Forged pistons causing double digit knock?

[ThikStik]

My new motor build has CP forged pistons. Piston to bore was supposed to be 4-5 thous. My machinist admits to 4.5-5.5 thous. I hear a near dieselly sound when real cold. After warm, there is a very slight thrashing sound. I have 22 deg of K at first gear. Ive discovered that a free rev to 4500 rpm's gives same. But, Car punches out great from 2nd gear, and on, with only 3-4 degrees of K. I suppose Im screwd as usual...that a machinist gave me too much slack on a cylinder or two. Ive checked;exhaust, tc bolts, motor mounts. It could be internal trans issue maybe. But main ques...has anyone heard of forged pistons causing terminal K triggering?

 

[lazaris]

With those pistons your better off with that clearance. I ran mine at 4 thou and showed some scuffing on the skirts. The car will run faster as well with that clearance. Yes there are a bit noisey on a cold motor but when they heat up the noise is mostly gone. As far as the knock sensor youll have to either dis connect it and run race fuel or use a chip that can ingnore it for a certain amount of time.

 

[ThikStik]

Sounds good Lazaris...I know these are fairly new pistons as far as being out there in the field. Good to hear your results. Does anyone know if K sensors get MORE sensitive as they age?

 

[VadersV6]

You're not running any less than normal piston/rod length combo are you? A short skirt'd piston tends to be noisy, but you shouldnt have that type of piston unless youve gone with some special setup. .004-.005 wall clearance will not make it sound like a diesel. There is something wrong. Hopefully the machinist Mic'd the skirt diameter on every piston, because quite often, someone mishandles it and you end up with one skirt crushed in .005 or so. Then you get a piston flopping around. You're also supposed to mic the cam profile of the piston (pistons are not round).
Did they put any sleeves in the motor, or put bushings in the lifter bores or anything exotic?
You can try a trick I did on my cobra. Pull the knock sensor out, and wrap the threads in teflon tape. Then put an o-ring at the base where it seats against the block. Install the knock sensor, but dont tighten it all the way till the o-ring squeezes out. Just tight enough that it wont be totally loose. This drastically reduced the sensitivity of the sensor on my cobra.

 

[broke1]

What kind of timing chain???If its not oem or a rollmaster,id say thats your problem.

 

[Blown&Injected]

Have a fresh engine, like 10 miles fresh, with forged pistons and started it the other day when it was in the 20's just to make some room in the garage - good and cold!!!!!!!! Did not make any knocking noise or anything different than the stock engine.

seems like something is very wrong - what's your oil pressure look like?

 

[lazaris]

Have a fresh engine, like 10 miles fresh, with forged pistons and started it the other day when it was in the 20's just to make some room in the garage - good and cold!!!!!!!! Did not make any knocking noise or anything different than the stock engine.

seems like something is very wrong - what's your oil pressure look like?

What brand pistons? Some forged pistons like TRW's require less piston to wall.

 

[Blown&Injected]

40 over Speed Pro's - the box is in the garage, will check the specs on the box later - got to move the cars to get the grill out anyway.

 

[bruce]

Good to hear your results. Does anyone know if K sensors get MORE sensitive as they age?

While I've never seen it, it might be possible if it was in an engine with a long term, and serious case of detonation. Vibrate anything long enough, and hard enough, and you generally can cause it to fail.

There are just so many possible causes. Anything from a serpentine belt to alt bearings, just to name a few of the odd things. Since you're in the frame of mind to seriously free spin the engine that hard, you might try taking the serpentine belt off for a min., and seeing if the KR goes away, if it does then you know it's then probably accessory related rather then engine mechanical. Do it quickly, and just once, IMO, free reving a street engine is being extremely cruel to it.

 

[lazaris]

40 over Speed Pro's - the box is in the garage, will check the specs on the box later - got to move the cars to get the grill out anyway.

These are the same as TRW's which require less piston to wall than JE's.

 

[ThikStik]

Sorry about my lack of response...I didnt get any emails alerting to thread responses and thought there were no more. Ive got a Cloyes timing set. I have no unusual rod/stroke , just stock, no sleeves or anything trick. I checked 2 pist/bore's and got 5.5 or so at skirt. I trusted the shop, so I didnt check all others. Naive.. machinist just apparantly dont like to send Buick turboes out the door correctly.
Since it sounds like just 1 sound, it may be that a skirt was damaged inward like you say Vader. Its not a hard sound just a busy thrashing..if all 6 did it , it would be way noisier.
Oil press is 18 @ and 55 at speed. Yea Bruce, I scoff at free revving but will try to eliminate accesories by belt removal.
Thanks

 

[VadersV6]

