QuestionQUESTION: I have a 650 unit construction.What would cause the back of the piston to start to drag on the bore. Just had it rebored, new pistons and rings. The oil pressure is good. Not sure what to look for.Started to mark the piston in line with the join in the oil ring. Thanks Ray.
ANSWER: Hi Raymond,
This is for Triumph cars but I happened to have been a motorcycle mechanic prior to my car experience and I use to race Triumph on flat tracks. Also, at one time I was a Product Information Manager for a major piston and valve manufacture.
On any engine, when the "trust" side of a piston is showing excess ware. You need to examine the ware patterns closely to see if anything provides an indication of the cause. Even though there are many possible causes the piston retains the most clues as to what was wrong more than any other part of an engine.
I will give you several things to look at and give you some of the causes when you examine the engine and give me some answers.
1. How many miles were put on the bike after the new pistons?
2. What brake-in procedure was used?
3. What mileage was the first oil change after the rebuild?
4. What kind of oil and what was the temperature in the area?
5. Are both pistons scuffed?
6. Looking at the top of the piston as a clock and straight forward is 12:00 o-clock, is there any scuffing at 5:00 and 7:00 and any signs of light scuffing at 11:00 and 1:00?
7. Any signs that 12:00 was starting to scuff?
8. Examine the underside of the piston crown on the inside of the piston. Is it darkened?
9. Closely examine the cylinder bore at the 3:00 and 9:00 o-clock.
Can you still see the criss-cross honing marks and are they crossing each other at 30 to 45 degrees to each other?
10. Run your fingernail up the 3:00 and 9:00 area of the bore. Can you feel glass smooth or rougher?
Answer the 10 questions and number each answer for me and I will try to give you my opinion as to what I think was the cause.
Let me know,
Howard
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QUESTION: Hi Howard,thanks for the help.
Q.1.350 km
Q.2 Two short runs,8 km out and 8 km back,Powered on every 2 km and backed off but didn't push it.
Q.3 NOT YET.
Q.4 Castrol gtx 20w50.
Q.5 Yes.Very light scuff just below oil ring on front Right.mark about 8mm wide and 20mm down under oilring gap at the back 6 oclock.
Very light scuff below oil ring on front left about 2 down allup.Scuff at back but not bad.Scuff as in just the shine off the piston.
Q.6 Just the shine off below the oil ring 1in.
Q.7 same as above.
Q.8 black under right piston but clean under left.
Q.9 Criss Cross still seems ok.45 deg/?
Q.10 Still feels as hone is OK.
The motor had a bit of a rattle after the two short runs.Retensioned the head after 250 km one push rod must have slipped out of pozzy as I backed the valves off so there was no tension on the head studs, pulled it back down,when I pulled it down to inspect it, one of the push rods is bent.There is 10 thou ring gap, even shine around the rings, it isn't out of round.The only thing I found is the Harris rings are square or as it seemed,but, when I pulled them out, the rings have a shine half way up so I measured them and they are 1.58mm on the outer edge and 1.55mm on the inside edge.The motor was using oil that is the reason for this as you know.The shine on the top ring was on the bottom of the ring when it was on the piston and the shine was on the top of the 2nd ring ,the oil ring has a shine all over.The motor has hot cams,not sure on the size I have to set the timing and adjust it when I am out on the road as you fine tune it to get it working correctly under load ETC.It has a TR6 Head as well.
ANSWER: The black under the head of a piston is an indication of detonation. (abnormal combustion) Detonation is an explosion not a smooth burn necessary for all gas engines. The burn of normal combustion provides only 100+ PSI to run an engine. But detonation provides a hammer strike on the piston at about 1000+ PSI which puts undue strain on the piston, rod and crank in a single shock and does not produce much horse power. Even though combustion temperature is high especially in an air cooled engine the uneven temperature of detonation is many times higher than a normal burn (thus the burning of oil on the bottom of the piston crown) This extremely high temperature overheats the piston and being aluminum expands quite a lot causing the piston to get too tight in the bore and can cause scuffing.
