Piston to wall clearance isssue on 455 with cast pistons

Dear All,

I’m rebuilding 1975 455 for street application with some improvement in performance, e.g. increased CR, ported heads etc.

I’ve ordered cast pistons from TA - 1607, which after delivery turned out to by Egge pistons (I’ve found out some critical opinions on them). I know that some people here are using those pistons without issues and TA has good opinion and perfect customer service so I think they will be ok but...

My machinist (with a reputable opinion) checked their volumetric expansion and said that they expand a lot. Thus during fitting them to block he used quite high piston to wall clearance. After getting block from him, I’ve measured piston to wall clearance and for all pistons instead of recommended by the manufacturer 0.0025 (+0.0005 - 0.0010), I’ve found that he used 0.0043-0.0055, and on one I’ve got even 0.007. Obviously, I’ve measured clearance near the piston bottom as recommended by the manufacturer.

To check if pistons really expand a lot, I’ve boiled one and found out that the diameter increase from around 77 to 190 Fahrenheit is around 0.005”.

This is my first rebuild so I really don’t have experience on that. Do you think that this clearance can be good for a street application? I’ve heard that on cold, pistons can slap with such large clearance but truly saying I would like to have a smooth and quiet engine all the time.

Or do you think that should I argue with him? as producer gave recommendations and he actually used double clearance… (I repeat he has a good reputation, however, he works mostly on european modern engines but physics laws on thermal expansion are same...).

I’m thinking also about switch to higher quality pistons like Autotec and Racetec forged ones which with custom made size could be easily fitted to block only with honing saving some $$ due to remachining cylinders.

At the same time, I’ve found that the forged pistons, in general, expand more than cast pistons. So assuming that expansion of my current pistons is normal, it can happen that the machinist will use even larger clearance with the forged pistons...

Thx for any advice.

Maciej

 

I had a block prepped and the shop thought I was using forged pistons. Block was already bored....he ended up talking me into using the new cast pistons I already bought and had them knurled to take up some of the clearance. Regretted listening to him as it had mild piston slap from day one and pistons ended up cracking in 22K miles.
Moral of story: cast pistons must be set up tight (.002 or so) as opposed to forged which expand more and need .004-.006.
Cast pistons can't take being slapped around.....

 

My 1st ever rebuild way back when was on a '69 Buick 400. Just honed the block & re-used the orig. pistons so wound up w/.005 (ish - some prob more) clearances. Never heard any piston slap, drove it for years & put many 10's of thousands of miles on it & sold it as a running engine.

That said, knowing what I know now would I do it again? Probably not. Ignorance was bliss @ 19 y/o.

 

That can't be good.

Cast pistons, as a rule, expand very little. HOW did he check their "volumetric expansion"?

You need a different machinist. That guy is reckless.

Or, you're not measuring the cylinders accurately. Let's hope it's your measurements, not his.

No.

IF (big IF) he really did give your cylinders that much clearance, you should not argue with him. You should get your money back and go somewhere else. Ideally, he'd buy you a new block that isn't honed so far oversize.

You should switch to better pistons. Yes, you may need "custom made size". Be sure that size includes the compression height; as Buick pistons tend to be enormously too far "down in the hole". It's not just the diameter you need to be concerned with.

The only way to know is to ask questions of the manufacturer, or a KNOWLEDGEABLE parts retailer or machine shop.

 

Order Autotecs. Give them your exact bore size and they'll make them with the proper clearance for expansion. Don't scrap the block...

 

The problem with boiling your pistons like you did is two fold.

1) with no rod on the piston you can in no way simulate the heat that it pulls out of the piston in the motor when it running .

2) cast pistons to not conduct heat from the crown area any where near as well as a forged slug, so in use in a motor they do not get as evenly hot as when you boiled them to check how much they expand with the result that any measurement you took near the pin area of the skirt where exaggerated!

Your in a tuff place money wise with what you have invested in just these pistons, but if it where me I would shelve them and go for off the shelf forged pistons , especially in light of the added power from porting the heads and the added rpm the motor will now see.
Also on the plus side of doing this is that forged pistons run some 100 degrees cooler up top and in the first ring land area.
Running a forged piston in conjunction with today’s poor octane levels is a nice safety plus for motors looking as yours is to make more power then stock.

 

Thank you guys for your opinions.

Well, I'm disappointed in him especially that he has a good reputation. He also argued with me that those v8 engines were not externally balanced by the manufacturer and it is the wrong way of doing this... Well, he could put the minimum effort and read some technical book about old GM v8 engines... Obviously, internal balance is more precise but my goal is a stock engine (for street) with some improvements. The worst thing about people working in the automotive business (at least in my country) is that if they don't know something they are afraid of saying "I don't know, I need to read/learn" (which is not a shame). Then they say some bullshit and pretend that this is the truth, the whole truth and nothing but the truth...

Unfortunately, I'm not the one who made an error here. I've checked everything several times and everything is perfect except the cylinders...

This is probably the solution which I will choose. I need to use the current block. Getting the new one to Europe would be way too expensive for my project. With a good (precise) measurement I could order Autotec pistons with the right dimensions without the need of honing.

Thx, Stevem for this nice explanation. I will go with the forged Autotec pistons with custom compression height to get CR 9.5:1. To my knowledge it should be ok with the iron ported heads (porting them was pretty cheap comparing to prices in the US).

 

Keep in mind the cylinder will also expand with heat, although less than an aluminum piston. You could find the expansion rate of cast iron in ppm, multiply by diameter and temperature rise and subtract that amount from the piston expansion measurement

 

Update!

The machinist admitted that it was his fault and that the clearance is obviously too large for cast pistons. He will buy those pistons from me and finish the job as soon as I get the new pistons. So I’m positively surprised.

I’ve already contacted with buyracingparts.com and ordered Autotec pistons + rings with custom bore size. Waiting for their response and just thinking if they can provide rings for this custom bore size… 4.3355

Another thing, as it is my first rebuild… the cranskshaft was machined to 0.010/0.010 (mains/rods) and clearance is exactly where it should be. However, I’ve found out that the oil passages were not chamfered (I didn’t asked for it). When I was assembling crankshaft for checking purpose, I’ve noticed that on the bearings (clevite P-series) in the places where there are oil passages, they left some marks (see attached picture). I think it’s not babbit layer but just protective layer (against corrosion during storage?). Those marks don’t affect measurements and obviously can’t be felt under finger. This protective layer is so thin that the gauge during diameter check leaves some marks.

So would you suggest chamfer the oil passages? I’ve read some stories that they can eat bearings if there is some burr...

 

1. Make sure you get the correct compression height along with the correct diameters on those pistons. You did give them the cylinder-by-cylinder bore sizes, since the machinist didn't make them all the same...right?

2. Sure, chamfer the oil holes. Clean the crank well afterward.

 

Are you suggesting he order pistons sized for each individual cylinder?

Bob H.

 

The individual pistons can have slightly different sizes. The machine shop should measure each piston and Bore/Hone match to each piston.

Gary

 

Right you match the bore to the piston not the other way around!

Bob H.

 

Guys, due to really long lead time for Autotec's - 10-12 weeks, I've decided to go on with the TA Spherical Dish 20cc forged pistons (1611B) as JW suggested me.
So after getting the pistons my machinist will do .038 over and hopefully this time everything will be fine...

Thanks to his mistake I've switched from cast stock replacement pistons to the forged pistons... paying only the price difference.


Maciej

 

Good to hear, I’m also guilty of running cast with near .007 and it’s been fine. Not saying it’s right, but it’s worked so far. If I had the money to correct it at the time I absolutely would have.