Team Camaro Tech banner
1 - 19 of 19 Posts

·
Registered
Joined
·
10 Posts
Discussion Starter · #1 ·
I have taken my block (350) to a machine shop to have a .030" over bore done. I thought it would be as simple as getting that done. But they said they would really like to have the pistons and size the bores to them as they reckon pistons can fluctuate by .005". My question is does it really make that much difference to a street motor? I was not banking on getting the pistons yet. Should I take their advice?
 

·
Premium Member
Joined
·
11,170 Posts
The machinist told you no lie. You need to have the pistons in hand to do this correctly. Final bore size will be determined by the piston and its material as they do expand at differant rates.
 

·
Registered
Joined
·
72 Posts
In general your machine shop is giving you good advise in that. Pistons should always be correctly fitted to the proper clearance (as suggested by piston manufacture) to the bore.
The catch is....used bores really need to be checked for wear before ordering new pistons because occasionally you might get a block with one or more bores that may not clean up all the way with a standard .030 overbore.
 

·
Registered
Joined
·
4,899 Posts
Follow your machinist advise. If you have not done this before (rebuild a motor) I can recommend a couple of books that are helpful if you would like that explanation about these issues (such as piston to bore, and how differences between cast, hyper forged etc. effect bore size). Otherwise a purchase of a short block would be your cheapest alternative to get back on the road. Many people assemble crate motors (full assemblies to short blocks) cheaper than you can buy the parts.

Larger Dave
 

·
Premium Member
Joined
·
22,122 Posts
While in theory the correct clearance is built into the piston, and all the machinist has to do is bore and hone the cylinder to exactly 4.030", a good machinist will NEVER trust theory, and will never finish hone without pistons in hand, and will always assign and mark each piston for one particular hole, and finish hone that hole to exactly the size required by that piston.

We call this blueprinting, and I'd bet you can trust that machinist to pay the same level of attention to your bearing clearances and other details, and to deliver an engine that will run great and not use oil for a long long time.
 

·
Moderator
Joined
·
3,563 Posts
Roger,
Your machinist is being careful, that is a good sign. What we "usually" do is bore the cylinders first, that way we know what size they will clean up to. A lot of small blocks will not clean up at 30 and need to go 40. Then we get the pistons in hand before we hone the block.

Now the reality of it ... modern manufacturing and cnc machining has made size differences in pistons pretty much a thing of the past. I cannot remember measuring a piston in the last 5 years that was not made exactly to the correct size. This includes everything from the stock replacement stuff all the way up. The other thing is .... if you are using a piston that has a skirt coating, like the Sealed Power stock replacement pistons, you really cannot measure them. You have to go by the bore size printed on the box label.

So depending on which pistons you plan on buying, there is a good chance you could get away with finishing the block to size, or maybe a half thousanth (.0005") or so oversize and you will be ok. But the best way is to always have the pistons in hand.
 

·
Registered
Joined
·
10 Posts
Discussion Starter · #7 ·
Thanks to all the responses very helpful information. I'm going to get the pistons and give them to the machinist. It makes sense to get more out of the cost of bore/hone. I would kick myself after the build if I had a problem with it.
 

·
Registered
Joined
·
323 Posts
Short but "true" story!!!

This was an occurence I had been involved in about 3 or 4 years back when we were doing more individual machine shop operations than building entire units. Like I stated above, true story!

Under "normal" circumstances AND the fact that we deal with some really
"savvy" customer's, I would do a block to a specific size without the pistons physically being in the shop. I did the size to the customer's request. Doing the high number of blocks we'd been accustomed to, it was easy to be at the correct size. Most times it's a straight-forward deal anyhow and as stated above the most pistons are "on the money"! I would base my decision to do it or not on how well or if I knew the customer. After speaking with this particular one I felt OK.

Now, here's the catch. Like I said 3 or 4 years ago a customer had this done here, picked up the block, went home and put it all together. A set of 8 TRW slugs, L2256F+030's! Simple deal right? Almost, they got 1 bad piston in the batch!! I never had the pistons in the shop to measure as I'd recommended.

