Inliners International Forums General Discussion Engines boring out a 250

Just cracked a piston skirt on my 250. There is some vertical scoring on the cylinder walls but it is minimal. The engine is already 60 thou over. The engine builder wants to sonic test the block and then if all is ok, take it out to 80 thou over. I need some opinions as to whether or not this is a good idea.
He also wants to install hypereutectic pistons.
This motor is installed in my '52 Chev hardtop, will not be raced, but will see duty towing my 19' house trailer.
Any help would be appreciated.
Thanks, Al

al,
the blocks are very thin. i would suggest finding another core. the towing is what scares me. the engine will see more heat than a normal car driver would. tom

Al:
I'm really curious about this..Is there any forensics on what caused this? Is this a common problem? I don't remember hearing much about failures like this, however I don't remember a lot of things any more

Really curious
Paul...aka xerxes

Al,

Does he charge for the ultrasonic wall thickness testing? It doesn't take long to perform. If not, ask for the set of readings before throwing away the block. You may have a better than average block.

I do not believe hypereutectic pistons are available for the 250, at least not in the US, maybe Brazil. It so, what manufacturer and model are the pistons?

The engine was originally built about 8 years ago and was in a roadster. It has TRW pistons, hastings rings, an RV cam, had been balanced, and has had some head work. It was pulled from the roadster and sat for a few years before I bought it and put it in the '52 Chev. That was about 3 years ago. It broke when I was pulling my house trailer. It developed a loud rapping niose coming from #6 cylinder. Grounding the #6 plug lead caused the rap to almost disappear. The builder says that because of the cracked skirt, the piston was rocking on the power stroke and causing the rap. He couldn't tell me what caused the problem. I think that the motor was driven very hard in the roadster, and it might have been weak when I got it. Pulling the trailer put more strain on a weak part and it failed.
He charges 45.00 to do the sonic test. I'll know by Tuesday.
He has got a line on some hypereutectic pistons out of California that are 80 thou over. Don't know the make right now. If the block checks out, I may go this route.
What do you think?
Al

At .080 over your running a thin wall. And when towing the motor will run much hotter then normal. As a race motor or a daily NON-towing driver it many be fine.Personaly I think you'd be much better off with a less over bore on the motor for a tow rig.And will run cooler ,Even if not towing with it.
Just my2Cents.

I'm listening to what Tom and Larry are saying. But I'm in a quandry. On the one hand, I don't like the thought of starting all over again building a new engine. It's time consuming and expensive. Just finding another good short block is tough up here.
On the other hand a rebore would be much simpler. Is it just a matter of a better cooling system?
I have a three core rad and dual fans now and have never had an overheating problem.
How about honing the cylinders to clear up most of the scuffing without a rebore?
I am really not sure which way to go on this. I am relying on your advice to make a final decision.
Al

If the cylinders can clean-up hone without leaving too much skirt clearance, then yes that would be a better solution. But from your description of the scoring, it sounds as if an overbore would be required to eliminate it. And as already has been mentioned, and .080 bore might not give the longevity and results you are expecting for your intended use of the engine.

If you're sure only the one is bad, leave the others alone.
Th difference in cylinder pressure between a +.060" and a +.080" is far smaller than the differences in chamber volume and ring seal between the "alike" cylinders: 1.02%.
No, it doesn't affect the balance.

I'll keep my thoughts to myself.

MBHD

Feel free to make any comments you think are appropriate - along with attribution to source and factual data.

I'd advise the same for you.

MBHD

That's so funny!

Exactly what part of my statement is wrong, other than who posted it?

1. factory pistons, regardless of size, are all the same weight (i.e., with normal tolerance, 5 gm. or so). Which factory is that which requires re-balancing after boring?

2. as I said (perhaps in your eagerness to make a negative remark you overlooked it?): the difference between an .060" cylinder and an .080" cylinder is 1%.
Did you do the math, and get different results?

Find me an engine, even a new engine, that has cranking pressure or chamber volume on all cylinders match within 1%.
Try making a warranty claim on a new Ferrari about a 1% variation. 1% is excellent in anything below F1.

