When rebuilding an engine and boring out to wider pistons and grinding the crank and rods for oversized bearings: Does the machinist get the new parts first, then measure them, then do the machining to match those measurements, or is the machine work done first?
In other words, how do machinists know exactly what to grind to dimensionally?
Thank you!
Good machinists will want parts in hand before machining.
Which can be a little bit of chicken and egg, when you don't know how much you are going to need to overbore...if any.
Often, it's a case of machinist measures/inspects what you have now, advises how far you need to go, then parts are ordered...either by you or them.
Andy Hollis said:Good machinists will want parts in hand before machining.
Which can be a little bit of chicken and egg, when you don't know how much you are going to need to overbore...if any.
Often, it's a case of machinist measures/inspects what you have now, advises how far you need to go, then parts are ordered...either by you or them.
As is usually the case, Andy nailed it.
Only for the pistons.
There's really no easy way to measure the bearings before the rod is sized, so they are machined to size. You could then put the bearings in, torque the bolts then measure that and use that dimension to size the crank, but generally you don't. Rods and crank journals are typically ground to a standard size
Peabody said: Rods and crank journals are typically ground to a standard size
Assuming you need to do so.
Which is where I was headed. Unless there's a visible issue with the journals, it's often not at all necessary or beneficial to do so. New bearings and maybe a light polishing may be all that's needed for a refresh.
Personally, that's all I've ever had to do. Of course, I usually break stuff long before it wears out. :)
Exactly... something to bear in mind is that you can't put metal back, so if you have the crank ground undersize (and it may be cheaper to just source another crank) then that is one fewer time that it can be machined.
I don't know how much machine shops cost in your area, but just the cost of overboring, honing, and decking the block would probably cost more than sourcing another engine, unless it has to be THAT engine. Mom and pop machine shops have either gone out of business or they would rather contract with Chrysler to fix 10,000 cylinder heads than onesy-twosy jobs, so they price accordingly.
And then you find that their flycutter had a low bit and your deck is anything but smooth now and you have to take it somewhere else to be fixed...
Andy Hollis said:Peabody said: Rods and crank journals are typically ground to a standard sizeAssuming you need to do so.
Which is where I was headed. Unless there's a visible issue with the journals, it's often not at all necessary or beneficial to do so. New bearings and maybe a light polishing may be all that's needed for a refresh.
But that's not what he asked.
The assumption was that it was already being done, and he wanted to know, how do machinists know exactly what to grind to dimensionally?
Whether you need, or even want to are different questions.
The way I have seen it done:
Measure the block to determine what the minimum overbore, if necessary, is going to be.
Order pistons and rings accordingly.
MEASURE the pistons.
Bore and hone the block to the desired clearances for THOSE pistons.
Gap the rings.
Same as Pete says /\
On crankshafts, it depends a lot on what you're building and whether it's been worked on before. I've had Mini cranks that I got that had been ground 10 under that had significant taper and had to be ground again. Mini cranks are available new, but even those need to be checked as I've seen a variety of flaws in them right out of the box.
In general, my shop straightens, resizes and hones the rods, then I order bearings to fit the spec of the crank - std, or undersized based on their final grind/polish.
I always deliver a new set of pistons with each cylinder marked on them and have the shop bore to fit.
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