Hone and run it, or machine shop?

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Arthur Sperotto

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I'm rebuilding my '78 360 and just recorded 48 bore measurements to try and determine if I can just ball-hone it at home to get it ready for the new KB107's and moly rings, or if a trip to the machine shop for an overbore is needed. I would highly prefer not to bore it, if possible. It's a STD bore STD deck engine, rebuilt about 1000 miles ago by previous owner. Currently has H405CP pistons, never burnt a drop of oil, never smoked, always had healthy and constant cranking compression across all 8.

For each cylinder, I measured the bore longitudinally and transversally at the top, middle, and bottom (giving me 3 different out-of-round data points AND 2 taper data points).
I'm uploading the table with my results.
The numbers you see are the wear, aka where you see .0025, read that the bore is 4.0025" at that spot.
Also keep in mind that the "top" measurements were taken just below the compression mark (or ring witness mark), not IN the mark.
I selected the worst out-of-round and taper value from each cylinder and wrote them down in their respective columns.

P.S.: I have read from a couple members that pistons usually come with the manufacturer's desired piston-to-wall clearance built into it. Meaning that if the KB107's are to be installed with a .0015" minimum piston-to-wall clearance, they won't actually be a 4" piston, they will be a 3.9985" piston. I have also heard that this is not true. Does anybody here running KB107's still have their diameter measurements from when you first bought them?

Screen Shot 2020-03-18 at 6.04.16 PM.png
 
I just measured 2 oversized stock-type pistons for Fords (2.3L and 2.8L) and they are .002" under the bore size for which they are fitted.

So you have a bit of wear in a couple of those bores. Of course, the piston fit diameter will not be all the way up at the top of the ring wear area.

Taper tends to wear rings out more rapidly.... 1000 miles is probably not showing this yet. If you are planning on keeping/running this a while, then I'd bore it. (But I never do hone jobs anymore... regardless. I am pretty strongly biased to starting fresh each time.... good bores is one of the key foundational items for a good engine.) I've re-ringed a couple of engine with more taper than you have..... and the results were pretty poor.

BTW, do you have a 4.000" standard to check your measuring equipment to?
 
You could check with your machine shop and ask how much to hone for minimum clean up, with torque plates and main caps torqued. If they are squeamish do as you said. in your 1st post. As for piston size, you will need to measure.
 
Is the piston change to get static CR up?

BTW, OP, know that if you go to KB107's, the bobweight changes by almost 75 grams..... you're going to need to re-balance the crank.
 
Thank you everyone, helpful replies so far!
I just measured 2 oversized stock-type pistons for Fords (2.3L and 2.8L) and they are .002" under the bore size for which they are fitted.

BTW, do you have a 4.000" standard to check your measuring equipment to?

So if the KB's also come slightly undersized, I might have massive piston-to-wall clearance on #2 and #8 at the top...
I used a Fowler bore dial gauge and Fowler calipers, both accurate to .0005".

why fix what ain't broken? Have you tried bolting on a torque plate, and torqueing the main caps and rechecking your measurements?

Because it's not what I wanted. I didn't buy this car to settle, I bought it to tinker and make it what I want. I'm not fixing, I'm modifying.
All those measurements were taken with the whole bottom end still installed. I was rotating the crank to get measurements. So if the main caps are still there and torqued, together with rods and pistons still installed, I'd assume that's the same as having a torque plate, correct?

Is the piston change to get static CR up?

BTW, OP, know that if you go to KB107's, the bobweight changes by almost 75 grams..... you're going to need to re-balance the crank.

Yes, it is. I'll have the assy. rebalanced, thank you!
 
2 mils out of round and 3 mils taper? If you were using cast pistons, unless your goal is building a 150,000 mile engine, I'd hone it and re-ring it. But with the hypereutectics, the 1 mil+ oversize at the bottom of the bore is of concern. I agree with nm9stheham, make sure your measurements are accurate before you make any decisions.
 
... All those measurements were taken with the whole bottom end still installed. I was rotating the crank to get measurements. So if the main caps are still there and torqued, together with rods and pistons still installed, I'd assume that's the same as having a torque plate, correct?
Yes, it is. I'll have the assy. rebalanced, thank you!

