Do I need to bore the engine, or can I hone and use the stock pistons?
 

On my last 400 engine, I blew my allowance and went full tilt on the rebuild. I was biting my tongue in anticipation of doing it again, but had something interesting happen on my current project.

I tore down a stock 68 400 YT engine yesterday to send it in for machining. I bought this engine a long time ago and it just sat in my garage with oil that I had squirted into the cylinders through the spark plug holes.

I don't have much info on the engine, but the guy I bought it from said it ran good and he thought it wasn't ever cracked open. To my surprise, when I removed the heads, the oil I squirted into the cylinders was still there. All of the cylinder walls looked very nice and of course were were smooth. I checked the pistons and they had no size indication on them, so I then did a quick check on two of the cylinders. Indeed, the block was never bored. I have yet to do a full check on how much wear there is on both the pistons and cylinders, but this got me to wondering. How much wear is acceptable to just do a hone and ring job and then reuse the stock factory pistons? Mind you, the car is only for summer shows and crusing. I just want it to run good.

It might sound silly to do a rebuild without replacing everything, but back in the day I did it several times and it worked just fine. Bye the way, the crank looks excellent and bearings were still the stock GM bearings. Still need to check it for wear as well.

Old Joe

Just Hone it out, Valve job it, re-ring kit and CLEAN everthing the best you can.......
It wont go 100,000 miles
But still should be a good Full rebuilder....some time in the future when you can, do it correct.
"Just like back in the DAY"

If you want it for cruise in duty and weekend driving in the summer on a tight budget, do a "backyard rebuild" on it like w.e used to do 40 years ago. Strip the block bare, roll it outside and run a ball hone through it about 15-20 strokes each cylinder, keeping the hone very wet. 280 grit. Buy some Hastings or equivalent PLAIN IRON piston rings. They are very soft and will give you the best chance of having a successful break-in. Get new bearings, freeze plugs, full gasket set. Clean everything to death, using laundry detergent, lots and lots of scalding hot water. Blow everything dry and throw it back together. It will likely run decent, probably better than 90% of the Barret Jackson cars. Expect it to last 30-50K miles before it begins to wisp some smoke and burn some oil. That is 10-15 years for most collector cars. Can do the entire deal for less than $1000.00 on a typical Pontiac V-8. That includes a new timing set, rings and bearings, oil pump, gaskets, plug kit, cam and lifters.

Or you can really do it right for 5-7K. Or anything in between.

I bought a '66 389 in similar condition. It was in a field installed in a rusty '65 GTO. It ran so good with a gas can attached that even the choke kicked down once it warmed up. No smoke and no fouled plugs.
I pulled the heads and had a valve job done and took it to a machinist friend of mine to check the short block. Said it just needed a hone so that's what he did. I had him install the cam and main bearings and rear main seal. Re-ringed it added new rod bearings and all the peripherals... it's been in a plastic bag ever since.
Funny, I just fogged the cylinders and turned it over a few times yesterday. It'll be quite some time before it winds up in a car so I left the valley plan unsealed and the intake off so I can get back into it and make sure everything is still lubed up as far as the camshaft goes.

Yup, I did quite a few cheapo rebuilds and only a few where I spent the loot to do it "the right way". All of them ran. In fact, some of the cheapo builds ran extremely well. Still, I would like to know what the wear specs are for simple honing and re-ringing. Back in the day when I did it, the engines were much younger and had less than 100K miles on all of them (just like me). Since I don't know the history of this engine, I don't want to assume that ringing it without making sure it is in reasonable tolerance will produce good results.

I guess this engine is kind of like me. Old and needs some love, but won't get many miles put on it.......................... :rolleyes:

When I blew up the motor in my 76 formula, I got a junkyard 400 from an old Pontiac wagon. Tore it down, cleaned it up, did all the things that mgarblik mentioned, put it back together with the original pistons and drove the car as a DD for about a year when I sold it to a friend who I know drive it for at least 3 years as a DD. There was nothing high performance about that motor but it ran good as a DD for probably at least 40k miles, maybe more.

And In fairness I should add that I have no idea how mine is going to run. I'm not an engine builder by any means.

Here’s what the factory wanted for piston to wall clearance.
As you can see as the compression went down and the exh gas temps went up the engineers wanted more piston to wall clearance.

I would suggest to you that since you are saving money with your rebuild by not boring, that you put a bit more back in by getting a Total seal gapless top ring and then run a reverse twist barrel face moly second ring like the motor came out of the factory with.

