question for automotive machinists

[383 240z]

Can you resize the big end of a connecting rod, with the piston on it?

I have a set of stock SBC connecting rods that have forged pistons hung on them, I would like to upgrade the rod bolts to ARP rod bolts. I figure the big end will be distorted after the old ones are pressed out and the ARP's pressed in. They would need to be resized after to bring them back to round. Am I correct in my thinking? Or will I need to have the pistons pressed off, the work done, then pressed back on? Keith

 

[shawnspeed]

Yes, you are correct in your thinking , will need to re-size the lower ...it depends on the equipment the shop uses to recondition the rod, weather or not the piston has to come off.. we used to use a sunnin manual rod hone , and I relive you could leave the piston on..( It's been a few years)...they will need a very good cleaning after though..

 

[airrj]

Agreed that you physically could hone them with the pistons still hung. The key item is to be sure that you get all of the honing oil out of the wrist pin. Having all of that microscopic steel in the oil wouldn't be good. I am sure that you could make it work, just finding a shop willing to try is the key.

Good Luck.

 

[383 240z]

That's what I was afraid of. None of the shops near me will say they can get the pistons off with out damaging them. These are fairly spendy pistons, about $80 a hole. Don't want to damage them if I can get around it. Are the shops just doing CYA or do they really damage them often? I'm hoping that once back in 1964 a new guy damaged 1 piston and now the shop warns everybody that it could happen again. Keith

 

[Tronyadorable]

********. Find a real engine shop.
Call Paul.He's cycle guy but he can do it( remove-reinstall the pistons). UPS will be needed
http://www.fastbygast.com/contact.htm

 

[shawnspeed]

Normally if you are pressing the pin's out of the pistons, you are scrapping them...sooo , most shops don't have really good fixturing to press the pins out for re-use...what is the application of said rod's & pistons???Shawn

 

[Nitroneal]

$80 @ hole forged?? You would think they'd be floating pins. I have changed rod bolts without resizing and got away with it. Just need to measure the hole carefully. I have done them with the pistons on the rod, real PITA. Fair chance of banging piston around while doin it.

 

[BMW Rider]

I would swap the bolts then have a shop check the big ends for size and roundness. The bolts typically do not press in overly tight so the pressing operation should not affect the rod. the biggest issue would be how the rods and caps index. If they are modern fractured type, they will always line up perfectly. Older rods that rely on the bolt shoulder to align the cap might see some variation after changing the bolts.

Honing the big end with the piston on is possible with the caveats mentioned by others. It certainly is much preferred to have them off. If you get a good shop and a good machinist, it should not be a big deal though. It's been many years since I did this and I don't know what the typical machinery used today is, but with the older Sunnen pinhole machine, we would set the stop bar so the piston was above the big end as it was honed to keep the majority of the oil and swarf away from the piston. Attention to cleaning afterwards is important.

As mentioned, removing the wist pin will damage the pin. It might be possible to get new wrist pins, but then you'd have to have them fitted to the pistons which would be more cost and hassle. That plus the cost to remove and reinstall the pistons on the rods.

 

[Kevin54]

There is no reason that you can't press them out and have new ones pressed in, piston on or piston off. As long as you have things marked as to what goes where, you should be good to go.

I would want the bore resized and new bearings pout in anyways if you have things tore down that far. they will put the rod and cap back together and hone it round to a particular size. If you haven't had the crank worked on, get the crank turned, then the rods honed to match the crank and new bearings.

For any engine shop, it's all in a days work for them and they do it every day.

 

[airrj]

So I have dealt with lightweight forged pistons on pressed pins before and shawnspeed is absolutely right. Most pistons that are pressed off are just going to be scrapped, so they don't have good tooling to remove pistons. I hand made myself a fixture to support the lightweight Wiesco pistons that we ran on our Street Stock. It took a little time but it worked well.

Not knowing everything involved in your project, I myself would make a fixture and remove the pistons to have the rods resized. And yes it is possible to put ARP bolts in and not need the big end redone, but far more of the rods that I put new bolts in needed resizing than not. So I would plan on doing it.

If you don't remove the pistons to have the big end resized, I would suggest taking some good quality grease and use it to seal off the pin ends and in the area between the rod and the piston to minimize the amount of honing oil that gets into the clearance area between the pin and piston. Then once you have the machine work done, you can wash off the grease and honing oil and you should be good. Just be sure not to dunk them in the nasty parts washing tank.

Good luck.

 

[Alta_Racer]

Are the pin holes grooved for a retaining rings? You could buy some lightweight pins, and have the small end done to float the pins. No ring, use buttons.

Isnt the saying "go big or go home" ?

 

[383 240z]

They are forged pistons. Went and looked they are only $60 a hole, they were more when I bought them, they have been sitting on the shelf for awhile.

http://www.summitracing.com/parts/slp-wl2491nf30/overview/

I was digging thru some boxes of parts and found them. Hung of 5.7" rods, with what look like stock bolts. No 12 pt nuts, and no center dimple for the rod stretch gauge.

They are sold as either press fit, or floating. They were installed press fit.

I have a good Gen 1 block and a lightened **** 9000 crank is only $250. I could toss a motor together pretty quick with parts I have in the shop. Get a new bullet ready for my '41 Chevy. I can't for the life of me, figure out why I would put forged pistons on a stock rod, with stock bolts in it. Makes no sense. Unless I bought/traded for them at some point. The have no marks on them, no rings and brand new STD size bearings. The rods show that they were balanced to the pistons. They are also marked for what cylinder they were matched to. But I don't seem to have an empty block here to go with them. Keith

 

[shawnspeed]

"They are sold as either press fit, or floating. They were installed press fit."
So , they have the grooves for the clips in them??? if so , I would convert the rods to floaters and get some new pins...but that is just me...but then again it sounds as if you would be putting it in an application where a stock rod bolt would be OK ...Shawn

 

[VDubJoe]

When I had my shop I had the tooling to press on and off with no damage. A good shop probably would have it. But i've also resized with pistons on. Used heavy grease on the piston end and a lot of cleaning. For sure on replacing the rod bolts, cheap insurance.

joe

 

[toplessHO]

heres a thought
replace the bolts, mic them to see if still round use plastigage to check the bearing crush
a simple snap gauge should work.