylinder. The lip is made thicker that the slot is deep so the liner will stick out above the cylinder when seated.

Depending on the engine use loc-tite or sealant around the liner and the lip and place a weight on the liner to hold it in place until cured.
One of my hit miss engines ran fine for a time and then started blowing bubbles in the water cooling sump. The liner was not held down during curing and that process moved the liner away from the bottom of the slot and created a very poor seal. I had to reinstall the liner correctly and then re plane the head.
After the sealant has cured the liner and cylinder are planed down evenly as shown below.

All of my pistons have been made from aluminum and use an o-ring. I tried making cast iron rings a couple of times but it never worked for me so why not just use something that is easy to use and reliable.
If your liners are not the same exact size or close then make your pistons to match each liner. Here again I am not hung up on trying to hit exact size for the piston. If I am with in a few thousands and it slides smoothly then that is good.

I make my o-ring slot with a cutoff tool that is 0.113 wide. My o-rings are only about 0.100 thick so there is ample room for the o-ring to squish into. I have never calculated the depth of the cut but just go slowly and install the o-ring to check for fit. I would like to have a rather snug sliding fit in the liner to start with as it will get looser while seating. I do like to exercise my engines at this point by chucking the crank shaft in the lathe and let it turn for a while. If an o-ring is too tight the slot can be cut deeper and rechecked.
If the o-ring is too loose or in other words you cut the slot too deep it can be salvaged. I have done this on a couple of pistons over the years just to see how it would work and I am happy with the results. I have some outdoor sewing thread that I used to make an umbrella for my tractor and I wrap that in the slot first. I make 10 to 15 wraps of thread in the slot depending on how much I screwed up and then install the o-ring and recheck it for fit. After several years of running the fix is holding up well.


Hope you have found this informative and helpful.

(it takes longer to write this up than it does to make the parts)
Thanks for the great write ups. I know I have asked you this before but what material O ring do you use? The last ones I bought were Viton and I do not think that they held up very long. I have tried quad rings, O rings with 4 lobes, and they did not work either. My problem could also be the cylinder liner. I used cast iron and used a .750 diameter steel dowel pin with lapping compound to get a smooth finish. Maybe the surface finish is not smooth enough. If I changed the O ring the engine would run good for a while. The next day I would have problems starting it. I gave up.

Roger L
Aargh. Yet another thread I should have never opened lest I add another project to my already too long list of wanna-do's. A model gas engine looks like fun.