Where on the skirt did you measure and get that .0055"? You should flip the piston upside down, and using a micrometer (not calipers!) you should measure about 3/4" from the end of the skirt. Dont measure the skirt at the end cause thats the crush zone. Yeah JE does recommend a bigger gap, but I always ran it tighter than they recommended. Usually .002"-.003" at the most. Sometimes as tight as .0015". Hypers, I usually honed to get .001-.0015" and TRW forged and Ross forged about 002-.003". They will usually manufacture the pistons in a way that gives you the proper clearance if you simply take the bore to the exact number. Say 3.830" is a .030 over bore, and the piston measures 3.828 3/4" down from the end of the skirt. Obviously you have a .002" gap and the manufacturer cut the piston to give you the gap they recommend.
A .0055 gap on a 3.8" bore is really big, but I think the knocking is coming from somewhere else, like a crushed skirt causing it to bang around or maybe a loose pin or improper cam slope cut into the skirt. Could be anything. A gap that big will bang around but not enough to set off that much KR. hopefully they didnt treat your main and rod clearance the same way. This isnt a 1967 327 small block chevy where you run big gaps to make the motor live at 7500 rpm. The TR motor makes ALOT of cylinder pressure at low rpms, and it needs to be gapped everywhere with that in mind. Too much oil will bleed off at low rpms with big gaps, which is right where you need all the oil pressure you can get. There are too many machinists out there who wont spend the money to buy the book with all the clearances for all the motors, and they end up doing everything by "feel".
I dont know what everyone else thinks, but that oil pressure (18psi) seems low to me. Try dumping that oil and throw in some valvoline SYN 20w50. I put that stuff in this weekend and I cant believe how much quieter my engine is now. In independant tests, that SYN oil outperformed and outlasted every synthetic they tested it against. I had 10w30 in there before and it sounded like a damn rock crusher. It was eating alot of oil too. I think whoever did my engine made some big clearances, and this quieted everything down and should reduce oil consumption. I dont have an oil pressure gauge which sucks, and Ive just sprung a huge oil leak somewhere in the front (before the oil change) and cant find it! There is oil and grease everywhere! YUCK.

 

[ThikStik]

Thats where i measured it, about 3/4 up from skirt using a mic, and for bore, using a telescopig gauge. History of this motor has gotton me to this point. It ran rich and washed cylinders for 9k miles before I tored it down from a loud rod bearing. (that was given chevy clearances). I removed this motor before losing anything. I told this crew to sleeve any cylinder that may need it because I have $700 in these 10 over CP's. They said that all would be fine and claimed that their plateau honing would only remove .0004. A finish hone only was needed. I did convince them to give 1.5-2.0 on recip assbly. They balked. I sent rods to Arizona to be resized (Mike Zimmerman). I did find a few clearances to be big at 2.6, but I dont think that is my noise. I bet a cylinder has a 5-7 pist/bore cleaarance, or , as you said , maybe a dropped piston with a bent in skirt. I also dont think this is causing the hi K. I have some race gas and will report back soon to eliminate real K.
Thanks

BTW-Reid, if your out there, I had my motor done at Howtons in Hueytown. Did you'res turn out ok.

 

[ThikStik]

I know this thread is going to the crickets, but figured Id post latest. Ive discovered something tht is consistent. I can avoid K on a hiway puch from 3rd if I have trans already in 3rd, and IF I ease into WOT. While quick 4-3 detents with WOT usually set off K, but not always.
Thanks

 

[eddc87]

I have a similar situation with my new motor. A diesel sound on first start up but after warm up the noise is quiter but never going completely away. Everybody that I have talked to says that the pistons have more clearance that the manufacture requests. I have Wiesco pistons that have a recomended clearance of .005. My machinst said he usually increases this by .001 to help with piston scuffing problems and uses wiesco's alot in circle track motors. I have checked most everything else that I can think of. I do have geat oil pressure (25 @ idle) and no knock. This noise seems more prevalent towards the rear of the motor. Could it have anything with the torque converter? Hope we can find some answers.

 

[VadersV6]

I have Wiesco pistons that have a recomended clearance of .005. My machinst said he usually increases this by .001 to help with piston scuffing problems and uses wiesco's alot in circle track motors.

What is it with engine machinists and their desire to be mechanical engineers when it comes to playing with other people's engines? There is a coefficient of expansion with that material that remains the same no matter what. There is no need to add .001 to the already huge clearance. .006" clearance will CAUSE scuffing problems and will cause more skirt collapse than you would like to have. Once the piston cocks to the side, the skirts crush, and then the piston cocks even more, and then the skirt crushes even more, and so on. Keep the piston tight from the get go, and it will break in properly. I would never go more than .0025-,003 on a piston like that.

 

[ThikStik]

Well, I may have found something. The comp 206206 flat tappet cam with only 14k miles may be killing the #3 exh lobe. Or, more correctlym the Buick block may be killingit! It has just enuogh difference in lift, that I can see it at idle. I tried to dial gauge it, but couldnt get it all jigged up. Anyway, it is known that exhaust valve timing is the most important cam event. A sooner valve closing causes cylinder pressure to rise..so, I imagine that my cylinder pressure is higher on that cylinder and it is the one therefore causing double K. I guess I could spend the rest of the night searching for what the semi flat lobe can do. My stock one did this and no-one ever had the same symptoms as mine did from looking on here.
I wonder how long this motor would go if I set a brick on the accelorator and went inside for supper. Getting tired of doing things 2-3 times.