Machining a block, how it´s going???

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360duster

360duster

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Hi Guys,

got some confusion around me, hope some of you expirienced Guys can help me out. I want a machine shop to bore and hone my block, called the guy today and he said it´s necessary to have the pistons at the same time. I said the bore is bad, don´t know wich size it must have to be clean, i think it´s more than .030 over(can i know it before?). He insists getting the pistons before. I planned to order the pistons if i know which size the finished bore is. who´s right?

Ok, i understand that the correct piston to wall clearance can only be measured if you have the piston you want to run in this engine, but i don´t know if he´s honest to me and just want to sell his stuff.

how do the machine shops in U.S. do this work? do they finish a bore with the biggest size needed to clean up the walls (and small as possible) or do they take a specific piston and machine the bore according to the piston?

thanks in advance

Greets Michael
 
That is wierd that he MUST have a piston. Then all these guys selling pre prepped blocks like Chenoweth dont know what the CRAP they are doing??
They sell blocks in a bag ready to build with your poison.........

I think your machinist if full of Chitttt!!
 
You need to have the bores checked to see what size over piston you need. From there you order them and the finish bore and hone to the pistons.
 
A good machinist will want the pistons so the final clearance will be in spec. Have him measure the bore so you can choose the appropriate next oversize. If it's std, you're likely good to .030. At .030, you might get away with a .040 piston.

Yes the wall must clean up to a size before honing would be my suggestion. I'd ask what size the machinist might suggest to get a fresh wall surface.

Not all .030 over pistons are the same size, they vary slightly and the type of piston will also determine total clearance. I have a set of KB107's that the bores had to have specific pistons in them because of size variances.
 
As was said.. it's normal to have to have them when the bores are finish honed. But in terms of boring they would normally simply bore out the damage to the next available oversize minus a couple thousanths. The hone takes the final couple thou off to get the right piston to wall clearance.
 
well the true way i learned to do it is u have to know what pistons ur using because of the expansion rates are different. i mean .030 is .030 over. then hone it perfect unless ur building a top fuel motor or something.
 
The machinist should measure the bores and recommend a oversize piston.
Once you get the pistons he will measure them and know how much material to leave for the finish hone and the proper finished bore size for the type of pistons you have. The machine shop that's doing my work cleans and checks everything out then gives me an estimate of what needs to be done. His list starts with things that must be done (bore +.030, turn crank .010/.010, surface heads, recondition rods, etc) then lists things that could be done ( decking the block, align hone the mains, etc. The last part of the list is optional depending on how much I have to spend and how perfect I want everything to be. toolmanmike
 
The block I dropped a few weeeks ago I measured and it looked ok to go .010 over existing, but the previous shop had bored and honed to .030 but there were odd shadows. I had the shop bore the bad spots out to be sure. I'd think a skilled eye with a dial bore gage could tell what the bore might have to be unless its close...
 
hi, If there is a doubt on bore sizes, then he needs to bore the worst one first. then he can tell you what size the motor will be.I have done this before, when confronted with a bad bore. test bore , then decide what the piston has to be.once you buy pistons, then block has to be bigger, you're stuck with pistons you don't need. just food for thought.
 
Depending which engine you have I wouldn't go over .030 without checking the block. (sonic check). I've heard too many horror stories of guys spending lots of time and money going .060 over and ruining the block or having overheating problems because the cylinder walls are too thin. The first thing I did after taking the block apart was to get out my micrometers and dial bore gauge and measure everything. I wrote it all down in a notebook and have been looking at the measurements. I am no expert but I've got an idea now as to what's needed for machine work. You may not have the tools to mike everything and analyze the measurements but the machine shop should measure to see what you need. I hope they write those
measurements down. They should! Anything out of specs should be machined if it's not too far gone to save. Good luck and keep us posted on your progress. Toolmanmike
BJR, tell us how it's done..
 
Hi Guys, thank you all for the input.

i borrowed a dial bore gauge today and will make some measurements tomorrow - then i know how bad the bores really are.

Then i will get the pistons and let the machinist do the boring / honing. Could be that everything´s ok then and the bores clean up. Worst case is that i have to order another set of pistons and do the machining again.........not really good for me because i want to buy a balanced engine kit.....new pistons require new balancing.....

wish me luck!

regards

Michael
 
a machinist that takes pride in his work and reputation will want both pistons and rings. DIfferent rings require different bore finishes.
Now about overbore... metals , including cast iron will expand and contract with temperture change.Some more than others. The cylinder wall needs to ne thick enough to keep this change as round as possible. The problem with oversize boring comes from the lack of wall thickness at the water jackets.
Uneven cylinder wall temperture equals uneven expansion.
 
