Blowby, undersized Piston Rings, and 1/2 second

-
jimmyray

jimmyray

Well-Known Member
Joined
Jul 17, 2007
Messages
948
Reaction score
31
I have whined a couple of times about the performance of my 408, which should propel my duster to 11.5's, but the best it's got is 12.13 @ 110.5. The car has always had some excessive crankcase pressure, usually pushing oil out of the dipstick tube, and sometimes leaking past the valve-covers. However, Sunday I was making a few runs, besting at 12.19, but I actually had som much CrankCase pressure it forced the screw-in PCV valve assembly out of the valve-cover, and blew oil all over the firewall. Thinking it was not screwed in correctly, I reinstalled it, and made a couple more runs, and it did it again!! Figuring something was not right, I packed it in and came home.

I've had the stroker in for almost 2 years, and it has over 120 passes on it as well as 5,000 miles, so it's got some use. I bought all of the parts for the stroker from Hughes Engines, thinking I should avoid E-bay and go with the "experts". Well when i got the kit, the crank was slightly damaged from handling by UPS, the main bearings were rusty, and the ARP head bolt kit was mismatched, missing 2 of the longer bolts.

Well, I am now researching the pistong ring issue, and according to the invoice, they sent me stock 4.00 rings with my 4.030 KB pistons. I'm not sure if thats what they actually sent, but that's what the invoice reports (see below).

I have not performed a leakdown test, but a compression test showed between 170-180 on all cylinders.

Any thoughts on the impact of putting 4.000 pistong rings with a 0.026 (corrected, I origianlly wrote 0.022) gap into a 4.030 bore cylinder?

Engine details:
Motor details:
408 cast crank stroker from Hughes
9.6:1 compression
93 Octane gas
mild port/polish on Edelbrock al perf heads, closed chamber, fresh valve job
M1 single plane airgap with gasket match (heads too)
Carter TQ - .102 & .161 jetting, 1.5” primary
Comp roller .544/.541 236/242 110 centerline
FBO ignition, ditributor curved to 16 inital, 34 total @ 2900 rpm
Hooker comp 1 5/8" headers (dented bottoms on driver side, naturally) 3" collector
3" pipes into/out of welded Summit 3" mufflers into a-body tips
4000 9.5” Dynamic Convertor
727 manual reverse
3.73 sure grip in a narrowed Dana

ScreenHunter_01 Oct. 26 18.02.jpg


ScreenHunter_03 Oct. 26 18.04.jpg
 
a .052" gap is no big deal.. and has nothing to do with blowby. the piston to wall gap is whats gonna bite
you in the butt if its out. should be around 0040" - .0050" clearance. if that checks out, then you need to be sure
the rest of the cyl.bore is to spec. what kinda pistons?
edit* nevermind seen its Kb hypers..
 
4.000 bore rings will not have the tension necessary to seal correctly on a 4.030 bore. Did you check the ring gap yourself?
 
360Z28 said:
a .052" gap is no big deal.. and has nothing to do with blowby. the piston to wall gap is whats gonna bite
you in the butt if its out. should be around 0040" - .0050" clearance. if that checks out, then you need to be sure
the rest of the cyl.bore is to spec. what kinda pistons?
edit* nevermind seen its Kb hypers..
Click to expand...

According to the recipt they are KB hyperutectics which should have a wall P/W of .0015 to .0045. The ring gap factor is .0075 which calculates to .030 for the top ring. .022 is way to tight for the top ring.
 
ramcharger said:
According to the recipt they are KB hyperutectics which should have a wall P/W of .0015 to .0045. The ring gap factor is .0075 which calculates to .030 for the top ring. .022 is way to tight for the top ring.
Click to expand...

Oops! My mistake, I gapped them to 0.026", not the aforementioned 0.022"

I was real careful to gap the rings myself, to the KB specs, with their factor of 0.0065", as per the below chart. Since they are the Hypertectics, they use a different gap factor, according to them.

I'm wondering if the difference is sizes cause an oval shape of the ring in the bore, thus limiting it's sealing ability, increasing blowby. But, I don't know if that amount is that big of a deal.

Thoughts?

ScreenHunter_01 Oct. 26 19.23.jpg
 
I used a ring gap factor of .0075 since you are drag racing it. .0065 is probably OK at .026 but personally it would make me a little nervous since you are running that much on the strip.

Anyway, the only way to know for sure that those were the right/wrong rings would be if you still had the box the rings came in. The amount of blowby you have is most certainly excessive. I would forgo the cranking compression test and do a leak down test. I'm betting it's well above 10% and you'll hear air rushing out the breather/pcv.

I'm sorry to say that you will most likely have to break that engine down and find out what the heck is going on. You may have a bad piston with that much blow by or the rings could certainly just be the wrong ones.
 
loss of tension? doubt that very much. oil rings will be the first to fail in that case.. and he'd know it from day one.
it would be worse then if he'd use 1/16" rings. my buck is still on one or more cylinder bores being out of spec.
 