In summary: if there were no other differences, the .080" cylinder and any .060" cylinder would be closer than any 2 .060" cylinders chosen at random as to power, vacuum, static CR etc.

I'm interested to hear a rebuttal.

I'm sorry, this is pointless, and rude of me to presume that an actual answer (rather than an opinion) would receive anything less than the usual response.

Replacing the piston sounds like a viable option to me. I've done that only to have a rod go out on another cylinder. A full tear down and inspection would be prudent. Considering the towing a sleeve might be in order. I would think there might even be a TRW piston setting on the shelf, left over from a set for bent thing.

Larry

al,
i am not against reusing the old block, but am against the 80 overbore.

had good luck once with my F450 diesel. one piston lost 1/2 of a skirt. the piston wobbled bad enough to allow tapping against the head. took the block in for a check and found all bores were in perfect round with no wear. bought new standard pistons and ran again. still pulls like a champ.

sleeving the bad cylinder is a option. i will bet the other cylinders will need the bore though. i am also not against putting a different size piston than the other bores. as in, you sleeve the one bad one and put in a standard piston. i am against boring the 1 to 80 over though.

there is probably a reason the piston gave up. did the engine ever ping under load?

I appreciate all the different solutions to my problem. That's why I enjoy this forum so much as there is usually more than one answer and you guys have them all.
At this point I am taking your advice and I will not bore to 80 thou over. I will ask the builder if he can hone out the scuffs, and if he can't I will consider the other options that you have given me. As we are in the middle of our memorial day weekend here,I will not be seeing him until Tuesday.
As to Tom's question above, the motor has never pinged since I've had it.
Al

A thought that hasn't been mentioned is sleeve the bad cyl.

Al,

My recommendation is still measure the wall thickness. If you can't measure, then by all means use basic guidelines like "no more than 0.060 inch overbore". When you have access to technology and measurements at low cost, use them. You can run closer to the limits while controlling your risks. The mold cores of the cylinder blocks do shift some during casting, so you could have a block with cylinder walls over a range of thickness, either good or bad.

One important question is what is the minimum wall thickness recommended for your engine and application? I have seen figures from 0.100 inches (European engines non-thrust wall) to 0.250" for American V8 special heavy duty racing blocks. One Jeep site listed 0.180" on the thrust side.

For what its worth, I have only heard from several places of boring the 250 to 4.000", 0.125 overbore, to install Chevrolet 327 pistons. Those walls must be mighty thin with the 327 pistons.

Winter poses a good question. Does anyone know the minimum wall thickness recommended for my engine?
I've just gone through Leo Santucci's book and cannot find this information. It would also help if I knew the stock wall thickness of the 250. This would give me a baseline when I talk to the builder.
Thanks again for your help.
Al

On all of our race 292's, we tried to make sure the minimum wall thickness was at least .150 on the thrust side for any overbore. Sometimes you will find that even at .030, you will be thinner than that. On street engines we found that a thickness of .125 was a fairly safe cylinder wall thickness for the finished bore size to use as a guideline. Quite often we had to offset bore a block to be able to use it because of the core shift problems that can occur when the blocks are cast. Sleeving a cylinder is an alternative,but only if you plan to bore the engine, because it distorts the adjoining cylinder too much to allow them to stay where they are without boring them back to a round and straight shape. We found that the Mexican blocks were always much thicker in the cylinders than the US blocks were, as were some Canadian blocks. So if you have a Mexican or Canadian block, that could be a big plus for you.

CNC-Dude,

Do the Mexican or Canadian 250 blocks have thicker walls than the US 250 blocks? If so, want minimum wall thickness does a stock Mexican or Canadian 250 block usually run?

Winter, the Mexican blocks we found were always thicker than the US blocks, and it was about 50/50 on the Canadian blocks. The Mexican blocks were over .200 thick at standard bore, so at .060 over you still were at around .170 thick. And occasionally, we had several end up being over .200 thick after a .060 overbore because they were really thick. Every once in a while a thick US block would come along, but they were few and far between.

Thanks for the core thickness info CNC. I will use it tomorrow when I see the builder.
Al

CNC-Dude,

Thanks for the information.