When I get a block in for boring, I first clean it and then use a dial bore guage to check the taper and the difference in bore roundness. Then I make a decision on what the finish bore would end up at. I too like to have the pistons in hand before I bore a block, as they differ in size reguardless of the type of material that they are made of. And true different rings take different finishes. Also too the amount of piston to wall clearance differs from cast to hypers, to forged. Anyway all this is determined after the block has been cleaned, and discussed with the customer. Then the piston choice will be determined by engine useage.
 
Redfish, BJR and moper are right on. Pistons vary in their diameter right from the manufacturer, cast, hyperutectic and forged pistons all have different clearance specs even from differing manufacturers and different rings need different finishes. You're machinist is correct. Let him tell you what size pistons you need if you don't have the tools and/or experience to judge what the situation is. :)

It's also a good idea for your machinist to use a torque plate as the bore does change shape when the head is torqued down. It's not life or death but makes a big difference in longevity and sealing when very high cylinder pressures (high hp levels, super and turbocharging, nitrous) are involved.
 
It is good practice to have the pistons to measure before finishing the bores. However, I would be interested in knowing what piston mfg wouldn't base their piston size on a nominal bore size ie. stock plus 0.030". You would think that they would vary the piston diameter to attain the correct clearance and not the bore size.
 
1MeanA said:
However, I would be interested in knowing what piston mfg wouldn't base their piston size on a nominal bore size ie. stock plus 0.030". You would think that they would vary the piston diameter to attain the correct clearance and not the bore size.
Click to expand...

Manufacturing tolerances vary. They don't mic every piston that comes out of the shop and a cursory inspection and a go no go gauge and it's the box. A well machined and blueprinted bottom end makes all the difference. It's also why two engines from the same manufacturer make different HP. It's all about tolerance stacking. Forging and casting dies wear and heat with use. Was the piston the first on the die of that day or the last? Was it the last piston ever forged or the last before it was replaced? No way to know until you mic 'em out.
 
The core block I just got has forged Venolias in it with no oversizew noted. Seller said it was .060 over and junk. I have a sonic tester so big doesnt mean junk all the time if it's cheap enough... Then I measured it. It's only .020 over right now. You never know... piston makers can do whatever. I Just ordered a set of std bore 340 pistons for the 410 I'm building... Because a .040 over 360 is the same as a std bore 340 with the 4" stroke...lol And that block is rough bored and waiting for the pistons to come in for final honing and square decking.
 
Your correct in the bore size being the same as the 340 but there's a difference in the CD on the 2 different pistons. Even the low compression 340 pistons would need about .055 off the top of the piston to make it 0 deck in the 360. So how are they going to work in the stroker engine? Unless there custom made, but then they aren't 340 pistons anymore. Or you have custom length rods to enable you to use the stock LCD piston. In which case the rod ratio go's to heck and back. Just courious?
 
No difference in the compression heights. Bobby. CD is 1.460 on them. It's not a factory stroke.
 
Those are correct assuming a factory stroke. I said it was a 4" stroke... so they share the same compression height.
 
360duster said:
Hi Guys, thank you all for the input.

i borrowed a dial bore gauge today and will make some measurements tomorrow - then i know how bad the bores really are.

Then i will get the pistons and let the machinist do the boring / honing. Could be that everything´s ok then and the bores clean up. Worst case is that i have to order another set of pistons and do the machining again.........not really good for me because i want to buy a balanced engine kit.....new pistons require new balancing.....

wish me luck!

regards

Michael
Click to expand...

Just curious Michael, where in the cylinder are you going to find the largest bore diameter if you decide to measure the cylinders yourself. I, and maybe yourself for all I know, map out all 8 cylinders by correct number and measure top of the bore just below the ridge, center of bore and 1 inch from bottom of bore. Then repeat them 90 degrees out. This will give you the original bore, largest bore, taper and out of round.

Terry
 
moper said:
No difference in the compression heights. Bobby. CD is 1.460 on them. It's not a factory stroke.
Click to expand...

Oh, OK non factory CD. That answers it. So at that point the 340's and the 360's can be counted as the same piston. Then they aren't 340's or 360's, there custom pistons. They just share the same bore and CD.
 
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