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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 8:11 am Post subject: Machining accuracy |
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What type of accuracy should I expect on a crank case decking job? I had a case machined to crank centre line and I'm wondering how precise a machinist could get it.
Asking as I'm not happy with the outcome of my case, it's actually worse than before it was machined. |
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Dale M. Samba Member
Joined: April 12, 2006 Posts: 20377 Location: Just a tiny bit west of Yosemite Valley
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Posted: Tue Jan 02, 2018 8:28 am Post subject: Re: Machining accuracy |
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Crankshaft center line or case center line.... Not always the same...... Any decent machinist should get it exact if he is not lazy or inept or distracted or his "machine" is not wacked...
How far off do you perceive it to be?
Dale _________________ “Fear The Government That Wants To Take Your Guns" - Thomas Jefferson.
"Kellison Sand Piper Roadster" For Street & Show.
"Joe Pody Sandrover" Buggy with 2180 for Autocross (Sold)
============================================================
All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ... |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 8:31 am Post subject: Re: Machining accuracy |
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I had it machined to crank centre, it was off to right side of case by 4 thou, it's now off 12 thou. |
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Paul.H Samba Member
Joined: April 03, 2015 Posts: 613 Location: England
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Posted: Tue Jan 02, 2018 8:31 am Post subject: Re: Machining accuracy |
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If you go to the trouble of having your case decked I'd want 0- 0.0005"between cylinders on the same side and maybe 0-0.001" across.There will be some tolerances on cylinder/rod/piston lengths. Main thing is the cylinders next to each other have same installed height |
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UK Luke 72 Samba Member
Joined: September 07, 2011 Posts: 2867 Location: Little Britain
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vwracerdave Samba Member
Joined: November 11, 2004 Posts: 15299 Location: Deep in the 405
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Posted: Tue Jan 02, 2018 10:41 am Post subject: Re: Machining accuracy |
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My 2276 was like that and I just put .010 more shims on the low side case 1/2. _________________ 2017 Street Comp Champion - Thunder Valley Raceway Park - Noble, OK
2010 Sportsman ET Champion - Mid-America Dragway - Arkansas City, KS
1997 Sportsman ET Champion - Thunder Valley Raceway Park - Noble ,OK
Featured in Dec. 2001 HOT VW's Magazine page 63
Watch my racing video's http://www.youtube.com/user/okvwracer/videos |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 10:42 am Post subject: Re: Machining accuracy |
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UK Luke 72 wrote: |
12 thou is too much, by a mile.
On a 2276, 40thou deck, you're talking ~0.4CR difference. |
Yes I know the mathematics, 4 thou was too much difference for my liking, which is why I had it decked.
It's annoying that anyone would return a case to a customer worse than how they received it. |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 10:45 am Post subject: Re: Machining accuracy |
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vwracerdave wrote: |
My 2276 was like that and I just put .010 more shims on the low side case 1/2. |
I was adamant I wasn't gonna run shims, for no reason other than my pig-headedness! It would have been the easy way though. |
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calvinater Samba Member
Joined: September 06, 2014 Posts: 3327 Location: 802 The Pointless Forrest
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Posted: Tue Jan 02, 2018 10:57 am Post subject: Re: Machining accuracy |
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Bummer. What did machinist say about this? _________________ "Albatross"! |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 11:06 am Post subject: Re: Machining accuracy |
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calvinater wrote: |
Bummer. What did machinist say about this? |
Spoke to him briefly, he knows it's coming back to him as it's not right. I'm a little bothered though as to how it happened and whether he's capable or his machinery is capable of getting it right. |
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Max Welton Samba Member
Joined: May 19, 2003 Posts: 10697 Location: Black Forest, CO
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jason Samba Member
Joined: August 07, 2002 Posts: 3444 Location: Garage
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Posted: Tue Jan 02, 2018 11:24 am Post subject: Re: Machining accuracy |
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How much did he take off other side? He probably took the same off both sides. |
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chrisflstf Samba Member
Joined: February 10, 2004 Posts: 3439 Location: San Diego
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Posted: Tue Jan 02, 2018 12:32 pm Post subject: Re: Machining accuracy |
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How exactly did you measure it? |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 12:38 pm Post subject: Re: Machining accuracy |
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I know he can't replace metal but hopefully it can be decked evenly and I can lose a little off piston tops to create deck where needed. How much can you safely remove from a mahle 'forged' crown in a n/a motor? |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 12:47 pm Post subject: Re: Machining accuracy |
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chrisflstf wrote: |
How exactly did you measure it? |
Used the 1 (long) cylinder in each location and used depth mic to measure deck, I had gone through the whole rod - piston - journal measuring process with this same set up previously and had managed to get things down to .004 difference from left to right sides by swapping parts around. |
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raygreenwood Samba Member
Joined: November 24, 2008 Posts: 21510 Location: Oklahoma City
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Posted: Tue Jan 02, 2018 2:33 pm Post subject: Re: Machining accuracy |
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baz76 wrote: |
chrisflstf wrote: |
How exactly did you measure it? |
Used the 1 (long) cylinder in each location and used depth mic to measure deck, I had gone through the whole rod - piston - journal measuring process with this same set up previously and had managed to get things down to .004 difference from left to right sides by swapping parts around. |
Wait...wait....I think either you misunderstood the question..or maybe I did.