The moly face top and second ring are why Pontiac cylinder walls last so long.

Be sure to take the hours that it will to break / lightly roll over all the 90 degree corners on all the rings .
This will stop these edges from digging into the walls and scratching them and also digging into the pistons.
Besides being rewarded with having no added ware taking place to the cylinder walls you will also have less friction and cooler water temps.

The ends of the oil ring rails being that they are hard steel are a big culprit in the scratches seen on cylinder walls.
If you do any rolling over of the corners of any of the rings then it should be these.

The thing about these old engines people forget is there is no diagnostic system turning on the "check engine" light for any one of 700 miniscule little problems. If you use soft piston rings to conform to the worn, tapered and out of round cylinders, lap the crap out of the valves so they have a complete mating surface and put new valve seals on, as long as the camshaft still has some bumps on it the engine will run. I had a student leave ALL the piston rings off a Honda B series 4-cylinder engine and put it back in the car. The piston to wall spec. is so tight the stupid little engine started and ran with no piston rings. Once it warmed up is ran terrible and began smoking. But it started and you could drive it. Backyard overhaul on a factory 350 HP 400 will probably make 300-320 HP with a quick ring, hone and lap job.

Check the taper in the bores and if not bad do what Mike Garblik said in post #3. The engine I've been flow testing carbs on for twenty years came out of a '61 Catalina more door I parted out. It had 80k miles on it and .003 or less taper in the bores so I did that rebuild on it but with an 068 cam and it has been a dependable little engine for almost nothing.

Thanks for the spec info. This reminds me of something I learned with my last engine. Originally I had it bored for 30 over pistons. The shop that did it cut and honed the cylinder walls for something other than the Old School TRW Forged Pistons I was using. I had those things in storage for 25 years waiting for the build! In the end, I had to have the block rehoned a few thousanths larger. The point I'm making is that forged pistons require a larger bore size (as shown in the chart your provided). If I could magically locate an old school set of standard size forged pistons, they might be a good match for this build. Of course, I haven't even checked out the bore size and taper yet so this is all just wishful thinking.

OJ

Thousands of years ago, I was in Trade School. We were told that .007 taper was the absolute limit, and only for a "grandma goes to church" engine--nothing with any performance aspirations.

The "Service Limit" for the 350 in my '88 K1500 was .001 taper. My replacement block had about .0015, I put it together and it's working fine.

There is no way I'd use plain iron rings. Inexpensive Moly-faced are as cheap as I'll go. Moly is porous, holds oil, reduces cylinder wall wear AND seals better.

Most pistons are supposedly set up with the clearance built-into the piston--you'd hone to the nominal bore size, and the piston would be appropriately smaller to give correct clearance.

But I won't guarantee that all pistons are made that way. My favorite machinist wants to have the pistons in-hand so he can measure them before honing.

Rigid honing should be done with a torque plate. Dingle-berry brush honing won't need a torque plate.

Don't forget to plane the head gasket surfaces square to the crank, and to end-up with appropriate quench/squish given the compression height of the pistons you select.

I don't understand how we got into the subject of forged pistions and the clearances needed for such when it seems you have a good stock usable Bore with stock cast factory pistions.

What am I missing here?

Ball hone it , wash it out 3 times over, clean out any crap in the ring grooves with the tool you can buy for that then re-ring it and have good summer fun with it.

Also please note that when your running a cylinder Bore that likely has a taper to it and will be a tad oversized after the honin, then you do not want to run a plain iron compression ring, you want a ductile iron ring.

The flexing that the rings experience by the out of spec Bore will quickly fatigued a plain iron ring and reduce its outward tension needed to seal.
A ductile iron ring will hold up far longer in a used Bore .

Here's the problem with the hone-ring thing.

Cylinder walls wear tapered, less at the bottom and more up where the heat is. This "works" the rings too much if there is taper in the bores and they loose their tension quickly. Second problem is the top ring land in the pistons. They wear a LOT faster than the lower ring land leaving the ring loose and it flutters all over the place.

So, IF your bores are straight and piston ring lands tight nothing at all wrong with honing the block and installing new rings. Piston skirt to bore should be fine as that's seldom an area of concern but common sense still applies here they have to be tight on the bores over everything rocks around too much and it will wear out quickly.

