Topic: serious white smoke after big bore

[HondaMan]

Out getting oil and filter for oil change today. No luck on finding amsoil, bellray or torco. Did find lucas zinc additive and an oreilys brand 20w50. I'll pull the plugs and let cha know what I figure out.

The Lucas zinc additive will likely mess up your clutch: it is a high-detergent additive that makes the plates slip. It is made for engine-only use.

[bluefields]

Sorry of my life 

[bluefields]

Ok one last idea before I admit total and utter defeat...would it be advantageous to get a set of wiseco rings and put them on my pistons? Or is my bore just too too sloppy to save?

[PeWe]

Ok one last idea before I admit total and utter defeat...would it be advantageous to get a set of wiseco rings and put them on my pistons? Or is my bore just too too sloppy to save?

Wiseco rings might not fit your pistons. The rings are often of different height.
Your rings are OK if you can measure the ring gap within the spec when you put the rings only in the bores (about 0.3mm). Use the piston as a tool to press the rings in correct angle thru the bore. Too small gap can also cause problems

http://dynomanperformance.com/engine/pistons/piston-install/65mmSOHC.pdf
Note! The piston/bore clearance here is for forged pistons that must have more clearance than cast pistons.

[madmtnmotors]

Ok one last idea before I admit total and utter defeat...would it be advantageous to get a set of wiseco rings and put them on my pistons? Or is my bore just too too sloppy to save?

Wiseco rings might not fit your pistons. The rings are often of different height.
Your rings are OK if you can measure the ring gap within the spec when you put the rings only in the bores (about 0.3mm). Use the piston as a tool to press the rings in correct angle thru the bore. Too small gap can also cause problems

http://dynomanperformance.com/engine/pistons/piston-install/65mmSOHC.pdf
Note! The piston/bore clearance here is for forged pistons that must have more clearance than cast pistons.

+1

You would need rings that fit the pistons. You can get them from the ebay seller that sold you the pistons. First things first though, pull the motor and then pull the jugs. Check for damaged rings. Measure the piston skirts and measure the bores. Report those measurements back here. If one or more sets of rings were damaged on install you might be able to re-ring if the cylinder was not too badly damaged. Let us know what you find.

[Bill/BentON Racing]

I know you don't want too.....but I have NOS freshly bored 836cc Wiseco kit, cylinder decked just to level, ready to install. Problem is they're  pricey these days. Let me know if interested. Bill

[Big Jay]

I know you don't want too.....but I have NOS freshly bored 836cc Wiseco kit, cylinder decked just to level, ready to install. Problem is they're  pricey these days. Let me know if interested. Bill

Best answer right there.

[bluefields]

Well I have pulled the motor apart and measured the ring gap within the cylinder. It it's 0.021 I'm in the process of building the 74 motor up stock so I'll get some riding in.

[TurboD]

I would continue to look for a problem. Again that ring end gap and or cylinder clearance is not the sole source (or a problem at all IMO). Since your original posting and my original reply, I built a 836 with a cruzin image kit using used cylinders that I had to hone out to .004 clearance to get them straight and round. I have been riding the bike around with no smoking or oil use, I runs good.

[bluefields]

Thanks TurboD, I'll keep at it.

[PeWe]

Valve guide seals OK? all 8...   any strange oil leak thru head gasket into a cylinder? Many new have small holes, not made for the 8 o-rings with dowels.
or if those are used, dowels too long when heead, cylinder block might been milled...
I have an old type of gasket and I think the oil leak outside the cylinder on left side is the result.  I'll pull engine coming winter anyway. I'll use a gasket with holes for the 8 o-rings and dowels...

[bluefields]

So the gap in the ring when placed in the cylinder is equivalent to the clearance, correct? Cause I'm confused now. The clymer book says stock ring gap can be anything smaller than 0.28? The gap in these is .02-so there shouldn't be a difference between bore sizes gap is gap

[TurboD]

Generally speaking, It has been common practice for many years to shoot for a minimum top ring end gap of roughly .004 per inch of bore to be safe so that the ring ends do not butt together when hot, causing the ring to try to lock in the cylinder thus scuffing the cylinder and often breaking the piston.

