Time For The Cobra To Get A New Engine - The 347 Build
 

What started this build is when I broke another rocker arm and bent another pushrod on the Cobra. With 90,000 hard miles I figured the cause was weak valve springs. So I pulled the heads and and while I had them off ported them. I then took them to Pacific Engine (well Guy did) and had them machined and a valve job done. While doing so Guy discovered that AFR had way too much spring pressure and that some of the springs only had .500 lift before going into coil bind. My cam with 1.7 rockers has .530 lift so surprising they lasted 90,000 miles.

While the heads were being done I started cleaning the carbon off the tops of the pistons and noticed a nick in one of the cylinder walls. With that discovery I really didn't want to just slap the heads back on so I started looking into the cost difference of rebuilding the 302 compared to buidling a 331. The cost diiference was going to be very little so I'm building a 331. I've been saving up to buy the stroker kit from Coast High Performance and will likely be ordering it this week.

This weekend I pulled the shortblock from the Cobra and disassembled it to see what caused the nick. What I found is that two studs that hold a splash shield on the intake manifold had come out and fell into the pan. They must have been in there for a long time. There were nicks on the bottom of the pistons, cylinder walls, crank, rods and one lobe on the cam. Here are some pictures of the worst piston.





Before I discovered the damage this weekend I was planning to sell the pistons and cam to help offset the rebuild but now I don't know. The threw the cam out but I'm still considering selling the pistons. They've obviosly held up fine in the Cobra pushing 11-12 psi and the only damage besides the nicks on the bottoms is the wear marks on the skirt of the one pictured above. What do you guys think? Would you buy pistons with nicks like that?

Oh sure, i've used pistons in worse shape then that. Those are cosmetic at best and wont effect longevity of the pistons. They're .030 over right? Those will sell no problem, hell if I didn't have 5 sets of forged pistons i'd probably grab them from you. Bring them down to the shop and i'll toss them in the washer, you'd be surprised at how little you'll be able to see them once clean.

Also dont toss that cam, it's still salvageable and can be refreshed for not too much money. Hell Joey might even buy it from you.

As far as 331, well, we've discussed that before :)

I threw the cam in the trash and it was picked up yesterday. I do need to have Dick take a look at the block though to see if .030 will take care of the nicks. If not I have another block but the one that with the nicks is a much cleaner casting and I would really rather use it.

I wouldn't care about nicks like that. Just put it in the ad and people can just choose for themselves.

Pics of nicks?

They are all in the same cylinder which is number 4. The dark spot in the first picture half way up the cylinder wall isn't a nick, it's just a stain. The deepest nick is the lower one in the top picture but I don't think it's .015 deep.

Are those at the bottom of the bore? If so those are no big deal.

Yes, they're at the bottom.

Dont even worry about them, the rings dont go that far so they arent a problem. Most aftermarket blocks dont even have material that deep. On my dart motor we removed a 1/2" piece at the bottom of each cylinder to clear the rods.

I ordered the kit today and as you can see from the change in the title I ordered the short rod 347 kit fro Coast High Performance. The original reason I was going to build a 331 instead of a 347 was because the 5.40 rod with the 3.40 stroke on the 347 puts the oil rings in the pin hole. This causes it to burn oil. With the short rod (5.315) it moves the pin lower to resolve that. So it won't burn oil and I'll have an additional 16 CI.

:clap:

You convinced me to go with it. They actually use the same piston as the 331 just with the shorter rods. Speaking of the rods, they also told me that they didn't have the rods in stock but they would upgrade mine to the 4130 I beam Scat rod. I looked it up on their website and it's only around $20 more than the Probe 5140 rods the kit comes with but it has a 5.325 length so the piston should only be down in the hole .010 or so and it uses cap screws instead of through bolts.

I have the same rods, and they were kinda "meh" i'd prefer their rods but either way i'm sure you'll be ok. You should see if they wont upgrade you to h-beams, haha.

