Piston knock...how much clearance?

[simonjrwinter]

I'm about to tear my engine (331 stroker) apart to trace a knock (only done 400 miles) I'll check all the usual things, but roughly how much clearance should I expect to find from my pistons at the bottom of the bores (trying to eliminate piston knock as the cause) I asked the engine shop to re-bore with 3thou clearance (recommended by the piston mfr)
Thanks
Simon

[simonjrwinter]

Anyone?

[John 550]

Why not call or email one of the engine builders on this site , they all seem to be great !

[scottj]

Originally posted by simonjrwinter

how much clearance should I expect to find ?

Anyone?

After only 400 miles, I think your answer is in your first post:

"I asked the engine shop to re-bore with 3thou clearance (recommended by the piston mfr)"

Scott

[misfit41]

I havent built a small block,but .003 sounds way to tight to me in general,especially if that is at the skirt,I would bet you will have piston scuffing causing the knock.
I dont have the specs in front of me,but I am thinking for a high performance application,it is more like .015-.020 total piston clearance.
Maybe some good engine guys will reply to your post,I know how you feel with no replies,try a thread that ditches someones car,or bickers about something if you want lots of input,when you really need help about something legit,not many wiill take the time,it isnt juvenile enough I suppose.heheh
good luck and happy New Year to all,,Tim k

[grumpy]

M41,
I doubt if it's PISTON knock, but a 331 rattles a bit to start with, those are very oversquare design plants with VERY SHORT piston skirts, good for 12,000 of street driving, at that point it might become unbuttoned skirt rattle. I would suspect it's more in the area of cam overlap/timing issue, not clearances.
grumpy

[Perf. Eng. Eric]

What type of pistons ? cast or forged. That will make a difference. Three thous. sounds like cast to me. Also check that rod side clearance,crankshaft end play,and piston to head clearance. Hope this helps.

[MJN]

Simon

Most modern pistons are barrel shaped or at least they are not round. This is so that when they reach operating temperatures they will fit the bore correctly. As you can imagine the crown or top of the piston will be hotter and therefore expand more than the skirt of the piston. The piston manufacturer will specify where the diameter of the piston should be measured to set bore to piston clearance. Also, forged pistons will typically be run with a larger clearance than cast or hypereutectic pistons. 0.003" sounds about right for a cast piston but too tight for a forged piston. I have Probe forged pistons in my 331 and the specified 'street' clearance is 0.0045" to 0.0050". For racing use they recommend a slightly larger clearance of 0.0055". Probe recommends measuring the piston midway down the skirt at 90 degrees to the wrist pin. BTW I can hear a little piston 'slap' in my engine when it is first started but it is gone within the first minute or so as the pistons heat up and expand. Knocking is not normally piston clearance but bearing clearance.

[scottj]

I must be reading the question differently than everyone else. As I understand it, the proper clearance has already been established, per the manufacturer, at .0030. After 400 miles this dimension won't have changed due to normal wear, so that is what you should find.

[HighPlainsDrifter]

Hi,
Mike is right, if it isn't a loose or broken rocker, then it may be a rod or main bearing, could be a 'spun bearing' or a 'piston wrist pin' with the wrong clearance. I would plastigauge all the bearings, should be .0015 to .002 for mains and .002 for rods. Racing clearances are .0005 more. Startup is when the assembly lube saves your bearings until the oil takes over, need LOTS of assembly lube on bearings or they can stick to the crank and spin.
Good Luck,
Perry.

[simonjrwinter]

Thanks for the advice guys, much appreciated and happy new year.
Simon

[simonjrwinter]

The shop machined the block to have .0032 clearance to the pistons, and yes they are forged. Is that too tight? (see my latest thread for pictures of damaged pistons)

[MJN]

Simon

I would say that is too tight a clearance for a forged piston. If you have not really leant on the engine you may not have done any real damage to the pistons due to the clearance because they won't have got to max operating temperature.

Also, the photo in your other post looks like a ring clearance issue. Either the ring end gap is too small or could be a ring that has siezed in the ring groove. If that particular cylinder was backfiring, or detonating as you noted then the ring may well have siezed or be pinched in the ring groove due to the ring land being deformed. If it was real bad then the knock may be the wrist pin clearance on that particular piston. It obviously took a bit of a hammering.

[simonjrwinter]

As an Update, I just received this from probe pistons.

"Simon, we run our pistons with skirt to wall clearance .003 to .0035. I
would say that perhaps your machinist measured the pistons in the wrong
place or measured the bore wrong. I have customers that are running the
pistons at .002 clearance with out a problem on naturally aspirated engines."

As my machine shop has measured the clearances at .0032, then it seems I'm stuck with nothing to do except paying out more money for pistons that might do the same again !!!

[wilf leek]

Simon:

two things;

1/ get a second opinion on the bore/piston clearnce measurements in your exisitng engine.

2/ check the rod side clearances, to ensure they are large enough to give sufficient splash lubrication to the bores. Someone elsewhere mentioned the fillet radius at the "ends" of the crank pin journals being large, which can cause this rod side clearance to close up - a good point.

HTH

[simonjrwinter]

Thanks Wilf, I'll check EVERTHING before I put it together this time

Simon

[MJN]

"Simon, we run our pistons with skirt to wall clearance .003 to .0035

Simon I am sitting here looking at the instruction sheet that I got with my Probe forged aluminum racing pistons and it is very clear about the piston to wall clearance.

N/A street cars 0.004"
Blown/Turbo/Nitrous 0.005"
Drag/Circle Track/Road Race 0.0055".

My clearance was machined to 0.005" and I have had no issues in 2600 miles of road and some track use. They also note that engines that are honed without torque plates will require additional 0.001" clearance. Also the piston diameter should be measured at the bottom of the skirt.

I can scan the sheet in and e-mail it to you if you want.

[simonjrwinter]

Mike,

If you could do that and send it to me, that'd be fantastic.
My email address is simonjrwinter@yahoo.com
Thank you very much.

Simon