I know this does not pertain to your application, but when we are building a motor for Nitrous or Turbo, we will normally shoot for .0065 ring end gap per inch of bore to be safe.

[kmb69]

So the gap in the ring when placed in the cylinder is equivalent to the clearance, correct? Cause I'm confused now. The clymer book says stock ring gap can be anything smaller than 0.28? The gap in these is .02-so there shouldn't be a difference between bore sizes gap is gap

Don't confuse ring end gap with piston to cylinder clearance. Two different things.

[Jerry Rxman Griffin aka MuthaF'er]

And don't confuse INCHES with MILLIMETERS. Use your units of measure guys. These bikes are metric and many talk SAE/inches.

[DMcD]

I'm confused, my 550 barrels are currently at the machine shop, to get the cruzinimage rebore.  He asked me what clearance I wanted, and I told him 0.002, as I Read another thread on here somewhere where a guy left 0.001, and it wouldn't turn over when warm! 

[Jerry Rxman Griffin aka MuthaF'er]

.002 WHAT??

[DMcD]

.002 WHAT??

I assumed 0.002 of an inch.  This chap has spent 40 years reboring engines, and asked if it should be 0.002 or 0.003.  0.001 did not come up in the conversation!  Think I'll stick with 0.002 and go for thicker oil . My race rep bike is only being built for parade laps so it won't matter too much if it likes a wee drink

I appreciate this is a mainly 750 thread, and mine is a 550 kit.

[TurboD]

Yes it is .002 inch.

I wonder what the OP found wrong with his engine??

I will say that I have been riding and racing my bike (HARD) every week with the .004 clearance and no signs of smoke, no oil use, and perfectly clean plugs..

[DMcD]

Good to know, I've even found a thread on another forum where two engineers are arguing about how to measure the gap!!!

They couldn't agree if 0.002 clearance spec was 0.002 between the piston and bore in one place, or 0.002 all around i.e. 0.004 difference between piston and bore!!!

[TurboD]

When measuring the piston you should always take the reading on the skirts at the widest point, typically mid way down on the skirt. The cylinder if bored and honed correctly will be completely straight and round.

[DMcD]

Excuse my previous grammar, they weren't so much arguing about how to measure, but whether (for example) a 4" piston specced to have 0.002" clearance would mean a 4.002" bore to have 0.002" at one side, or a 4.004" bore to allow 0.002" all around.

Both argued they were right, and had been building engines for many years, and teaching others to do so.

[bwaller]

Those are cast pistons. 0.002" may not be excessive clearance but you certainly can tighten that up some if you choose to. When I hear guys talk under 0.001" I pucker for sure. That said I just bored a set of 550 cylinders to 0.0017" for forged pistons with a skirt coating.

[joe p]

I had a 1428 Kaw FBG build with .0015 I raced that motor for 5 years no problem. It had 310 psi on all 4 when I sold it.

[HondaMan]

I rebuild all of the SOHC4 engines (cast pistons) with .0010" piston clearance, .0080" ring gap on the solid (upper) rings (and .0060" if 1-piece oil rings) and .0040" on the 3-piece oil rings. The only deviation I will take is with the near-limit bores, like the 590cc for the 550 or the 836cc for the 750: then the piston clearance is .0012".

If the pistons are forged and the clearance is less than .0020", then yes, the engine will "stick" if it is run a short distance and stopped (like a trip to the gas station from home?) until it cools off a bit. This is because the forged pistons heat up and cool off slower than the cylinders do, as their materials are different. If you know this and are willing to  put up with it and "stay with the program", the forged pistons can work for you. The main "rule" to remember with them is: if you don't fully heat up the whole engine, don't shut it off and try to restart in a couple minutes. The pistons will be temporarily bigger than their bores. Some machinists freak out at this, and start adding excessive clearance to "fix" it, which in turn makes for almost no oil contact between the piston skirts and the cylinders: this then overheats the pistons and makes them melt.

So, it's just that you gotta know what you have with those forged pistons, and ride accordingly....