"how exactly did you measure it"
Meaning....how did you measure the case decking? Not saying that your measurements are wrong...just that only measuring it that way may provide LESS info than the machinist needs to get you what you actually need.
Typically before and after having the case decked...you place one half at a time on a machine table, granite slab or granite plate and use a depth gauge or dial indicator to measure from the cylinder seating area to the case centerline on the slab. In a pinch you can use a machine bar laid diagonally across the case to put the indicator probe against.
Outside and inside...3:00 9:00 o'clock inboard and outboard and repeating at 6 and 12....as well as a simple comparison of a machine straight edge laid across the seating areas...will tell you if it NEEDS to be decked.
Measuring the crank bearing saddle depth on each half with a dial depth gauge can tell you how centered the crank bore actually is in the case...this measurement is important....I;ll get back to it.
Measuring again on each case half to the granite plate or bar when it gets back tells you how much they took off.
While doing it the way you are doing it certainly is less involved and simpler....it can also be Waaay off....if you are measuring as deck height....for reasons that have nothing to do with how the case was decked.
If the crank throws are off a bit from past machining issues....thats one stack up. If the main bearing saddles were off center...which is also common...and whoever set up your case decked it to ALSO set the case centerline to deck equal....that can be a problem as well.
Ray |
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mark tucker Samba Member
Joined: April 08, 2009 Posts: 23937 Location: SHALIMAR ,FLORIDA
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Posted: Tue Jan 02, 2018 2:59 pm Post subject: Re: Machining accuracy |
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I wouldent messure off the case, mesure off the main bores. either with a mic or caliper. or if you have a straight 2" square bar to set the case on and square up case half then mesure to the bar from the spigot hole. 1/8 radii on the corners of the bar would be nice too but it needs to be persice. we always decked all blocks off the mains. most v8's are done off the oil pan serface at the factory .some are close some are horrifically off.Ive seen over .020" end to end and twisted too...wtf it will still run and thats all they cared about as do many machine shops. when you messure with cylinder and piston there are a few things that also come into play, like piston rock,rod aliginement, piston quality and pin aliginement.cylinder seating on the case correctly.etc.as well as oil on bearings.pin fit, pin quality...and just plain wore out crap being used..and yes new crap thats worse then some of the wore out crap. so beofre you chew real hard do some checken to be sure your findings are truly your findings...I wood also check to see if the decks are cut square to the crank....but the crank is round square to crank&cam. thats top to bottom and front to rear. If it was .004" I would of left it alone.after all it's a vw. ( wacking the piston is the rong thing to do!!!!unless your making it the same as the other pistons... the other pistons not the other messurements..)if you equalize the deacks by wacking pistons and the cylinder tops are not the same hight but the piston decks are equal now....how you gonna machine the heads to match the stagger on the cylinders?? or you just gonna bend the head to match.....in the big skeem of things the .004 is squat unless it's a trick race motor. yes machinest fuck up sometimes.Ive done it, it's eazy to do. I have 1 .010" shim under my #4 cylinder due to me shorting the cylinders and....brain fade I get it about .004 too much....so i went to then next shim size I had...yes shit happens, and that was...about 8 or 9 years ago..possiably 10 or 11...but I know witch cylinder I did it on.why i screwed it up I have forgotten. |
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vwracerdave Samba Member
Joined: November 11, 2004 Posts: 15299 Location: Deep in the 405
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Posted: Tue Jan 02, 2018 4:23 pm Post subject: Re: Machining accuracy |
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.004" I would have built the damn engine and not even wasted my time worrying about it. _________________ 2017 Street Comp Champion - Thunder Valley Raceway Park - Noble, OK
2010 Sportsman ET Champion - Mid-America Dragway - Arkansas City, KS
1997 Sportsman ET Champion - Thunder Valley Raceway Park - Noble ,OK
Featured in Dec. 2001 HOT VW's Magazine page 63
Watch my racing video's http://www.youtube.com/user/okvwracer/videos |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 4:31 pm Post subject: Re: Machining accuracy |
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Thank Ray and Mark, this .004 has been consistent throughout dozens of mock ups and two sets of rods, OK it wasn't quite an even 4 thou at the beginning of my mock ups, but I used the small variations in rods and pistons to give me almost perfect 1+2 and 3+4 only difference was 4 thou more deck on one side of the case.
I know I don't have 2 pistons out by anything near that amount, same goes for rods so it's either the case or the crank. I'd love to be able to accurately measure my crank to eliminate that. If I had another case I guess I could see how it specs out in it.
Case was sent for decking with my findings, guy measured case and said crank was almost dead straight just 4 thou to one side of case so I asked him to deck the case to eliminate this difference.
I'm gonna try measure up some more next few days as advised above. I'm not ragging on anyone either I just want to get it right. |
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challomoner Samba Member
Joined: October 13, 2010 Posts: 1211 Location: IRELAND
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Posted: Tue Jan 02, 2018 4:33 pm Post subject: Re: Machining accuracy |
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vwracerdave wrote: |
.004" I would have built the damn engine and not even wasted my time worrying about it. |
Yep me too.... |
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