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Piston Clearance ( Economy Engine )

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MalcB25/03/2020 17:02:22
257 forum posts
31 photos

Hi all,

I am at the stage of boring the CI liner. The nominal design size is 42mm diameter.I have the rings supplied in the kit. I am intending aiming as close as possible to dead size after boring and honing.

Piston body is aluminium so what clearance twixt piston body and liner should i be looking at please?

Also when boring/honing the liner and trying rings in what sort of gap should i be looking at in the rings, I would guess at say 0.06mm min to 0.16mm max or is this too tight?

Malc

Jim Nic25/03/2020 22:28:16
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354 forum posts
200 photos

Bump

Emgee25/03/2020 22:51:35
2146 forum posts
265 photos

Malc, what type of engine is the Economy engine ?

Emgee

John Vietti26/03/2020 00:52:17
23 forum posts
3 photos

Malc,

I asked a similar question a while back. A variety of answers. If it were me I'd fit the piston rather tight and if it proves too tight the piston can easily skimmed.

MalcB26/03/2020 06:35:52
257 forum posts
31 photos
I am particularPosted by Emgee on 25/03/2020 22:51:35:

Malc, what type of engine is the Economy engine ?

Emgee

Its a half scale open crank hit and miss engine. There are a couple of builds listed on here but not the info i seek. I am following the one that Nick built on here ( yellow painted ) as a reference.. mine is the std alloy casting version with cast iron liner and cyl head,

Have done the twin flywheels, machined all the main body casting, manufactured the crankshaft, con rod, main bearings and con rod bearings, machined the main hopper/cylinder casting ready for the cylinder liner. Have finish machined OD on cylinder liner, rough bored thru’ 1.5mm undersize and ready to finish bore and hone.

When the piston assembly is complete my next stage is the gear train assembly.

Henry Brown26/03/2020 07:59:03
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472 forum posts
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Following with interest, I've not got this far yet!

JasonB26/03/2020 08:14:36
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21284 forum posts
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I rarely actually measure the pistons on my hit and miss engines once I get close to size tending to machine them until they "feel right" when the honed cylinder is offered up to them. But if you aim to get the cylinder on size after honing then make the piston something in the region of 0.05mm or 0.002" smaller on that size bore with an aluminium piston..

These engines don't run that hot as the revs are quite low, also as most are run with no load they will miss far more than hit so the piston is just pumping air in and out of the cylinder which helps cool it and reduces the piston thermal expansion.

Ring gap 4-5 thou.

MalcB26/03/2020 10:10:36
257 forum posts
31 photos
For mePosted by Henry Brown on 26/03/2020 07:59:03:

Following with interest, I've not got this far yet!

Henry, if you havnt already done so, do a search on here for Economy Engine and you should come to Nicks build on one. I would bookmark it on your device.

Very informative and quality workmanship, it has set the benchmark for me. Also good input from Jason on it as well.

Jason - Many thanks for your advice I was hoping you would come along having such a wealth of information.

Perfect!

For info I will try and get a pic on today of progress so far, waiting for an Asda delivery so fingers crossed.

Edited By MalcB on 26/03/2020 10:12:35

Edited By MalcB on 26/03/2020 10:13:05

JasonB26/03/2020 10:24:17
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Looking at the piston drawing there is plenty of metal there if you also wanted to add a viton O ring below the CI rings or instead off if you find you are lacking compression.

May also be worth cutting a shallow Vee groove around the piston that lines up with the middle of the wrist pin and allows the oil to flow down from the lubricator to the pin. Quite a few engines also have a ctr drilled hole in this position so that some oil can work its way down to the small end which would also be worth thinking about.

Edited By JasonB on 26/03/2020 10:25:19

Henry Brown26/03/2020 11:00:07
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472 forum posts
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Posted by MalcB on 26/03/2020 10:10:36:

Henry, if you haven't already done so, do a search on here for Economy Engine and you should come to Nicks build on one. I would bookmark it on your device...

..For info I will try and get a pic on today of progress so far, waiting for an Asda delivery so fingers crossed.

Yes I saw that and the build here: **LINK** both very helpful!

I've just fettled and lined out the two main castings, made a few studs, milled the bearing caps and started on the base so far. I look forward to seeing your pictures! I've ordered some 316 stainless for the valves, just wondering what you have used?

Cheers, Henry.

Henry Brown26/03/2020 11:07:07
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472 forum posts
104 photos
Posted by JasonB on 26/03/2020 10:24:17:

Looking at the piston drawing there is plenty of metal there if you also wanted to add a viton O ring below the CI rings or instead off if you find you are lacking compression.

May also be worth cutting a shallow Vee groove around the piston that lines up with the middle of the wrist pin and allows the oil to flow down from the lubricator to the pin. Quite a few engines also have a ctr drilled hole in this position so that some oil can work its way down to the small end which would also be worth thinking about.

Thanks Jason. The gudgeon pin has pads at either end to locate it. I was going to make them a light press fit so the groove might not work unless a small hole through them was added. I do like the idea of the feed to the little end though, just have to remember to put the piston in the right way up!

MalcB26/03/2020 12:22:32
257 forum posts
31 photos

My kit with the bit of progress so far below. Gotta face liner to length, finish bore and hone. Will bring up bore to suit ring gapping when aiming for dead size and if it works out then machine piston to suit.

Jason - I do have in mind to do additions on piston to supplement oil transfer and thanks.

Henry - Not starting cylinder head yet and will order valve material then, but may well be 316 but will research a bit.

d188e7dd-c733-45b7-884b-af6a7e056690.jpeg

Henry Brown26/03/2020 13:37:06
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472 forum posts
104 photos

Looks good Malc! It took me ages to dress that crank guard...20.03.22 crank guard.jpg

Howard Lewis27/03/2020 14:40:29
5224 forum posts
13 photos

Regarding ring gap.

For a 42 mm bore, with Cast Iron rings, I would suggest 0.05 to 0.042 mm.Under NO circumsatnces let the ring ends butt!

The rings should be close fit (of the similar size, or even smaller if compatible with freedom ) in the grooves.

This should minimise loss of compression, or blowby.

As the engine starts running, the rings will bed to the bore, and lap themselves to the ring grooves.

If the piston is Aluminium alloy, the fit of the ring to the groove needs to be closer, since the groove will expand faster than the C I rings, and increase clearance, when the engine warms up.

Howard.

MalcB28/03/2020 08:31:49
257 forum posts
31 photos

Thanks people all the info needed, stay safe.

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