Somehow one skirt was "pre-collapsed" from TRW. Not a sign of any damage, no marks at all. The unit had been installed, had one bad case of piston-slap on one cylinder. I got involved "after-the-fact" and actually got a $500.00 credit from TRW for our part in the repair. In this case they acknowledged the bad piston. It was not the customer's fault BUT he was out all the labor with the installation because it was told directly to him from TRW that the piston should NEVER had gotten installed. What they gave us was a courtesy-credit for him!

Thanks, Gary in N.Y.

P.S. Follow all the recommendations above especially the one about the block-plate. And don't be offended if the machinist asks for or even demands the pistons. If the shop is supplying the pistons it becomes another story, it would be made right before it left!
 

·
Banned
Joined
·
21,341 Posts
How the heck can a piston, "pre-collapse"???

Never heard of such a thing..........

I would think like onna the suckers was a miss-stamped .020" in w/ a set of .040"s tho...

Gary, please tell me more about this "pre-callapse" deal.........

pdq67
 

·
Registered
Joined
·
113 Posts
If you drop a piston it can ding the skirt enough to collapse it. If the whole box was dropped it might have hit on the end and not marked the box enough to notice, but banged up a skirt. Last shop I worked in EVERY piston was miked before boring to catch things like that. If it isn't too severe, it can be "adjusted" with a hammer handle.

I got a 'pre-broken" cam once...

Good Luck!
 

·
Registered
Joined
·
323 Posts
How the heck can a piston, "pre-collapse"???

Never heard of such a thing..........

I would think like onna the suckers was a miss-stamped .020" in w/ a set of .040"s tho...

Gary, please tell me more about this "pre-callapse" deal.........

pdq67
Hi Paul, more than likely it started out at the correct size from TRW but after I had gotten involved I found THAT one skirt measured 4.022" at the very bottom, all the others were at the "norm" 4.028". This unit had gotten into the car and ran for only about an hour or so. This customer, the one I did the block work for, called me and I immediately told him to remove it and break it all down. He did a quick recommended test and confirmed the location of the problem. I was figuring a bad pin hole. This I had seen before on mass-produced pieces, the boring operation makes a hole in the piston! Either way, it had to come out.

I believe like the post mentioned above at some point in transit it probably was dropped and "shocked" the skirt. Only a guess. It did get checked out by TRW and they had no issue with the credit. Evidently they found no physical damage either. We did agree with each other that the piston shouldn't have gotten as far as it did with the installation.

The "pre-collapsed" terminology is my own. This was how I had been seeing pistons removed from abused stock SB's over the years. The skirts would always be "down".

You have to remember, even a "custom" piston can be dropped. It may not have indications of damage by appearance alone, being packed in the box and taking a "hit" could hide it!


Important: Aside from the issue above, another reason I like the parts in front of me is this, it gives me an opportunity to check ALL the spec's. such as the wrist pin press-fit/clearance in the rods, the clearance in the piston holes, the ring grooving, C.H., etc. A "good" shop stops to check all these numbers way beforehand!

Thanks, Gary in N.Y.

P.S. This is why it's really to the customer's advantage to have the pistons at the shop. I measure ALL 8 here on every unit. Habit!!! TRW never ventured a guess as to the cause and I didn't push the issue. It did turn into a total build for me including the TRW credit! I'd like to tell you also here, "it ain't easy gettin' credits on parts like pistons from the big guys", you must be correct in your argument!!!
 

·
Registered
Joined
·
13,315 Posts
Due to the way pistons are made sized to get a precolapsed piston is impossible....
Yes dropping a box could cause such damage, but it would have to be dropped from quite a hi height onto a hard floor.
The Common issue is the bores Not being sized to each piston on the final hone...and the pistons marked/numbered for each pot.
Depending on the type of piston, the tollerance is 2 to 3 /1000, measured at the base of the shirt...(note pistons are not round)
If the tollerance is slightly too tight, on the 1st fire up the piston heats and expands faster than the block, the skirt just needs to momenturaly 'bind' to the pot wall, this will collaspe the shirt...15 to 25/1000
If the collaspe is great enough, you can have a knock, maybe not heard from the top, but definately from underneath the car...a mechanics stethascope with tell u which pot.
A collaspe of upto 12 or 15/100- is unlikely to be heard, 20/1000 plus will be.
A collaspe under 15/1000 in unlikely to show or may show scoring that could be atribituable to piston install.
Over 20/1000 will also show scoring on the piston just above the top ring.
Any collaspe 10/1000 and above , the gudgen (wrist pins) will most likely be very tight.
If the gudgens where ok when the rods where connected, and tight when dismantled, this is also a sign that piston/bore was no correct when machined.

