Author Topic: Horizontal Air Cooled Engine  (Read 12285 times)

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #195 on: January 07, 2022, 06:26:02 PM »
Today I'm just piddling around putting in time. I had to look thru all my various bins of junk to find something, and what I actually found was a plethora of pistons. I see two with Viton rings, one with cast iron rings, one with no rings, and one that had been lapped into a cylinder without even any ring grooves. I have no idea why I saved these, nor what was wrong with them. I guess it's part of the "Never throw anything out" syndrome that I have. I'll keep them. Most of them have a thick enough wall that I could perhaps use them in an engine with a 31/32" bore some day.

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #196 on: January 15, 2022, 05:10:07 PM »
Today I went digging thru my stock of brass, and found a length of 1.25" diameter brass. This will be big enough to make a 7/8" external lap from. I may make a new piston from cast iron, and lap it to be a very precision fit into the cylinder bore. This will definitely help with the compression issues. I will still make new cast iron rings for the piston, when my length of cast iron specifically for making rings from arrives here from Wisconsin. Right now my aluminum piston has about 0.002" diametral clearance in the cylinder. I should be able to bring that down to 0.0002" diametral clearance  with the lap and a cast iron piston.
« Last Edit: January 15, 2022, 05:13:39 PM by Brian Rupnow »

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #197 on: January 15, 2022, 08:20:17 PM »
So---this is the external lap for a 7/8" piston. It is drilled and reamed to 0.875" thru. I've stuck a 7/8" piston into one end of it for "show and tell". In actual use, there are no rings installed on the piston. The sawcut lets it open to greater than 0.875", and lets it close to less than 0.0875". The single shcs in the center provides the clamping force to close it. In use, the piston is mounted on an arbor of lesser diameter that fits into the counterbore in the piston and cross-pinned thru the piston pin bore, and is held in the lathe chuck. The brass lap is coated with 600 grit lapping paste on the inside and held in your hand, and is worked back and forth on the rotating piston at about 70 rpm. You start with a slightly oversize piston and work it with the lap until it will just fit (somewhat tightly) into the cylinder. then everything is disassembled and brushed with dishsoap and an old toothbrush to clean away any remaining lapping paste. You have to be ready to quickly let go of the lap if it "grabs". Don't want your arm wound up in the lathe!!! If it does "grab", you let it go, shut off the lathe, and then walk everything out to your bench press, press the piston out, then back into your lathe to finish it up.

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #198 on: January 16, 2022, 04:34:43 PM »
When I made this external lap, I reamed it with a 0.875" reamer. After I put in the saw cut, it closed up a bit  due to internal stresses in the brass and reads about 0.870". Next step will be to coat a piece of 7/8" round cold rolled in my lathe, add some lapping compound to it, and lap to an even 0.875" inside diameter. At that point I will tighten the cross bolt and close the sawcut up a bit more, and again lap it on the 7/8" diameter rod until I reach a point where the i.d. is 0.880" inside diameter when the clamp screw is backed off. This way I can make my piston .002" to 0.003" oversize, then using my external lap and lapping paste bring the piston down to an exact 0.875" diameter for a snug fit inside the cylinder.

Online Roger B

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Re: Horizontal Air Cooled Engine
« Reply #199 on: January 16, 2022, 07:51:01 PM »
Don't make the pistons too close in diameter. An aluminium piston in a cast iron/steel bore needs some space for expansion. I used a cast iron piston in a cast iron bore for my diesel, around 40 Bar compression, to reduce the expansion problem. My horizontal engine initially would start and run without piston rings (ali. piston in ci liner) but would then sieze as it warmed up. I had to take a few 0.01mm off the piston and add ci piston rings to get it where it is today.
Best regards

Roger

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #200 on: January 18, 2022, 10:50:27 PM »
Okay--Back to the external lap. When I made this lap, I drilled and reamed it with a 7/8" reamer, and it was a perfect 0.875" i.d.  Then after I put the sawcut in it, it closed up to something less than 7/8". I thought about this for a while and decided to set it back up in my mill and run the reamer thru it again. That worked, but the lap still wanted to close up a bit. I mounted a piece of 7/8" cold rolled steel in the lathe and turned a 1 degree taper on it. Then I coated the cold rolled with 600 grit aluminum oxide paste and with the lathe turning about 70 rpm I worked the lap back and forth  slowly over the tapered end by hand until it slid freely over the 7/8" cold rolled steel shaft. This was done without the clamp screw in place. After it slid freely over the shaft, I took it out to my bandsaw and ran the blade thru the existing cut again. Then back into the shop to run it over the shaft a few more times to knock down any internal burrs from the sawcut. Cold rolled steel comes in about half a thou undersize, so I know that at this point my lap is 0.8745" inside diameter. Next step will be to gradually tighten the clamp bolt and continue running the lap over the cold rolled with the 600 grit paste. When the clamp bolt has been tightened up enough to close the sawcut, then I know that when I release the clamp bolt the lap will spring open to something greater than 0.875". Not sure how much greater, but then it will be at a point where I can begin lapping my cast iron piston from "oversize" to a "perfect fit" in the cylinder.


Offline Admiral_dk

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Re: Horizontal Air Cooled Engine
« Reply #201 on: January 19, 2022, 12:23:01 PM »
Good that you found out why Brian  :ThumbsUp:

I think I would not close the clamp all the way for the last pass, as that will not give you total ajustability / possible compensation for gritsize ....

Per

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #202 on: January 19, 2022, 07:21:00 PM »
The external lap is finished. When the clamp bolt is loose, the lap measures about 0.880" inside. When the clamp bolt is tightened, it measures 0.870" inside. I will machine my cast iron piston to 0.880" diameter, and then with the lap I will lap it down to a point where it is a snug fit into the cast iron cylinder.


Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #203 on: January 20, 2022, 03:19:08 PM »
A bit of explanation now, on what I've been doing. I was very surprised when the engine didn't want to run with the new cast iron rings I made for it. And then, I figured out that I had used the wrong kind of iron to make the rings from. This was surprising, because the rings were made from the same stick of iron that made two or three sets of rings that did work in the engines they were made for. The shop I buy my iron from didn't seem to know if this was ductile iron or the other type of iron. I found out after the fact by seeing how easily the rings bend.  My current piston is aluminum and is 0.873" diameter, riding in a cast iron cylinder. When the engine wouldn't run, I made a second aluminum piston the same diameter with a viton ring for a reality check, and the engine ran fine with a viton ring. I then ordered a stick of the correct type of iron for making cast iron rings from Wisconsin, and it showed up here yesterday. I have a choice now--I can make new rings and put them on the existing aluminum piston----or---I can make a new cast iron piston which is a closer fit in the cylinder and put new cast iron rings on it. This is why I made the brass external lap, to lap the new cast iron piston so it would be a closer fit in the cylinder.-not an air tight seal, but much closer to it than the aluminum piston was. I'm not worried about thermal expansion, because the piston and cylinder will be the same material so if the piston grows a bit from heat, the cylinder will grow as well so it shouldn't seize on the piston. At the same time I was waiting for my new iron, I started a new thread about the possibility of using my Toolpost Grinder from Little Machine shop to grind the piston to the exact outside diameter instead of lapping it. I got so many varied answers to that question that I decided not to do it. So, armed with the knowledge I have collected, I will now make a new lapped cast iron piston from the old stick of cast iron from my local supplier and new cast iron rings from the new correct material from Wisconsin.

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #204 on: January 20, 2022, 07:35:49 PM »
Meet the new piston--Looks an awful lot like the old piston. However, it's made of cast iron, not aluminum, and it's 0.003" larger in diameter than the old piston (the one with rings on it).  I had hoped for 0.005" larger in diameter, but you work with what you get. If it works better than the old piston, we're golden. If it doesn't, I had the material anyways and it's 2 1/2 hors out of my life that I can spare. Sometimes I'd swear that my lathe is bewitched.--It never takes off less than I wanted it to, but frequently takes off more!! Tomorrow I'll make some new rings and heat treat them.


Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #205 on: January 20, 2022, 10:25:10 PM »
Things didn't go the way I had it planned. My plan was to turn the new piston to about 0.880" diameter, and then use my external lap to bring it down to around 0.877" for a snug fit into the cylinder. On the last pass, which was intended to bring the piston down to 0.880"', the machine Gods laughed and when I measured the piston with my micrometer it was 0.876". Didn't leave enough on the diameter to even bother using the lap. I think it's probably time to pull the topslide off my lathe and tighten up the nut which the cross-feed screw engages with, and then tighten up all the gibs. The new piston is good enough to use, but I would have been happier if things had worked out the way I planned.

Offline Jasonb

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Re: Horizontal Air Cooled Engine
« Reply #206 on: January 21, 2022, 07:08:28 AM »
What are you measuring the bore with ? a few posts up it was "a perfect 0.875" ?

Brian, did you read my post else where about TCGT inserts or sharp HSS, you will be able to sneak up on the size far easier with them as a standard TCMT insert being blunter tends to ride over the surface when a small cut is applied until you sudden lt find it cuts but will be several times more than the feed.

Two small grub screws in the back of the cross slide nut to adjust backlash but that is not likely to be the cause of your overcutting.

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #207 on: January 21, 2022, 02:17:15 PM »
Jason--thank you for posting. I do read your suggestions.--Brian

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #208 on: January 21, 2022, 06:40:02 PM »
Okay--I'm going to run with what I got. The new cast iron piston is marginally smaller than I had intended, but it is still within the recommended 0.002" smaller in diameter than the cylinder bore. The piston rings will be cut from my new piece of iron from Wisconsin, and the o.d. will be turned to match the cylinder bore of 0.878". The o.d. of my piston is 0.876" dia. and I cut the ring grooves to be .046" wide x 0.045" deep. So---the rings will be cut to 0.045" thick, and 0.876-(.042+.042)=.792" i.d.  That will give 0.001" total side clearance when the ring is setting in the .046" wide groove, and 0.792" i.d. will give 0.003" radial clearance between the inside of the ring and the bottom of the groove on the piston.

Offline Brian Rupnow

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Re: Horizontal Air Cooled Engine
« Reply #209 on: January 21, 2022, 09:41:04 PM »
When I part these rings off from the parent stock, there are a few things to consider. First, I know the ring thickness I want. My target figure is 0.045". Second thing is that I must know the thickness of my parting off tool, which in this case is 0.039". Third thing to know is that I want to make the rings about about .004 to .005" thicker than my target figure, because each side has to be lapped against a cast iron surface plate. Fourth thing to keep in mind is that every time I part off a ring, I must sand and deburr the end of the parent metal before I cut the next ring. That means that I only have one face on each ring that must be lapped. So, in my case, it meant that every time I parted off a ring, I had to move the carriage 0.088" before I parted off the next ring. Then I go thru all the parted off rings and check to see which side is the "bad side" and needs to be lapped. When all of the rings are brought down to "finished thickness" they have to be split. Then the ends are filed so that when the ring is in the cylinder there is a 0.004" gap between the ends. Then they go into my oven, mounted on a special fixture that spreads the ends apart to a "target dimension" for a four hour heat treatment to "set" the gap .