Torque plates simulate the stress of having the heads torqued when your engine is complete. It can make a difference. I always use them in any build I do, from a 2.0 Neon to a 1957 341 Desoto Hemi.
 
Seems like it's about $300 for a torque plate. Isn't that approximately how much a shop would charge for a bore and hone job? Decisions just got a little harder to make now lol :BangHead:
 
Seems silly to try and skip a proper bore/hone. The dingle ball might work well for antique tractors or refreshing a high-mileage low-performance engine on a super tight budget. KB107's don't typically got in super low-performance builds, so trying the super-cheap route doesn't make sense in my head.

My personal rule is that if the rings leave the bore, the engine is going to get new bearings/pistons/bore-hone at a minimum. IMO, it's not worth risking $300-500 worth of new parts and tens of hours of labor to save $100-150. Look at it as an investment. Once the shortblock is solid you can swap heads, manifolds, cams etc to make it YOURS, but having a sketchy short block is a recipe for disappointment.

Besides, 4.04" bore is just more natural for a SB mopar ;)

If you call Keith Black they can probably tell you what the actual piston diameter is for their advertised sizes. They list their pistons by BORE size, so I would assume the recommended p-t-w clearance is built-in. However, I've learned the hard way that assuming is guaranteed to get you in trouble.
 
Torque plates are not at all the same as the mains torqued in place; they will change the roundness of the bores near the head bolt holes, near the top. Having said that, street engines rarely get that treatment, and it is not any issue of note, that I am aware of.

I raced 4 bangers, turbo and N/A, for a long time in more endurance type of racing (rally) and it seems to work out fine without torque plates, but the engines get rebuilt after maybe 1000-2000 full-blast miles, and HP is not at a premium like in an all-out drag race or circle track engine. However, excess blowby due to out-of-roundness was never any issue that I ever observed, and the Opel 1.9L blocks will go out-of-round by as much as .003" in the #4 cylinder when everything is torqued.

I worry more about taper (constant flexing on the rings) and clearance (piston wobble), than torque plates. New pistons is also a plus IMHO, with good tight ring grooves, to keep the rings stable and flat, and you get that without or with the re-bore.

And while your calipers may be accurate to .0005", that is not the same as calibrated to .0005". It won't effect your relative measurements (taper and out-of-round) but it can throw your absolute numbers off (piston-to-bore clearance). That is a good reason for a machine shop to always ask to have the pistons in hand before boring.... it removes some/all of the absolute accuracy errors in the measuring equipment.
 
those measurements are not bad for the streeter
If brought to the shop I'd have stuck it in the ck-10 and given it a minimum plateau hone with torque plate
save the bore -save-the block only so many bores
but 4 u a mid grit rigid hone a few strokes will tell you if the bores are straight
if ok hit it with a fine brush research ball hone and clean clean clean

not for here but...
A Lateral Move — Total Seal’s Gas-Ported Rings May Be A Game Changer
 
Update: I got an email from Trevor Cleveland from KB, and he told me the KB107.STD measures 3.9995" at the dial point. He also confirmed I should only have 1.5 to 2 thou clearance for a street engine.
The dial point however, is farther down in the bore than where I was getting my "top" measurements from, per the attached picture. I was measuring ~1" down the bore, but the dial point looks like it's ~3" down the bore, much closer to my "mid" measurements. So if I use those, we get an average of 1.5 thou to 2 thou, right on the money!

Screen Shot 2020-03-20 at 11.11.41 AM.png
 
Your all set chief! I have the same KB pistons. My machine shop set my bores @ .25 thousands and have ran my 360 for 16 years never a piston slap, wierd noise or smoke/ blowby.
 
Usually a machine shop will check the bores for free and answer any questions you have. Bring the pistons in and tell them what you "want" to do and get your answer.
It's a good idea to have the block checked anyway if you're gonna be putting any sort of money into it in my opinion.

If the machine shop won't do that, find another machine shop.
 
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