Basic rule of thumb is to look at the "ridge" at the top of the cylinders. If none present you are tyipically looking at a lower mileage well cared for engine that didn't get a lot of dust/dirt ran thru it. Those make for excellent re-ring jobs in most cases. If you find a lot of ridge at the top of the bores you don't need a qualified machine shop to tell you it needs bored to the next size.

Same with crankshafts. Most to NOT need much more than polished if the engine was running fine when removed from the vehicle. WIth split precision bearings the bearings never touch the crank so most are in really nice shape if the engine was well cared for and didn't grind up a cam lobe and pump millions of metal particles thru it. Once again common sense applies here and if the bearings look good, crank isn't all scratched up and tore to hell you are good to go there as well.

Valve guides are often in perfect shape with Pontiac heads as they use low lift cams and gentle lobes without a buttload of spring pressure. Still need to be checked to make sure they are really tight. If so you can simply "touch" up the seats, surface the valves and back in business. I always replace the valve springs vs shimming old ones, but will re-use Pontiac retainers and keeps every single time, they are very strong parts. So are the rocker arms and pushrods and can go right back in if not heavily worn anyplace.

Cams and lifters are risky to use again, but then we have all this outsourcing with those parts these days so it's a crap shoot to get good USA made parts to replace them. As for oil pumps and timing sets I always put a new Melling pump back in place, but NEVER just drive the pick-up on it and think it's good to go. This new metal doesn't have any "memory" and you've got to take an extra step to retain the pick-up on the pump or it's coming off after you run the engine in.

As for timing sets I use and prefer the stock 3/4" wide Morse chain set-ups. They are at least 5 times stronger than a cheap double roller variety and will last the life of the engine. The high-end billet double roller stuff is OK, but I've seen enough failures with them in high mileage engines that I woln't use them here for that sort of work. They do make the cam easy to degree and fine for most of these engines based on the way the vehicles are typically used. Most are lucky to see 1000 miles in the next 10 years so using a junk bicycle chain set-up to drive your camshaft isn't really much of a concern and we don't hear about how bad many of those parts really are.......FWIW......

PS: I'm pretty sure I have a Speed Pro stock bore ring set in the parts room. I'm doing a MAJOR shop clean-up and have been scrapping all sorts of parts.

Tried selling some but folks on the websites are like black birds picking corn out of cow poop, plus shipping is expensive, taking pics and advertiseing time consuming and eating into my retirement fun. I don't have time for all that so just throwing most of what's left in the scrap pile instead.

If they are still out there I'll run them for sale again, cheap.....

Steve,

The engine is a stock bore, but I have not yet measured the individual bores for wear in any way. I mentioned that, but perhaps you missed it. If the bores and pistons turn out fine, then yes, I probably will go with a re-ring. but if it turns out there is too much wear, then I have to go another route on this build, hence my comment about wishful thinking and forged pistons.

I started the thread asking a simple question about what acceptable levels of wear are for a ring job and that's really all I was looking for. While I'm no where near the level of engine builder as many folks on this string, I've done around a dozen engines, but that's over nearly 50 years and not all of them were Pontiacs so I have a lot to learn in my retirement.

Now if I could only remember how many molecules in a mole.

OJ



.

That is an easy one: 6.022*10^23


:)

Back about 50 years ago when you would only put about 10,000 miles a year on your only auto, we would put Hastings rings on anything with about .003 taper. If the cylinders were tapered more we would think bout using Continental rings. Continental had some interesting oil control rings. The rebuild would last a couple years and that would be it. I used some Hasting rings on a re-ring on my spare 400. It ran great but I only had it in for about 7000 miles before I went with my .030 over 455.

With all of this fine advice, I checked the taper on the cylinders. As Cliff noted, engines with little ridge on the cylinders are often lower mileage engines and make for good re-ring jobs. This engine had by far the smallest ridges on the cylinders of the Pontiac engines I've done. The taper came out very nice. Two of the cylinders had .0015 taper and all the rest were less than that, so the wear was very even. Still need to check the pistons and crank. I expect the crank to come out fine as the bearings have nice even wear that's not excessive and the crank looks great just eyeballing it. Not sure how the pistons will come out. Crossing my fingers.

OJ

Check the original pistons very carefully for cracks in the ring lands. Almost sprung for a re-ring job cause the bores were still decent but after another careful inspection i found three cracks on no. 7 piston. So since i had to buy new pistons anyway, i went ahead with .030 overbore and forged pistons. Some of the cracks are really hard to see. I used a magnifying glass to spot. From now on i run octane booster and 93 gas.