The shine on the top ring being half way up is correct as the lower part of many new rings apply more pressure to the cylinder wall than the top part of the ring face. These are called scraper rings. They hold compression but scrap oil off the wall and keep a reservoir of oil down below that ring. This is especially so on the second ring. Many engines use a scraper ring on the second ring down but rarely ever use on on the top. If the second ring has the shine on the top half of the ring it was acting as an oil pump forcing oil up not down, this will over load the top ring with oil and even though it may have been scraping oil it can not handle the pressure of extra oil coming from the second ring. Look closely at the rings and note if there is a dot or a number close to the gap on any of the rings.
Tell me about the shine on the face of both pistons rings and was there a dot or number on the ring and which way was the mark? Facing up? or facing down?
Oil has a very low octane rating and even when high octane gas is used the oil burning lowers the octane rating of the gas and thus detonation. Plus the fact that oil burns much hotter than gas does.
Very few new parts in an engine need a brake-in except rings and pistons. Rings and pistons need to be broke in. Many bike racers years ago use to run in a new rebuild and tear the engine down and fine sand all shinny spots on pistons and run it again and tear it down again and fine sand the shinny spots again. This would provide a larger evenly distributed load of the piston against the cylinder wall on the power stroke and thus help prevent scuffing in a race.
The new technology of pistons and rings makes that not necessary on todays engines but you get the point. The pistons need time even today to conform to the exact shape of the cylinder when warm. If you measure a piston in the ring land area you will see it is round but measure the skirt and you will see it is egg shaped, not round. When a piston is heated it expands and not evenly so it must have time to ware in to the correct shape and it needs a continuous supply of oil in the correct places.
Let me know about any marks on the rings,
Howard
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QUESTION: Hi Howard, It's me again just home from work.
Right side-Top ring, dot up
Sec ring, dot down
Left side- Top ring, dot up
Sec ring, dot down
There is a half of what should be a dot but just looks like a mark on the ring.That would be why it is going through oil and as you said black under the piston.
I had a look under the pistons I took out and they are both black underneath as well, could that be the reason why it stopped the first time?The rings are still in the pistons with the numbers up as well. 'Top'is written on it also.This puzzles me as to why it happened in the first place.Or could there be something else amiss? Regards Ray.
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AnswerRay, you are not making it clear. You say the dot is down on the second ring but then you say the numbers are up and it has the word top. The piston industry has always had a standard of EITHER putting a dot OR a the number (which is the size) on the face near the end gap and that dot OR number always faces up. Sometimes when there is a dot and the word "TOP" on only one of the rings it means that ring goes in the top groove. But the word "TOP" and the dot will be on the same side which goes up.
If the second ring down had a ware pattern on the upper half of the ring and the dot was down IT FOR SURE WAS PUT ON UPSIDE DOWN and that was the cause of the oil being pumped into the combustion chamber and the detonation that blackened the underside of the piston crown. And damaged the piston skirt.
The hammering effect if allowed to continue could have destroyed the rod bearings, piston pin bushings and piston.
Providing that the cylinder walls do not have any aluminum deposited on them from the scuffed piston and the skirt damage is not extreme, you can usually just sand the scuff marks smooth. Don't try to sand them completely away, just so they feel smooth.
You should try to move the rods up and down to see if the detonation has knocked out the rod bearings. You should not feel any up and down play.
Low RPM detonation can be heard as commonly called "Valve Rattle" (nothing to do with valves) However, higher RPM detonation can not be heard and that does the most damage.
You should also double check the ignition timing as it must be correct. Because you had a bent pushrod, you need to put a fluid in the ports like diesel fuel or such to see if there is any leakage at any valve. The reason is that you get overheating form the detonation and if there was not much clearance between the valve stem and guide, a valve would have stuck thus the bent pushrod and now possibly a bent valve. Hold the head up so you can fill the intake ports with a fluid and look for seepage around the valves. Lightly wet around the valve after setting a while is usually ok but dripping as soon as you pour the fluid in is not acceptable. Check the exhaust ports the same way.
Howard
