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Balanced crank
https://www.ausmini.com/forums/viewtopic.php?f=2&t=25479		 Page 1 of 1
Author:  Wombat [ Fri Dec 01, 2006 2:56 pm ]
Post subject:  Balanced crank
Just got the crank back from a complete overhaul , grind and balance -

Gee they took some metal off at places -

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If it needed that much off it must have shook rattled and rolled the rest of the setup :?
Author:  drmini in aust [ Fri Dec 01, 2006 3:22 pm ]
Post subject: 
Amazing how far out some are when new.
Also amazing how some places pretended to balance cranks in the past by just hitting `em with the grinder... anywhere will do!:shock:
No names, no pack drill.... :wink:
Author:  Wombat [ Fri Dec 01, 2006 3:26 pm ]
Post subject: 
I suppose by the time they had straightened , stroke corrected and index ground the thing the balance may have shifted from original :roll:
Author:  drmini in aust [ Fri Dec 01, 2006 3:56 pm ]
Post subject: 
Wombat wrote:
I suppose by the time they had straightened , stroke corrected and index ground the thing the balance may have shifted from original :roll:

Nah, geometrically they were not far out of plumb when new.
But, some people were less than careful when regrinding them. One of my EN40B S cranks had a slight problem, they had not deburred the front centre before grinding the mains previously.
OK now... thanks to Spiro at St George Crankshafts. :wink:
Author:  sports850 [ Fri Dec 01, 2006 4:07 pm ]
Post subject: 
Doc , should the rods and pistons be done at the same time as the crank ? I've always matched the weight of the ends (big ends and small ends) of the rods and the pistons any way and have normally had the crank ground and balanced but wondering if there is any benefit in getting them done professionally or do they just do what I do anyway .

For those wondering , by matching I mean make a solid bracket to support one end of the rod and let the other rest on a scale , weigh all the big ends like this and carefully file (not grind , grinding causes heat) the cast marks off the others till they weigh the same as the lightest one . Then switch them around and weigh the small ends and do the same . Also weigh all your pistons and file some of the casting marks off the inside of the skirt so they all weigh the same as the lightest one . This takes a bit of time but the motors always seem to run nicer . I have also used an ancient wheel balancer (basically a cone with a spirit bubble at the top , the cone sits on top of a spike and finds it's own level , then check the bubble and file bits off till it's true) in the past to balance the flywheel/clutch assembly .
Author:  Lillee [ Fri Dec 01, 2006 4:16 pm ]
Post subject: 
Side note: graham doesn't do this, he thinks it's not necessary so long as all the rods are the same weight total within 1 gram. This could just be the case for road motors and not race motors so not sure there.

Balancing of backplate, flywheel, diaphram and clutch plate is done independantly then assembled and balanced as one unit.
Author:  Mike_Byron [ Fri Dec 01, 2006 4:55 pm ]
Post subject: 
I had my pistons, rods, crank, harmonic pulley and clutch sent away to be balanced when I build my present 1310 motor.

The rods and crank had almost no metal removed - pistons, I dont know about coz they were on the rods and in the block when it all came back from the machinists. The clutch and pulley has noticable amounts of work and red paint lines to line it all together.

It was a large journal 1275 non cooper crank btw.
Author:  Lillee [ Fri Dec 01, 2006 5:55 pm ]
Post subject: 
RE: pistons, they are generally very very close to the same weight... esp hyertechs or other well manufactured brands like Omega
Author:  drmini in aust [ Fri Dec 01, 2006 6:26 pm ]
Post subject: 
A connecting rod is more complex than you think, as far as balance is concerned the big end is rotating weight and the small end (along with the piston) is reciprocating weight.
The classical `correct' way to balance rods (for reference see Phil Irving's book, Tuning for Speed, and other writers) was to treat the ends separately-
Support the rod's little end bore on a knife edge and weigh the big end on an accurate scale or pan balance.
Lighten the heavier big ends to match the lightest of the 4.
Then do all the small ends the same way, by supporting the big ends on a knife edge and weighing the little ends.

These days you can get digital scales that read to 0.1g pretty cheaply.

Having said all that, A series rods, whether smallbore or large- are all forged steel, and there is not a real lot of weight difference between those in a set. For road use it's not essential to balance them.
My current 1360 and the 1310 before it both use standard S rods, the motors are smooth to 7500 and beyond.
I have balanced the 1412 stroker's rods though, using Graham's method.

More important than balance really is straightness, all 4 of the rods for the stroker were bent to some extent. This is a common problem. But they are now straight. Thanks, Graham....:wink:
Author:  Lillee [ Fri Dec 01, 2006 6:37 pm ]
Post subject: 
Too right!!! out of 16 rods that Aaron had, only 4 were straight, the rest were WAY off, some off by 100 thou! :shock:
Author:  Wombat [ Fri Dec 01, 2006 8:37 pm ]
Post subject: 
I also had the rods straightened and picked them up today as well. I'm picking up a set of digital scales tomorrow off ebay ($15) but I'm wondering if I should have balanced before straightening or doesn't it matter?
Author:  drmini in aust [ Fri Dec 01, 2006 9:44 pm ]
Post subject: 
If you start lightening them overall they can move, but if just balancing the end bits they should be OK.
If worried, get them checked again after balancing.
Always polish lengthwise on the rod, don't put grooves across it- they are stress raisers. :wink:
Author:  graham in aus [ Sat Dec 02, 2006 3:58 pm ]
Post subject: 
See my other thread about how notto lighten conrods!!

He He :lol:
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