02GF74
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| posted on 1/2/08 at 02:44 PM |
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offset bore
What does it mean?
Context; frod crossflow can eb take up to 1690 but 1760 bocls are avaialble, these have offset bore.
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tegwin
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| posted on 1/2/08 at 02:46 PM |
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It means that the four cylinders are not perfectly in a line.....
Its more like a V4 where the cylinders are out of alignment...
it basically means you can have a shorter block...or, thicker materials between cylinders
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02GF74
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| posted on 1/2/08 at 02:51 PM |
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ok - thought so - so like a v4 but with vertical bores.
surely then TDC for 2 cylinders is no longer at TDC, if you see what I mean.
I wonder how many degrees out it is?
**** Anyone know how kuch offset the 1760 bocl would have?
[Edited on 1/2/08 by 02GF74]
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tegwin
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| posted on 1/2/08 at 03:07 PM |
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The cylinders are still vertical....they are just offset...
So TDC is the same
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02GF74
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| posted on 1/2/08 at 03:11 PM |
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no. that cannot be. the TDC of the offset cylinder no longer occurs at the same position of the crank as it did with non offset.
or am I going mad? ..... 
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tegwin
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| posted on 1/2/08 at 03:19 PM |
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Im no 100% sure on this, so best you take someone elses advice!
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dhutch
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| posted on 1/2/08 at 03:23 PM |
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Ofcause for the distances involved it might be a tiny bit academic?
- Does anyone have a photo. Are the bores all dead parallel. Or are they acrtaully just slightly vee'ed?
- Ive just accepts that off set means there slightly out of line without really thinking about it, but he has a point!
Daniel
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02GF74
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| posted on 1/2/08 at 03:28 PM |
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quote:
Originally posted by dhutch
Ofcause for the distances involved it might be a tiny bit academic?
aggreed 100% I haven't done the sums but even with one or two degree it will make some difference otherwise there would not be this palava about
having vernier timing gears to get withing 1 degree of the recommended timings.
I wonder if it is adjsuted in the pistons themselves? who knows......
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onzarob
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| posted on 1/2/08 at 03:35 PM |
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your making assumption that the offset is in the cross block plane and not the inline.
ie the pistons are not equal spaced apart
I hope that makes sense
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dave-69isit
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| posted on 1/2/08 at 03:38 PM |
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offseting the piston will not change tdc
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dave-69isit
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| posted on 1/2/08 at 03:42 PM |
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the stroke stayes the same
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02GF74
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| posted on 1/2/08 at 03:51 PM |
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yes, stroke is the same but the point at which the piston is highest in the bore is no longer at 12 o'clock position of crank i.e. the orignal
TDC..
I drew it out on paper to see it is so.
If you did the same, and picture below is a feeble attempt, the red line will be longer than the black line.
[Edited on 1/2/08 by 02GF74]
 
Rescued attachment offset.jpg
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omega 24 v6
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| posted on 1/2/08 at 04:04 PM |
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You are of course correct that they will no longer have the same TDC. It will be extremely small perhaps depending on the offset of the crank.
However if both cylinders were offset to the centre line of the crank by the same opposing amounts then TDC will be the same.
If it looks wrong it probably is wrong.
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Peteff
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| posted on 1/2/08 at 04:08 PM |
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I thought the offset was to avoid the bores being too close to each other, it was on the Mini engines. The bore was done on a different centre to the
original, hence the offset
 
Rescued attachment offset.jpg
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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02GF74
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| posted on 1/2/08 at 04:11 PM |
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^^^ yes, know what you mean, but I think the bores on crosslfow are evenly spaced so you would need to move the two middle ones apart and then the two
outer ones apart even further.
The pistons will need to be paired and have 2 offsets.
maybe that is what is done and they are in line? I don't know hence the question.
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onzarob
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| posted on 1/2/08 at 07:42 PM |
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your diagram wont work as the TDC will change and the comprssion as it wont go as high in the bore.
Just done a bit of searching and all I can find is that if you take a 1600 out to 85mm you get a 1760. If the bores are even then an offset
wouldn't be required. Unless there are water/oil ways that need to be avoided.
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r1_pete
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| posted on 1/2/08 at 07:56 PM |
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This is how I understood it was done: middle 2 are drawn a bit close together but the smaller bores are equally spaced, so are the larger.
[Edited on 1/2/08 by r1_pete]
 
Rescued attachment offset.jpg
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