Twins with Twins, Anyone?

[kidturbo]

^^ Spoiler Alert ^^

Yeah I actually took some video of one rocking to share with you on Saturday, then got side tracked by deer camp related activities yesterday. But late last night I managed to pull a single piston to get a better look. Gotta grab another paint marker before yanking the rest.

They certainly seem a bit looser now than I remember on assembly. Looks like TBC coating took most of the hit, and matches up with the cyl scuffs. Gonna see if I can get a buddy to run by with his bore gauge and set of mic's later today.

On a brighter note, first rod journal and bearing looks clean.. :alcoholic:

 

[kidturbo]

Pulled heads on second engine last night and piston tops look to be in better shape. However, the cylinders have same scuff marks as first. This engine only has couple hours run time.

What I did find interesting is how on one piston I found a spot where TBC looks to be bubbling up. So maybe this is how it starts.

In last pic I noticed this spot where some liquid was laying in the bowl. Anyone seen stain like that before?

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[DAVe3283]

I've seen those stains on most engines I pull apart, but I never fully drain the block of coolant, and I always figured it was dripping down as I pulled the heads. But in your case, I'm not sure where that came from. Interesting.

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[kidturbo]

Yeah I had a similar stain on another piston but it had the coolant to go with it. That one looks to have been in there for a day or two. Have to examine the heads are gaskets yet, just snapped those when I pulled it apart late last night.

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[MarkBroviak]

Probably a bit of fuel from shut off that dripped from tip of injector, we see it all the time when pulling heads for gasket jobs, not uncommon to see.

 

[kidturbo]

Is what I thought, a drippy tippy..

Checked bore to piston clearance today and it's .006 average across the board 16 holes. They do rock a bit, and the coating doesn't seem to of harmed anything beyond cosmetics.

So now who wants to de-ring 16 pistons and keep em all in order for me?


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[kidturbo]

Main bearings also checked out fine.

https://youtu.be/kE-Bq2elQGc

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[Fingers]

The marks are from the combination of larger PTW clearance and the additional head size from the coating. There will not be any long term issues from it, but would have liked to see closer bore size.

I am fairly sure that the pitting of the coating is coming from impurities leaching out of the piston base metal. Classic coating failure does not bubble. It tends to flake.

 

[kidturbo]

Thanks Jon,

I'd tend to agree with your impurities theory cause this second engine likely saw more water, higher cyl pressures, and EGT, but shows less coating lifting. So run time is our biggest variable between these two examples on the top side. Somethings up with the casting.

I also spoke with a west coast friend who uses Swain coatings on 1400hp forged piston BBC marine engines without issue. These average 1400EGT extended periods also. His only issue has been with thicker skirt coatings when trying to improve PTW clearance. And that failed where piston protrusion runs below the cyl bore on BBC. Not seeing any issues with the factory skirt coatings on ours.

I'm calling Swain today and will see what they have to say on removing this TBC.

These pistons are running a bit loose, but as I recall, we set everything at max GM tolerances for a reason.

Mark and I were pretty amendment about always bringing these engines up over 150F before putting them to work. Problem was, we had to spin some R's and apply load to get them up there.. So my thoughts are the crowns are rubbing worse at cold temp and probably about right under power.

Main thing is, nothing damaged. Just looks like crap, and could probably lead to some hot spots on the tops as things progress. Coating over the top edge seems to be pretty useless for this compound at least.

 

[Chevy1925]

Thanks Jon,

I'd tend to agree with your impurities theory cause this second engine likely saw more water, higher cyl pressures, and EGT, but shows less coating lifting. So run time is our biggest variable between these two examples on the top side. Somethings up with the casting.

I also spoke with a west coast friend who uses Swain coatings on 1400hp forged piston BBC marine engines without issue. These average 1400EGT extended periods also. His only issue has been with thicker skirt coatings when trying to improve PTW clearance. And that failed where piston protrusion runs below the cyl bore on BBC. Not seeing any issues with the factory skirt coatings on ours.

I'm calling Swain today and will see what they have to say on removing this TBC.

These pistons are running a bit loose, but as I recall, we set everything at max GM tolerances for a reason.

Mark and I were pretty amendment about always bringing these engines up over 150F before putting them to work. Problem was, we had to spin some R's and apply load to get them up there.. So my thoughts are the crowns are rubbing worse at cold temp and probably about right under power.

