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Hi All- in previous post "calling all mechanics," I posted issue of 70% leakage in #3 cylinder audible from the lifter valley (manifold off). Puilled pan, no metallic signs evident, pulled head and piston and saw piston scuffing, and some on thrust side of cylinder. Took head and piston to machine shop and said scuffing was the "black mark of death." I mentioned that I could never find a vacuum leak but it always ran 13" and slightly unsteady. He told me it was most likely leaking at bottom of intake and was making the cylinder run lean and hot, causing rings to lose spring and their sealing properties. I did the leakdown test by loosening the rockers all the way but did not note the position of the piston as I've read it can be done anywhere in the stroke. No measurable taper detected (4.280" and top and bottom), and the piston size did not seem to be affected half an inch up from the bottom of the skirt (4.2765") but the scuffing is evident. Is it possible to have a psi of 205 dry, 235 wet, and have that kind of leakdown? The scuffs are just barely detectable with your finger on the cylinder but never catches a nail. The piston is more detectable but still does not catch a nail. Pictures are posted of each. The machinist said he did not need to see pics of cylinder from just looking at the piston. Outside (thrust side) of #3 cylinder Outside (thrust side) of #3 cylinder, 2nd pic thrust side of #3 #3 inside (opp thrust side) scratches are visible.
The motor ran strong but had a choppy idle (110 centerline), but the ticking was really bothering me. I was not sure about the vacuum reading because of the cam. I'm pretty sure most will recommend rebuilding but my main question now will be...
should I reuse all pistons that are not damaged if the bores are good, and just re sleeve any damaged bore back to .030 over... or
go .040 over and replace all pistons. Why buy more pistons if they are mostly all good?
I clearly need to remove the other head and check out all pistons but if this was due to a vaccum leak, I'm anticipating others may be fine.
I don't mind spending the money if necessary on new parts, but I don't want to keep boring out more and more.
All thoughts are welcome...Thanks!
Those pistons look bad.... even where the skirt is not galled, it is worn badly. You can see the shadows left behind where the skirt machine marks are just washed out.
Also don't think you will get the vertical scoring out of that one hole without ending up with excess piston to wall clearance.
Either 1 of those 2 things alone would make me bore it and get all new pistons in any engine I cared about being right.
Yes it is possible to have good cranking compression and high leakdown on the same hole. Compression tester has a valve core in the hose and is just a momentary measurement that gets held and pushed up until the cyl can no longer pump more..... leakdown measures the volume of air entering the cyl vs the volume need to constantly enter the cyl in order to maintain the set pressure in the cyl, thereby giving you a percent thats by being lost.
Will
Last edited by rklessdriver; 07-12-2019 at 09:11 PM.
Definitely needs to be bored/torque plate honed and new pistons. .030” should clean that stuff up if it’s still standard bore. If not, .060” over is fine and shelf pistons will be available.
The bore might clean up with a hard stone hone. I would try that first. You may have caught it early enough. If tbey can get it smooth and the clearances are still within spec, I would run it.
Hi Jackson-
I have read that this technique is only temporary and will not last but I'm not absolutely sure about that- I don't mind spending money on new pistons needed, but if I can resleeve and keep the bore at .030 over like it presently is, I'd like to reuse any piston that is still good and just buy any new ones needed. The machinist recommended going .040 over to clean up and buy all new pistons. I guess this is my main question. Why buy all 8 if not necessary, Is there a downside to re-sleeving only the affected cylinders?
your guy bores one for $ ??... all 8 for $ ??
your guy installs sleeve for $ ?? Does his job require decking after sleeve $ ?? Both decks $ ??
your guy rebalances assembly for $ ??
some years back ... local racer running supertruck (essentially an asphalt latemodel with a fglas pu body) and a x603 crate (like an x602 but based on ZZ4).
Anyway, he smeared a piston and we had to get it ready for the next weekend. Only thing we had on hand was a used 6" rod & accompanying flattop.
So, 7 holes had 5.7" rod-piston and one had 6" combo ... ran fine ... no pronounced imbalance. It got straightened out in off season.
