shot peening

[morrisman]

Does anyone have experience of shot peening engine parts?

I have a NOS set of old Morris 4 ring pistons, a aftermarket brand . These are heavy pistons which I think means they have a high iron content. These type of pistons normally have a thin tin coating , the tin coating aids the running in process. In this case the tin has come away or fallen off after many years of storage.

Years ago I shot peened a set of used chevy iron pistons , I actually used a fine sand but I think the professionals use glass or ceramic beads . Anyway the chevy pistons have been running without any problems. I am wondering If I should do the same with these Morris pistons?

371404

[Grahame Collins]

Hi Morrisman,
You are not going to leave us all hanging, are you? Let the other boot drop.

What was the long-term result from shot peening the Chevy pistons?

The sand once used in commercial sandblasting was pretty much outlawed by the various safety bodies. Silicosis was a very big safety issue at the time.

The old machinery department came into the refinery and the sandblaster crew had to change to a garnet based pellet.

I recently purchased a sandblasting cabinet and the grit that came with it appears to be rutile which is as slow as buggery but gives a nice fine finish.
Is running in that style of motor an issue ,these days, given vehicles run up to 15,000kms to the first oil change,or at least my Hyundai does?

Or is the Morry motor a diffrent kettle of fish?

Grahame

[morrisman]

hi Grahame

I think they peen engine parts because it relieves surface tension, or something like that . I know they do it to con rods to minimise any potential fatigue cracks. I am not a auto engineer by any standard.

The tiny little indentations also hold oil .

The chevy pistons are still running without any problems , fingers crossed.

The old Morris engine is a 1930's era motor.

[Grahame Collins]

Hi Morrisman,
The Morris pistons should not be a drama then. I think the clearances and the quality of the oils these days are the difference.
The pistons remind me of a Chrysler straight 8 engine I saw stripped down.
My uncle who owned it reckoned it would pull a caravan up a cliff face, but being a big slow revving deep skirted long stroke motor I understand it would be very torquey.

Grahame

[OxxAndBert]

I think you might be getting shot peening confused with bead blasting. Proper shot peening is typically done with metal (usually steel) shot and in my experience done for fatigue resistance as you suggested. The time/coverage and intensity of the process is quite closely controlled to ensure that the part receives the required amount of peening. Do a search on Almen strips for more info

Bead blasting using glass bead or similar is mainly a cleaning process, but does produce some compression of the part surface - enough to cause distortion on thin flat panels but generally no issue on more solid parts. We used quite a bit of glass bead blasting for corrosion removal on aircraft.

IMO there'd be no point shot peening those parts, but using glass bead or similar to clean them would be fine.

Steve

[Retromilling]

Shot peening was done in my day to mainly conrods not pistons . Sand would just clean them nothing else. Shot means hard steel balls sometimes tungsten and the idea is it shallow cold forges the surface reducing forging stress at the surface that could lead to the start of fatigue cracks . Only iron , steel and titanium rods would get any benefit . Alloy rods it would be a waste of time and money . Polishing alloy rods would be more useful .

[Grahame Collins]

These type of pistons normally have a thin tin coating , the tin coating aids the running in process. In this case the tin has come away or fallen off after many years of storage.

I am wondering If I should do the same with these Morris pistons?

The above is the gist of Morrismans inquiry as I see it.
Morrisman wants to remove, what is now a partial tin coating of these pistons.

The blasting / peening will do this.The light peening action will also assist in retaining oil during the run in .

I don't see a problem with that.

Grahame

[Hunch]

I'd probably have a word to this bloke if there's nobody you know down that way MT&C Engineers - Home Page (http://members.ozemail.com.au/~mtce/) always found him helpful with ndt, peening and friction questions.

I've been using plastic shot on some stuff like that, start to have some concerns about the ring lands if over-enthusiastic.