It's a great facility and everyone was really great, last time I was there picking up my kit the owner saw me trying on this shirt and told me to keep it.

Oh BTW, saw this today
http://forums.corral.net/forums/susp...r-control.html

I'd love to buy that but with this build there is just no way I could.

Just passing it along, I'd like to have it too but it wont work with my control arms. I'll probably hunt down some stock cobra ones.

Little late replying but I might be a bit hesitant reusing the pistons with nicks in them. Worst case scenario, if they are made of a high silicon alloy like hypereutetic castings or a 4032 forging, then they are brittle and you probably already have cracks in the pistons which will propagate over time. Best case scenario would be if they are a 2618 forging; it is a much tougher material and the nicks would only act as a stress riser. Blending out the damage would help but now you don't have a compressive layer from the shot peening process. It is tough throwing out parts that are 99% perfect but it is really just a coin toss without doing some sort of non destructive testing like sonic and penetrant inspection. By the time you get that done, you are probably better off dollar wise getting new slugs. My 2 cents.

The old pistons are Probe SRS 2816-T61 and have been in my 471rwhp Cobra for 90,000 miles running 11-12 psi without any issues. I listed them on Craigslist with pictures of the worst looking piston. I don't think they will be an issue if someone buys them. If no one does though I'll take them to the scrap yard.

Well if you're gonna scrap them....
;)

Those pistons aren't going to be a problem, this isn't on the deck its underneath, where there is the least amount of stress. If the valve reliefs or face or something was all nicked up then yeah i'd toss them. But not on the bottom side. Hell, we use forged 289 pistons all the time cause they're half the cost of the forged 302 pistons you just have to remove a tiny amount of material near the wrist pin. I've had 5 motors like this. One with well over 200k and no issues. Shawn has done countless like that.

Disclaimer: Please don't think I am trying to start an argument. Tone is a difficult thing to portray through he interwebs. Metal is just something that I am extremely interested in and I don't want my enthusiasm to give the wrong impression.

The only problem with the nicks being on the underside of the piston is that that side experiences predominantly tensile stresses versus compressive stresses. Cracks will propagate under lower tensile stresses faster than higher comperssive stresses. There is a good chance that the pistons will be fine for quite some time, but I'm worried about the collateral damage and time to replace if one of the damaged pistons let go. Also, you mentioned that they have 90K on them. Aluminum doesn't have a fatigue life. All aluminum will eventually fail under any sort of cyclic loading; higher stresses and higher temperatures will expedite that. I would be delighted if I got 90K@12psi especially considering there was damage to them.

Not arguing, just stating my experiences. Probe extensively tests their pistons, something like 100k hours at red line. In some of the high winding motorcycle engine ive built I'd break a timing chain and the valve would leave its full impression on the piston. That bike is still going strong with the same owner who bought it from me 6 years ago. This was a 15:1 compression 36whp single cylinder thumper. Small impressions like that are nothing and have personally ran much worse for a similar amount of time. Gashed or grosh material removal anything where you can see that metal has been "moved" is the kind of stuff to look out for. Those are really no big deal. Its no difference then when a piston skirt has been shaved or valve reliefs have been cut or the dish has been opened up. I wouldnt worry about them even slightly. Hell the diesel guys take the 6.4 pistons and cut the dish open nearly 5cc then polish the edge and run 70-80psi on a hypereutectic.

Plenty of light and not any heat in this thread. The metallurgic stuff is fascinating. Continue on. No one's taking a piss.

Piston manufacturers typically have one as forged piston that can be machined into many different configurations. An area that has quite a bit of variability is the valve reliefs and so there is typically extra material in that area to accommodate all the different combinations. Having the valve hit in that area is one of the better ways to make contact with a piston. Unless you had huge reliefs because you had valves that barley fit in the head and a really high lift cam, you probably had some extra material. Also, that area is going to be predominantly in compression so that works in you favor too.

