Projects Yet another Vicky project

Oops! Either too much timing or the compression was a little higher than I thought. Beat on it a bit yesterday and noticed a bunch of blowby out of the breathers when I parked. Ran a compression test and #7 was down about 60 pounds. Skirt broke and cracks in a couple spots, bore seems good. Goodwrench 350. Rated at 8.5:1, read where they may actually be closer to 7.8. Cranking compression on one cylinder as high as 155 , expected them to be more like 130 or so. Guess I should have checked it before.

 

Man...I hate when that happens!

 

Knocked the second land out, and the top land is ok? If it were detonation, I would expect it to take out the top land, not the second. Is the rod bearing insert in that hole still tight in the rod? And 155 should be a-ok, even on crappy US 92. My gut reaction is something else happened here.

 

About to head out the door so can't take any second looks but upper bearing seemed fine in rod. Thought it could be too narrow ring gaps till tearing it down but top of piston intact and bore doesn't look scarred. Figured detonation rocked piston and broke skirt. 100+ out and 6+ pounds of boost with 34 degrees of total timing.

 

Ah-ha, didn't realize it was blown. Still, if it's been detonated hard, aside from knocking the top land down, I would expect the upper insert to lose some crush. Is the base of the valve stem blued? Any specks on the plug electrode? I assume the Pistons are hypers?

 

Top bearings tight but are a little burnished, no copper showing though. No evidence of valve contact or specs on plugs. Hardly driven since blower added and usually only in evenings. Probably just on the ragged edge of detonating and I pushed it too hard with hot air. Pistons are just run of the mill thin cast aluminum with steel reinforcing struts.

Gonna weigh one with pin and see if I can find a close match. Preferably with a little less compression and file fit the rings.

 

Well that sucks. Sorry to read this, but we were having fun, right?

 

Cast pistons, how high did you rev it? I think the skirt broke first, unless there are other indications, I dont think it was detonation, I think you over-stressed the stock cast pistons. They are REALLY marginal in any performance application, especially with a blower. You look at the inside of a forged TRW stock replacement piston as compared to the oem casting, they are a LOT beefier around the area where the skirt attaches around the corners of the oil gash.

You know you dodged a bullet there, right? It coulda kicked the rod out, and youd be picking it up with a broom.

 

And a band of blue discolouration around the valve stem just above where it flares into the head is a detonation indicator, not valve/piston contact. I was looking for other detonation traces, its hard to tell via the internet, but it doesnt sound like there were any.
If it was detonating you should at least see shiny specks on the plug electrodes.
I really think you are playing russian roullette if you bang another cast piston the hole and put it back together. I think you are looking at skirt failure, not detonation. I know this isnt what you want to hear, but I would pull it out, tear it down, and go over the other seven pistons with a fine tooth comb for cracks.

 

Yep. Should have it patched back up in a few weeks. Gotta learn somehow.

 

Never considered this thing bulletproof, more of a why not. Seen a handful do OK with mini blowers. Rev limiter set at 6-6200? Gonna replace all 8 pistons and rings and try again. Sounds ghetto but I have a backup if this goes boom again. Trying not to get serious or things tend to snowball and get out of hand and I've got another not so fun car that's only good at going fast.

 

See if you can round up a set of used forged TRW's. No more worries, and I have been getting them cheap lately. Bought a set of used forged +030 351C flat-tops on polished shot-peened rods with sps bolts for $100, nos in the box +030 327 chevy flat-tops for $150, and a set of used +060 283 pop-ups on 327 rods for $150, all in the last couple months.
The old ones are better anyway, for the last couple years they are machined in India, and there are problems with the pin bores out of square and junk ring lands. Try to find a set of used trw forged dish tops, just look for nice skirts and lands with no scoring, and you can pin that bastard anytime you feel like it, no fear!

 

Got my eye on an nos set of forged dished pistons and am going to try and get one of mine off and weighed tomorrow. Love to avoid a complete teardown and rebalancing the crank.

 

If you use the TRWs you shouldnt need to re-balance, they should wiegh the same as the cast stockers.​

 

That would be sweet.

 

Dang you weren't shittin'. Good old fasioned TRW forged L2403 is only about 4 grams heavier than the stock GM cast dished piston. A dollar bill weighs one gram. Have pistons, and a new converter. About to order the rest of the parts and get started on it by the weekend.

 

I HAVE done this before you know...

 

I know but I'm a believe it when I see it type.

Just ordered my re ringing stuff and an 1103 Summit cam 214/224 444/466 to upgrade a bit from my wee little 211/220 432/456 Crower, both at 112 L/S. Also got a Hughes converter made that should stall a true 2500/2600. Nothin' trick but I'd still swear by the seat of the pants that this car was already a low 12 high 11 second car with the blower and tame as could be. Hope the converter doesn't kill the mileage too much. Was still getting close to 15 before.

 

whether the converter will hurt mileage depends a lot on driving style.