68raggtop

First off, I don't now any of the history of this engine. It's a 2 bolt 427 NOM. I bought the car last summer and it ran good, but a little hot. When I pulled the engine (just to see what was inside) I noticed some scuffing on 4 pistons. 2 on each bank and all on the inboard side of the piston. Judging by the numbers on the pistons they are TRW forged +.03 (L2300F on top). The bores all measure 4.2805 and 6 pistons are 4.277(3 scuffed) and 2 are 4.278 (1 scuffed), the bores look pretty good yet, no taper or scoring. So basically I had .0035 bore clearance, which I thought was about right for forged. Federal Mogul (TRW) lists the bore clearance on the web as .0015 for L2300F30 pistons. Any opinions on if the bore clearance was to loose. IMHO .0015 seems tight for forged.

:smash:

WESCH

Hi

CHILTON gives following limits for piston clearance specs :

427 CI 390/400 HP is .0024 - .0045 this is the L36 & L68 engine
430 HP .0058 - .0080 , this is the L88 engine
435 HP .0040 - .0065 , this being the L71 & L89 engine

You can see that the clearance is increasing with the HP ratings.

The only smaller clearances are found on the 327 CI engine being .0005 - .0025 and 350 CI engine being .0020 - .0036

I would say that the .0015 for a big block is too narrow. :U

Gunther

lars

Your clearance is fine. Non-coated pistons always tend to scuff the skirts slightly on initial startup, even with plenty of assembly lube on the skirts. This is really no problem and is not an issue.

68raggtop

I don't know how old the pistons are and was wondering if maybe they have changed the material to a more modern recipe? :confused: Could that explain the difference? IF so, would any one know the actual size they are made to? I am a little reluctant to reuse the scuffed pistons, but don't think the bore's need anything done. So, do I
1) replace all 8 with the same L2300F pistons.
2) order 4 new pistons and see if the size matches
Any other suggestions are welcome.
:cheers:

Fevre

You doubt Lars!
:eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek:


If I where you I would to the gods and pray for forgiveness so that they don't strike you (or more importantly your corvette:)) down!




[Modified by Fevre, 4:14 PM 3/25/2004]

Freepop

:lol: :iagree:

lars

Not sure why you want to replace the pistons, since brand new ones will scuff the same way upon initial startup... If the pistons are running at .035" clearance and the bores are good, you gain nothing by replacing them.

Pistons are sized by the manufacturer to run at the recommended clearance in a nominal bore. In other words, you can take a 4.25" bore block (427) and bore it .030" over to 4.280 without having the pistons in hand. The piston maker will provide the pistons sized to run correctly in the 4.28 bore. Forged pistons typically run at about 3 thousands clearance, so your pistons are the right size. That same piston maker will provide a cast or hypereutectic piston that's sized to run at about 1.5 thou clearance, so the forged pistons are actually made smaller than a cast piston - both piston types will run in the same bore size.

There has not been any significant change in forged piston materials technology in the last few decades. The biggest change has been the advent of the hypereutectic pistons. Many of these have the coated skirts to eliminate the scuffing issue that you're seeing, but you certainly don't want to mix hypereutectics in with your forged slugs. I am not aware of any forged pistons with the coated skirts - the slight scuffing on the forged skirts is considered normal and acceptable in a forged piston motor.

LemansBlue68

Do you have pictures of the "scuffing"? What some consider scuffing might just be normal wear or scratches on the piston skirts. V-engines typically will exhibit wear on the inboard sides of the pistons and bores because after many engine shutdowns when warm, a portion of the oil film left on the cylinder bore walls drains first around the circumference of the bores to the outboard side of the bores and then down in to the crank case. The oil is like any liquid, it will flow to the lowest point it can by taking the most direct route. When the engine is re-started the piston take a few strokes in a marginally lubricated bore until enough oil is slung up in to the cylinder to restore the oil film back on the bore walls. This is even more pronounced in extremely cold environments. This why OEM's plate their pistons with tin, lead, or more recently, scuff resistant coatings. These materials have inherent dry lubrication characteristics that aluminum alone does not have.

Your 427 Corvette probably hasn't been seeing much extreme cold weather service so I'd guess what you are seeing is probably more normal wear. Post pics if you can just to be sure .

Another possibility that just came to mind is that with some head gaskets, the manufacturer may recommend the drilling of additional coolant passages in the deck face of the block. I think Gen V and Gen VI blocks have a slightly different coolant hole pattern and coolant passages than the Mark IV block. If these holes weren't drilled, or your gaskets aren't correct, you may have a coolant flow issue. My machinist ran in to this with my engine. I refused to allow him to drill the holes. He looked further and found me a correct set of correct Mark IV head gaskets where the drilling was not necessary; not some of the one size fits all types that are available. Are the pistons that are scuffed in positions 3,4,7, & 8 (the rearmost 4)?

68raggtop

I will post pics as soon as I get my web space worked out. I didn't mean to sound like I doubt Lars, :jester I have read a lot of your posts and am impressed with your insight. I am a "little ****" with putting things back together that I could make better. I was considering new pistons for the very reason you gave "The piston maker will provide the pistons sized to run correctly in the 4.28 bore. Forged pistons typically run at about 3 thousands clearance, so your pistons are the right size." My point was, these are the same part number forged pistons as I took out, and they list a .0015 piston to bore clearance now. I am gratefull for all the info.
Also, LemansBlue68, The scuffed pistons are #4,5,6 &7. the worst two are 5 & 7. in regards to it being normal wear, that is possible. this is the first set of forged pistons I have had. I have a pic but like I said, I'm working on web space to post them (forgot my user ID).
Thanks all. :cheers:

Wrencher

A picture would help, but I agree that the clearances are fine. The pistons often look worse than they are from normal use. I look for major scoring, ring grove damage, dome degradation, etc. On the other hand, change 'em if you don't like the look of them!

Hans

68raggtop

Well, i can finally get it posted (I hope)
Soooooo,what say you all now? Put them back in? BTW that is the worst one.
:smash:

LemansBlue68

'68 raggtop,

I would say based on your picture that these pistons are scuffed due to overheating. There is no doubt in my mind. You need to look further into your cooling system to see what went wrong. Your motor is going to need a rebuild and the bores at least honed if not bored to the next oversize. Of course you will need new pistons. While you are at it, check for head gasket mismatch as I described in the last post. I'm not a big block expert but I did not like the idea of adding additional cooling holes in the deck of my Mark IV block because these would tend to short circuit coolant flow reducing circulation past the rear most cylinders. I don't know how the Gen V and Gen VI blocks avoid this.

When having your block honed, make sure the shop uses deck plates. This won't avoid a future overheat or piston scuff but will assure round bores and better oil economy. Also make sure you get your block and cylinder heads crack checked. Get your new pistons before the shop hones your block. Your machinist will hone the block to give you any piston to bore clearance you want based on the pistons you have on hand for him.

I also noticed that I used the Ford cylinder numbering convention in my previous post (for shame, for shame....tsk, tsk). The rearmost in a Chevy are 5,7 and 6,8 so you've scored 3 out of 4 and one more (#4) for extra credit.

Sorry about the bad news. :sad:


[Modified by LemansBlue68, 2:04 PM 3/26/2004]