So to summarize the nominal minimum wall thickness range found for a stock Chevrolet 250 bore, would you state?:

US 250 Block: 0.170" to 0.190"

Mexican 250 Block: 0.200" to 0.230"


Leo Santucci states in his 'Power Manual', page 13:
"There seems to be no particular year or series that is structurally superior, although many racers favor the early years (1963-71), and others favor the Mexican-made block."

Winter, I would say that those numbers are a fairly accurate analogy of our findings. We would ocassionally find blocks that contradicted our previous findings, and while we did sonic check a large number of 250 and 292 blocks, that number still represented a tiny segment of blocks still out there. There might be a large majority of blocks still available that don't even meet the minimum thickness of blocks we measured, or even could exceed our earlier findings. But we always tried to make a Mexican block our first choice for a race block, I can't say I ever remember one of them being thin in the cylinders.

My problem is solved and I will be able to sleep tonight!!
I was reluctant to go 80 over, but armed with the info from CNC Dude, I talked to my builder. He told me that he remembered that he had an old 250 block under his bench that had been sitting for a few years. He pulled it out and checked the bore and it was standard size. He will take it out to 30 thou over, and with my speed goodies from the old block, and new rings pistons and bearings, I should be good to go. This builder has built many 250's for "Old Time Racing" here in B.C. They use 250's in their race cars. I am confident that he will do a good job.
Maybe this is a time for Larry's lump ports.
Thanks to all of you that responded to my post.
Al

Al

Al,

Glad to see you came to a comfortable decision. CND-Dude's information was very informative for us all. Did your engine builder mentioned the make and model of the eutectic pistons for the 250?

The builder says that the hypereutectic pistons are made Seal Power or Speed Pro.
Al

I've seen standard cast aluminum pistons (low silicon, not hypereutectic) for the Chevrolet 250 by Sealed Power (Federal Mogul). Speed Pro(also Federal Mogul), does offer hypereutectic piston with content of silicon 16 - 18%. But I have not seen any offered for the 250. Do you have a model number? I would like to order a set if available.

When the builder and I were talking hypereutectic pistons, it was when I was considering going .080 over. He said that a set that would fit .080 was available. We never discussed them for any other size. When I see him again I can ask for the model but you may be right in that they are not available for standard bore.
Al

Winter, Keith Black - Silvolite has a listing for the #3454

http://www.kb-silvolite.com/spistons.php?action=details&S_id=216

If this link doesn't work go to their web site kb-silvolite.com, then under Products - Piston - Diametrical Listing, you will find it listed by bore, compression height, and wrist pin. Nice list if you know your piston spec.s and were you can modify. They show standard. I've contact the Customer Support thru the web site and got results.

Larry

That is the worst 250 piston I ever saw.

Super low compression,hardly any quench area,top ring land is thin.

OK for a stock set-up I guess???

I'm bored.

MBHD

This is a little http://www.kb-silvolite.com/spistons.php?action=details&S_id=334

but not by much


MBHD

Now I can't edit my mispells or incomplete sentences,,,Damm :-)


MBHD

Is sleeving an option?
Bob

Oops, my bad! Only read one page of replies, disregard previous remark.
Bob

I called KB Pistons today. The Silvolite pistons offered for the Chevrolet 250 are standard aluminum alloy cast pistons. They are not hypereutectic aluminum (silicon 16 - 18%).

Could we get together with the Pontiac 250 OHC and Chevrolet 307 owners and order a single run of hypereutectic pistons in one bore size, say 0.040" over? Flat top pistons, no top edge chamfer, with valve reliefs for either the Pontiac 250 OHC or the Chevrolet 307.

If that were the goal (a multi application piston) would it be better to order them with blank crowns (no valve reliefs)?

The decision on blank crowns or valve reliefs would have to be worked out. My thinking was that for a larger market group, pistons with no machining required may be preferred. Of course the blank crowns would work for the Chevrolet 250, or any piston that works in the other two engines for that matter. The 4 valve reliefs of the 307 pistons may work with the Pontiac 250 OHC. Note that the Pontiac pistons have fairly deep 2 valve reliefs.