There are other issues that may cause skirt collaspe
1/ way over advanced on firing up..thu very unlikely because this makes impossible fire up and load on the starter.
2/Way to rich, but again unlikely for the same reasons to a leaser extent as above.

Watch some machinists they can get lazy, they will measure 1 piston , then final hone all pots to that size...
When pistons are made, on the production line they are automatically weight sorted before packing...it is therefore possible to have enough variation in piston size to cause binding thru incorrect tollerance.

Pre skirt collaspe is not an issue on engine machining...because IF the machinist is doing the job correctly, he will immediately pick up on the undersided skirt out of spec, and a piston to pot tollerance way above the 2 to 3/1000 tollerance, before the final hone.

Please....I would like comments, corrections from those more experanced than me in these issues.
If you think ANYTHING is a load of crap SAY SO...if agree also please confirm.
I have a current issue at the monent between the machinists and piston supplier at the moment pionting fingers, that has meant a full rebuild/balance of a new engine

Im sure pdq67 would be interested in comments to
 

·
Banned
Joined
·
21,341 Posts
Thanks Steptoe for chiming in.

I dropped onna the piston/rod assemblies in my 496 putting it together b/c it flat slid out before I could carch it. Scared h-ll outta me too!

I filed off a skirt ding, slid it in a couple of holes, felt it a couple a times by trying to cock it sideways, part way down as well as I did the same thing w/ a different assembly and I couldn't feel any difference so I just put her together.

ROSS forged low dome pistons here w/ solid oil ring grooves w/ little-bitty holes, not open-grooved like some cheap cast pistons I've seen.

NOT the best by no stretch, but I didn't build a race engine..

pdq67
 

·
Moderator
Joined
·
3,563 Posts
Steptoe,
We try to measure every piston if we can. I cannot remember the last time that there was any difference in a set. Meaning they were all within a couple of "tenths" which is hard to measure on a barrel skirted piston anyway. I think that manufacturing methods have improved to the point that its hard to mess up unless you are buying cheap "noname" pistons. To be honest with you I am amazed that the Sealed Power $8 pistons will all be within 1/2 gram of the same weight in a set, yet the $800 JE's will vary 3 grams or more :(

Just curious what type of "issues" you are having with the pistons you have ?
 

·
Registered
Joined
·
13,315 Posts
before I could carch it
There was a thread a week or so ago, along the lines "what have u on the workshop floor" I have old 2nd hand carpet
 

·
Banned
Joined
·
21,341 Posts
And UMC's Machinists shop area in the old Civil Engineering Lab used to have the neatest parquette 5x5 inch square oak flooring stood up on end so that if they dropped something, it didn't get hurt.

pdq67
 

·
Registered
Joined
·
113 Posts
Steptoe,
We try to measure every piston if we can. I cannot remember the last time that there was any difference in a set. Meaning they were all within a couple of "tenths" which is hard to measure on a barrel skirted piston anyway. I think that manufacturing methods have improved to the point that its hard to mess up unless you are buying cheap "noname" pistons. To be honest with you I am amazed that the Sealed Power $8 pistons will all be within 1/2 gram of the same weight in a set, yet the $800 JE's will vary 3 grams or more :(

Just curious what type of "issues" you are having with the pistons you have ?
That's because of the sheer volume of the SP pistons made. It's easier to weight match parts when you have large numbers to choose from. I agree, machining consistency isn't an issue with the CNC equipment pistons are done with these days.

Good Luck!
 
1 - 19 of 19 Posts
Top