360Z28 said:
a .052" gap is no big deal.. and has nothing to do with blowby. the piston to wall gap is whats gonna bite
you in the butt if its out. should be around 0040" - .0050" clearance. if that checks out, then you need to be sure
the rest of the cyl.bore is to spec. what kinda pistons?
edit* nevermind seen its Kb hypers..
Click to expand...
Let me get this right, did you say that a .052 ring gap has nothing to do with blow by??? Youre kidding right??
 
w2360Duster said:
Let me get this right, did you say that a .052 ring gap has nothing to do with blow by??? Youre kidding right??
Click to expand...
nope not kidding.
if the gaps are staggered like they are suspose to be acording to the FMS or what the manufacturer says.
its a non issue on the top ring. too big of a gap is better then to small of one. especially when its on the
power stroke.
 
O.k., I had the same issue on my 408, however my endgap was not the problem. MASSIVE blowby, and 180 psi crank pressure. Endgap @ .020. I had the top rings installed upside down, meaning the internal chamfer was pointing down instead of up. It blew my front main seal outward. My car still ran 11.98 @ 113. I knew something was wrong, and that's what I found. Rebuilt it after my EXPENSIVE learning mistake and problem solved. Not saying you made the same mistake, but it sure acts like my car did!!
 
It would have been harder to get the ring over the piston, and it wont have as good a tension to hold against the wall. But it's not fatal. I think it would be wise to get into it and check what's up with it.
 
69Chrgr said:
O.k., I had the same issue on my 408, however my endgap was not the problem. MASSIVE blowby, and 180 psi crank pressure. Endgap @ .020. I had the top rings installed upside down, meaning the internal chamfer was pointing down instead of up. It blew my front main seal outward. My car still ran 11.98 @ 113. I knew something was wrong, and that's what I found. Rebuilt it after my EXPENSIVE learning mistake and problem solved. Not saying you made the same mistake, but it sure acts like my car did!!
Click to expand...

Interesting. I was "sure" that I put them in according to the directions supplied with the rings, but then...

I plan on pulling the motor in a couple of weeks, and going through it. I recently had Bobby at BJR go through the heads, and have eliminated them as a root cause. I am thus left with these possibilities:

1. Uneven surface of the block deck causing blowby past the head gaskets - Bobby and I did notice there was carbon black between cylinders 4&6 and 3&7, as well as in other places, on the block side of the gasket.

2. Piston slap, out of round cylinders, excessive gap between the cylinders cuasing slight rocking of the piston or poor ring sealing

3. Ring issues - installed or gapped incorrectly, the undersize issue, or perhaps they were collapsed at start up due to a lean condition or running hot.

As you all note, the only way to know for sure is to pull it and disassemble it. However, I am going to try a leakdown test first. Incidentally, I have issues with leaking out of the dipstick at higher RPM from the beginning, it's worse now, though, since I have been inching the shift point up from 5500 towards 6500.

Hey 69Chgr, when you rebuilt, what changes did you make, and what was the result in 1/4 times?
 
I took pictures of the entire build, and here is a piston with the rings installed. Note that the dots face towords the top of the piston, and the gaps were indexed 180 degrees per the reccomendations of Hughes. I installed the rings with a spreader, did not spiral them on. Also included is a pic of the cylinder wals prior to assembly, nice and honed, but could be out of round or too big.

ScreenHunter_02 Oct. 27 16.42.jpg


ScreenHunter_03 Oct. 27 16.45.jpg


ScreenHunter_04 Oct. 27 16.45.jpg


ScreenHunter_05 Oct. 27 16.46.jpg
 
Did You Have The Block Squared Or Decked? I Don't See Any Machine Marks In The Deck Surface. I Just Built A 340 & The Deck Was Off 24 Thousandths From Side To Side.
 
mikedevore said:
Did You Have The Block Squared Or Decked? I Don't See Any Machine Marks In The Deck Surface. I Just Built A 340 & The Deck Was Off 24 Thousandths From Side To Side.
Click to expand...

Of course not! Silly me, but I did not know what I did not know... Incidentally, that is what Bobby at BJR is leaning towards as the root cause of the problem.
 
No, I can't remember what it was, but when I saw the pics on my computer, instead of the camera and by eye, I got concerned. I had to go back to the garage and check it out!
 
jimmyray said:
Of course not! Silly me, but I did not know what I did not know... Incidentally, that is what Bobby at BJR is leaning towards as the root cause of the problem.
Click to expand...

My last 360 was off .015 from front to rear from the crank centerline. I had to about beat this one machinist over the head before he did the work, as he insisted that my pistons would be out the top of the hole. I tried to explain over and over that I had mocked this engine up and I could go .020, but that prick didn't get it.

Thank God my old machinist's Busch contract is up and he can now work for the general public again. :cheers: Plus he just bought a brand new Rottler. :-D Nothing like dropping off my old crappy 4L block next to about 6 dry sump 850HP engines. :toothy10:
 
You must log in or register to reply here.
-

Back
Top