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Centaur compression ratio

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AJW29/12/2018 20:29:31
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Did a bit of 'fettling' today to enable assembly of parts that I have machined on my Centaur engine.
I trial fitted crank with con rod and piston into engine frame with water jacket and cylinder.
It suprised me to find the piston protrudes out of the cylinder top by 1/8 inch?
Checked through the drawings and it appears to be correct - meaning the piston will be entering into the cylinder head space?
Does this sound right, I just assumed the piston would rise to be flush with cylinder top.

Alan
JasonB30/12/2018 07:57:37
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Adding up the sizes on the drawing I get that the piston should be Flush the end of the liner at TDC

Bed 4.500" + Hopper 2.250" + Liner lip 0.125" = 6.875"

Crank Throw 1.000" + Conrod 4.312" + 3/16 + Big End Bearing 0.375" + Pistion 1.000" = 6.875"

 

Looking at your photos the wrist pin does not look to be in the middle of the pistion

 

Edited By JasonB on 30/12/2018 12:29:51

not done it yet30/12/2018 08:26:49
3344 forum posts
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JB,

Yeahbut, the other pic, with the piston t’other way up, looks like the piston pin is closer to the other end.smiley

Hoptical illusion at work?

Nick Clarke 330/12/2018 09:53:23
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Jason - While it doesn't affect your argument I can't get your figures to add up to your second total??

Edited By Nick Clarke 3 on 30/12/2018 09:53:48

JasonB30/12/2018 10:11:18
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Thank's Nick, it was a typo on the bearing which should have been 0.375, I've edited the post.

JasonB30/12/2018 10:17:40
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NDIY, It does look a longer piston in the other photo, ring and distance of pin from ring groove looks about the same in both. Drawing has a 2" long piston with the pin centrally placed eg 1" from either end.

AJW30/12/2018 11:48:15
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Thanks for your replies, must be an optical as the gudgeon pin is in the centre of the piston!
Jason, I can confirm the dimensions for the non moving and some of the moving parts but the drawings I have for the connecting rod give an overall length of 4 5/8 (4.625), gudgeon pin c/l to end where big end is secured, which gives the 'moving parts' overall of 7.000in?
Which is what's giving the 0.125 bit sticking out the top.
I guess it's supposed to end up flush, I will have a measure up of how much material thickness is in the piston crown for a possible mod?

Alan
JasonB30/12/2018 12:28:18
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Just looked at the ME series and that gives the conrod as 4 7/16 + 3/16 and overall as 4 5/8 but sheet drawing shows 4 5/16" + 3/16" but with a total length of 4 5/8" which is 1/8" different!

AJW30/12/2018 12:37:18
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Crikey, what happened there?

Think I might cut 1/8 out of the con rod and super glue it.

Alan
MichaelR30/12/2018 14:46:31
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I know it's to late now for you, but I took the length for con rod / piston off the job when making my Centaur, the piston finish's flush with end of liner.

AJW30/12/2018 15:55:36
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Hindsight eh!
I've decided to shorten the connecting rod, I silver soldered the big end flange on (quite nicely I thought!) So will remove it, I have a centre in the bore of the shaft so I suppose it could be worse!
Wonder if there's any more problems round the corner!

Alan
Howard Lewis30/12/2018 17:01:23
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Shortening the Con rod will produce more side thrust on the piston, and so more friction. It will accentuate the tendency for the piston to "rock" within the bore clearance, which could lead to problems with the piston skirt and the rings, risking scuffing!

My advice would be to shorten the top land, if possible. If the drawing calls for a taper, it would be advisable to maintain the clearances, to minimise the risk of problems.

H T H

Howard

AJW30/12/2018 17:09:46
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274 forum posts
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Thanks for your info.
I did look at reducing the height of the piston but there isn't really enough material at the crown to do this although this is obviously the easiest option.
By shortening the con rod I agree it would impart more side thrust to the cylinder/piston but this will then comply with the other drawings which are around.
Just annoying!

Alan
Howard Lewis30/12/2018 17:30:06
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At the risk of teaching granny to suck eggs.

You don't hold the Piston in the chuck to face off the crown.

You make a dummy Gudgeon pin, with a radial / diametral tapping on the centreline.

You then turn up a stepped "bung to be as close a fit as possible to the bore at the bottom of the piston, and drill it to clear a drawbar with the same thread as in the dummy Gudgeon pin. Having done this do not disturb anything..

Insert the dummy Gudgeon Pin, place the Piston on the "bung and clamp it in place with the drawbar. It should now run concentric, so that the crown can be faced off to reduce the compression height of the piston to the correct dimension above or below the top of the liner.

Howard

AJW30/12/2018 17:35:59
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I realise this is the way to turn material from the piston top, but there is insufficient material on the crown to be able to do this.
It would end up too thin to be reliable!

Alan
AJW31/12/2018 18:29:15
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274 forum posts
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Today saw removal of the big end flange from the connecting rod, shortening it the required amount and silver soldering it back together.
Didn't seem to take as long as I had feared and because I had a centre in the flange end of the rod it was a straightforward modification.
Oh well, onto the next part!

Alan

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