Main thing is, nothing damaged. Just looks like crap, and could probably lead to some hot spots on the tops as things progress. Coating over the top edge seems to be pretty useless for this compound at least.

i dont think .006 is max tolerance. i have not really found a "max tolerance" that GM has written down, just a assumed tolerance by taking the max bore service limit and subtracting from the smallest piston size gm allows. just seems like alot for a cast piston and its only going to grow bigger with wear.

 

[kidturbo]

I read over bunch of old posts, and inquired with couple buddies like Sting who've spun em hard and high. 6 seems to be a decent number for cast, and bit tight on forged. Our ccp was nilly, and no signs of oil burning, so gonna try and raise operating temperature and let em ride...

But the crowns mic'd at 4.034 vs skirts at 4.071 on a 4.077 bore. Guessing that top swells a good bit normally, and with coating, not so much??



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[Chevy1925]

I read over bunch of old posts, and inquired with couple buddies like Sting who've spun em hard and high. 6 seems to be a decent number for cast, and bit tight on forged. Our ccp was nilly, and no signs of oil burning, so gonna try and raise operating temperature and let em ride...

But the crowns mic'd at 4.034 vs skirts at 4.071 on a 4.077 bore. Guessing that top swells a good bit normally, and with coating, not so much??



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Top doesnt really swell that much. i mean its going to cause thats seeing the brunt of the heat but the wide skirts are to keep the piston top "floating" in the bore so they dont wear the sides where the piston rings ride and cause them to stick or keep from sealing. even a higher rpm engine doesnt really want real big p to w clearance because that piston will be slapping around much more. if you keep fuel duration short, timing sane, and get air down in there, you shouldnt have much to worry about as far as piston growth under high rpm. if it were forged pistons, it would be different. Your not hitting them with nitrous or anything weird and unlike street trucks, you have the ability to keep an even cooler supply of "coolant" to the engine which will slow expansion.

 

[kidturbo]

Your correct. I was thinking bass ackward again for some reason. So our 126F oil temps needs raised and leave coolant temps alone.

Here is what they rock like at room temperature.

Excuse the heavy breathing, had headset on with a head cold. Lol

https://youtu.be/FM8iUznnUOk

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[Ne-max]

When I built my last engine fingers suggested staying close to bore specs that came with pistons. After 3 years of hell cylinder walls and pistons look great. For some reason everyone thinks looser is better. I think there is a fine line per application.

 

[kidturbo]

Our thing is sustained high rpm running. Like minutes at 3500r loaded.

I thought we ordered the blocks .001 over, but been so long I'd have to check my notes.

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[Fingers]

One of the biggest advantages to the cast is that they don't expand nearly as bad as the forged. And the skirts even less. So little, that you can keep very tight piston to wall clearances that will work in essentially all applications.

 

[Chevy1925]

Our thing is sustained high rpm running. Like minutes at 3500r loaded.

I thought we ordered the blocks .001 over, but been so long I'd have to check my notes.

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The tighter ptw will like the higher rpms. Like half of what your running. It’s going to flatten the piston skirt against the wall more, distribute the coating against the wall better and glide on the oil film/coating better. That should reduce the amount of scuffing you see greatly.

At this point, I wouldn’t really change it. Like fingers said, it’s going to survive just fine (besides that piston coating). If you don’t have to get new pistons, I’d run it till the next service/major tear down, then re-bore and tighten up that ptw.

You probably have slightly more rock in that video going side to side over the wrist pin since the skirts are not on the direction of rocking and the piston is thinner there

 

[Fingers]

No, the wrist pin will keep them steady side to side for the most part.

 

[kidturbo]

Unless someone looking to run forged pistons has a pair of fresh straight up +.020 LML blocks to trade for these, I'm sending it...

Jon and spoke briefly about developing a piston stretcher tool for just such ocasions. But I've all ready committed to helping out with Mark's brass magnet idea. So once that ones ready to market, we can revisit the stretcher idea, and this build.

Till then, waiting on call back from Swain on the TBC issue. I got the feeling they don't get many calls from people asking, "Hey how do you remove your TBC coatings?" :rofl:

 

[gmc502]

Till then, waiting on call back from Swain on the TBC issue. I got the feeling they don't get many calls from people asking, "Hey how do you remove your TBC coatings?" :rofl:

I think I used a wire wheel to remove the coating from some pistons,,but it was a long time ago and brains a little fuzzy on it.it was that or I just glass beaded it off.