Do what you can, with what you have, where you are. Theo. Roosevelt
I'm curious as to why re-sleeving is a more invasive procedure than over boring. Do they have to remove a lot of material to fit the sleeve in? This is the original block so I want to do as little as possible to modify. It currently is .030 over (30,000 miles ago) and has TRW forged pistons. It sounds as though it will be better to bore all at .040 unless they will indeed clean up with a hone. I suppose I should go ahead and take out all pistons, assess, and get back to all those who were good enough to give me their input.
Thanks again!
Kirk
sleeve is at least 1/8th wall thickness. so .250 overbore. INTO the water jackets. then seal it with sleeve. AND sleeves are designed around OE stuff. so they all come standard bore. is this a forged piston build or cast. Cuz if cast, you are doing too much to save 7/8ths of a 200 buck set of pistons. if forged, you need piston numbers and see if match is available in 30, 40 and 60 over. you need to know what is available before you do any more than sticking a 3 stone hone in that bore and seeing what it comes up like.
Last edited by derekderek; 07-13-2019 at 07:56 PM.
The cylinders are most likely out of round.. a hone job will not fix that. You can see the shadows in the cylinder walls. Needs to be bored so the cylinders will be trued up. .040 or .060” over is fine, then do a good torque plate hone job on it. Spend the money and fix it right the first time or else you’ll just be doing it again when you’re tired of it guzzling oil and lacking power/efficiency. That engine/car is worth fixing it right.
I did the leakdown test by loosening the rockers all the way but did not note the position of the piston as I've read it can be done anywhere in the stroke.
This is not a true statement for a leak down test. If your piston being tested is not at TDC the 100 PSI air input will rotate the motor. You have to be at exactly at TDC.
So did the motor rotate while doing a leak down? Also if you truly did have over say anything over 15% blow by the rings you would have excessive oil consumption and the spark plug for that cylinder would have signs of oil fouling. Scuffed pistons is usually a sign of excessive ring ware allowing the piston to rock more in the bore.
I forgot to say that the 4.280 means that it was a .030 overbore. so it has been rebuilt in the past. You could do a quick cross hatch honing and just re-ring and go back together. Those appear to be a heavy cast piston. You can clean them up or go with some lighter forgings and rebalance the motor. You should consider higher compression pistons to get you up over 10 C/R since you have a better than stock cam. Just figure out your DCR if you know the cam specs
I'm curious as to why re-sleeving is a more invasive procedure than over boring. Do they have to remove a lot of material to fit the sleeve in? This is the original block so I want to do as little as possible to modify. It currently is .030 over (30,000 miles ago) and has TRW forged pistons. It sounds as though it will be better to bore all at .040 unless they will indeed clean up with a hone. I suppose I should go ahead and take out all pistons, assess, and get back to all those who were good enough to give me their input.
Thanks again!
Kirk
Yep pull it apart and take it to your machine shop and see what they can do. Kind of looks like the piston to wall clearance was to tight when it was setup last time.
I had the motor in gear during the test so motor stay pretty much in place. The #1 next to it was 9% and #5 was at 12% loss. The piston was a little carbon fouled but no more than the others on that bank. The plug was not perfect tan in #3 but it was not fouled with oil either. It was firing and you would not feel a loss of power. It was not a baby smooth idle but I did not expect one with my 110º lobe centers. Vacuum at idle was only at 13" and would drift but not jump. The machinist who looked at the piston said he suspected a vacuum leak below on the intake runner that caused it to overheat. i looked at the gasket in that cylinder location and did not note anything different that the other locations for that bank. I don't know if a leak for a long time would show up there. I was really puzzled when the test showed such a huge loss since the plug did not really look all that bad.
I'm curious as to why re-sleeving is a more invasive procedure than over boring. Do they have to remove a lot of material to fit the sleeve in? This is the original block so I want to do as little as possible to modify. It currently is .030 over (30,000 miles ago) and has TRW forged pistons. It sounds as though it will be better to bore all at .040 unless they will indeed clean up with a hone. I suppose I should go ahead and take out all pistons, assess, and get back to all those who were good enough to give me their input.
Thanks again!
Kirk
Resleeving requires the deck sirface to be milled. If the cylinder is on the passenger side, you will loose your matching numbers. Even if it was the drivers side, that operation is shooting a squirell with a shotgun here.
the real reason for leak test at or near tdc is that is where the bore wear is worst. at middle or bottom it will seal much better, giving a false good result.
07-12-2019, 05:31 PM
Advice for L-36 rebuild