The OEMs are pretty well known to throw metal at pistons; it is cheap insurance. It doesn't surprise me the diesel guys are able to use their stock pistons in heavily boosted applications.

Machining aluminum is different than upsetting aluminum. Machining metal cuts and removes the material like when you opened up the valve reliefs or shaved a piston skirt. As long as a dull tool that generates a bunch of heat isn't used, everything is fine. Upsetting the material imparts strain into the part. That is when things get fuzzy. Different process conditions can produce vastly different results. For example, certain alloys will crack if they are upset at low temperatures versus high temperatures. Also, putting stain into a parts acts like an aging process. Aging is how you get the different -T8 or -T6 tempers after the same quench operation for the same alloy. The main reason that I get worried when I see damage is because I know of affects it MIGHT have but I don't have enough technical experience to be able to quantify if it happened or to what magnitude.

After all that hot air, the pistons are probably fine.

I picked up my kit today. Next up is to clearance the bottom of the cylinders for the new stroke and then drop the block off at Pacific Engine for the machining.

I clearanced the cylinders and dropped the block off at Pacific Engine. Of course I forgot the pistons so I'll be making another trip out there so they can machine the block for .004 piston to wall clearance.

Checked my log book. Mine came in at .005 I got it confused with the spare forged internal motor that was already bored but wiped out the bearings. Came right under. 007

When we did Eric's 331 I had it set to .0045. I'll run it by Dick when I drop off the pistons to see if he thinks .004 will be enough. I think it will be but I'll see what he says.

Should be fine. He wanted to run a bit more because of the amount of boost. The other motor had some rust on the cylinder walls and he cleaned it up without boring it so thats why it has so much

I dropped the new pistons off with Dick today and he had already bored half the block. We measured the bore on the other side as well as the old pistons and the clearance was .005 which worked well. I didn't have piston slap with the old combo and there weren't scuff marks on the skirts besides where the rivet hit. So we are clearancing this build at the same. Hopefully I'll be able to start assembling it this weekend.

I'm going to call Joey today to see about the cam

Cool, text me once you talk to him. Hopefully he will have it done so I can pick it up when the block is finished.

He said friday

Hopefully he sticks to it this time.

I'm not holding my breath

Call him tomorrow and tell him I'm picking the block up on Friday and that I will be by to pick up the cam. Maybe a little pressure will help.

I've been putting pressure on him, every day, stressing how much I need the cam so I can assemble the engine. It's how he's always been.

I picked up my block and cam today (thanks again for picking the cam up Guy!). The new Probe pistons measured 4.026 so the block was finished honed to 4.031 for .005 clearance. Cam specs for my American Custom Cams cam are .555 lift with my 1.7 rockers, 218 at .050 with a 114 LSA. Tomorrow I'm going to start building.

Glad to pay back a favor

I didn't get much done today because I ended helping my neighbor install the trans in his Chevy 1500 P/U.

I started on the pistons after though but only got 4 done. I'm smoothing out all the sharp edges. Believe it or not, it took about an hour per piston.
Before.




After.

I worked more on Sunday. I started by washing the block. Then I chamfered the oil holes on the new crank and washed it. After that I measured all the journals. I then moved back to the block and installed the main bearings and then the crank. Main bearing clearance is .0025. I'm also using a Milodon windage tray so I had to tap the threads for 2 and 4 main caps and then installed the studs for the kit.

After I finished installing the crank I started assembling the pistons and rods. I know I mentioned this back when I help Eric build his 331 but I hate polylocks! So I got all the rods and pistons assembled and then start file fitting the rings. The CHP kit came with prefit Mahle rings (Perfect Circle) and I must say I'm glad that I don't just slap them on and install the pistons. They ranged from .008 up to .016 gap. There instruction sheet recommends .024 - .026 for both the top and second compression rings. So I set the top rings at .026 and the second at .024.

I didn't have much time tonight but I did finish file fitting the last four rings and then installed them onto the pistons. I also measured the side clearance which was .002.

Here are the only pictures I took.