Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board New pistons question

Something I noticed when I was checking the 0.060 JCC pistons I put into my '66 T120R is that there is no taper to the skirt. Clearance specs show a nominal 0.009 at the top of the skirt and 0.005 at the bottom. Yet my pistons measure the same top and bottom. Further evidence were burnish marks on the upper skirts beneath the valve pockets.

My old dealer buddy Lance is telling me he has not seen tapered pistons since the early 60's. Yet they still make them that way for cars.

So, is this normal now? Or should these JCC pistons be tapered at the skirt?

Thanks,
Rob

Seem to remember something being mentioned about the new pistons being reduced around the ring land area rather than being tapered?

Rod

Yes, they are a copy of the last iteration of Hepolite 650 pistons.

My pistons are relieved in that area. I'm talking about the top of the skirt, which is below the ring area. The manual talks about clearances at both the top and bottom of the skirt. The reason I'm asking is due to the burnishing I found on my pistons in the upper skirt area. My pistons measure the same, top and bottom.

Here's a pic.



So, are you saying it is normal for the top and bottom of the skirt to measure the same?

I understand that the last Hepolites were made in Taiwan. Could that have been behind the change since I believe that JCC are also made in Taiwan.

regards,
Rob

OK the last Hepolites were not made in Taiwan!!!!!!!!!!!!!!!!!!!!!!!!!!!! Most were made by Federal Mogul in Italy. Hepolite (the real company not Wassell) never made pistons in Tawian.

The reason for the piston taper from top to bottom had more to do with the available production machinery "in the day" than than where the needed extra clearance it provided could be produced economically. If you put the piston on a Shadow Graph you would see that the area at the top of the skirt (just under the oil ring) is a slight radius.

Because the width of this slight radius is narrower than the width of faces of a micrometer, and so slight, it is hard to measure. With modern CNC machinery, with only a few lines of code, one can place the taper exactly where it is needed. Where in the past the only way to economically get that clearance under the oil ring was to cut a taper across the face of the piston. While the taper was never needed for the length of the face of the skirt the technology did not exist to economically mass produce the pistons any other way.

The real question is why is this feature needed in the first place and it has to do with heat and how it moves through the piston. Heat flows where there is material for it to flow through. The piston pin bosses, with their greater mass, get most of the heat that flows out of the top of the piston. This is why four corner seizures are common. But there is a thick section of the piston just behind, and below, the oil ring. There is just enough of it to cause some mischief - seizure.

Where there is heat, and aluminum to expand from being exposed to it, you need more clearance. This is why you need a bit more clearance just below the oil ring. The area behind and below the oil ring is quite thick as compared to the skirt. There is enough to require some extra clearance in this area. The taper in the old days, and now the short radius serves this purpose.

The thin piston skirt acts like a resistor which blocks heat from flowing down it. Certainly some does and we need room for the piston to expand, but not as much where there is a lot of aluminum for heat to flow. Also the piston skirt is a bit flexible and can endure some abuse.

Now, your burnishing, well below the are we are talking about, is a different story. To make an intelligent comment one has to check the piston pin for signs of excessive heat (you can also look at the top and inside, of the piston dome for signs of detonation and excessive heat (sand blasted look on the top and/or carbon burn on to the surface under the dome).

Under normal conditions the pin will remain the same color as it was installed. If the pin is the original color this would indicate that combustion temperatures were normal and your polishing (which is really a seizure it just hasn't fully developed) was because of lack of clearance. The specification for these pistons in the range of .0043 to .007". In this case the piston was not over heated and normal expansion caused the piston to fill the bore.

If the pin shows any signs of discoloring - light straw to dark purple sometimes black, this is a sign that there was abnormal combustion and the excessive heat abnormally expanded the piston to fill the bore.

Yes, JCC pistons are made in Taiwan and the Hepolite's were used as a pattern.

Thanks for a very comprehensive answer, John.

Looking the piston damage book that TT posted, I tend to think that perhaps the burnishing is related to an overly rich mixture. There is no "wear" to the piston. It is simply polished, albeit gray. All that is missing are the tool marks from the original machining of the piston.

The point about using CNC machinery to make the pistons is a valid one. I will take a much closer look at the pistons when I pick up my jugs and pistons tomorrow. A bright light and a straight edge can be a poor man's comparator. I'll make that check when I have the pistons back in my hot clammy little hands tomorrow.

regards,
Rob

Bear in mind that they are successfully making pistons.

I understand that, TT. But I also know that quality control in Asia is less than what we are accustomed to in the US. So, I could have gotten a bad set of pistons.

I talked to the shop yesterday and one jug cleaned up with a 0.0003 honing. So, hopefully so will the other. So if I am going to use those same pistons, I want to make sure they are right.

regards,
Rob

Eeeeehm, JCC quality control looks not so bad to me.
Taiwan is not mainland China, and their's improved a lot during last years.
Don't you have an oil filter in this engine?

I can't understand why anyone would want to fit cannon ball weighted pistons to their rods. The new hepolites have a good name and other quality brands can be saught.

What do you mean by that Allan ( cannon ball weighted ) ?
Aren't you mixing Taiwanese JCC with Australian JP pistons ?
These could be really heavy .

Well certainly a possibility I think it is a stretch. Damage caused by oil dilution is not usually confined to just the area in question. The damage is rarely, if ever, as subtle as your polished spot. Piston damage from oil dilution is usually much more aggressive and the piston and rings are in a much more damaged state. The polished spot is used more to describe problems with clearance.

What you are looking at in the polished area is aluminum that is in an early stage of melting (seizure). The aluminum has gone into a plastic like state. In the process the machining marks are flattened. While clearance problems are typically seen on both faces of the piston, in the early stages the symptom can be confined to the thrust face. This because this face has the most pressure against the cylinder wall and will heat up sooner. Often this is the extent of the damage and it can be "cleaned" with some 400 grit wet-dri paper.

Diagnosis of piston problems vary greatly depending upon a lot of factors. What is called a four corner seizure where the piston seizes just fore and aft of the wrist pin in a car is typically caused by the wrist pin fit in the piston. In most cars the wrist pin is a press fit in the rod, and must be able to rotate freely in the piston. If the fit in the piston is too tight the friction heats the area causing the piston to expand more than the available clearance. Most motorcycle piston wrist pins are fully floating in the rod and a four corner seizure is indicative of abnormal heat introduced into the pistons by abnormal combustion. A car guy looking at a motorcycle piston will often misdiagnose the problem. Before diagnosing a piston problem one must look at the application as different kinds of engines suffer different types of piston problems with common symptoms. Then there are ones that are typical to a brand of piston or model of motorcycle. This has been the case with Triumph 650 engines since the introduction of Hepolite pistons to replace the ones made in House by Triumph.

My opinion that it is more likely a clearance/heat problem problem is based upon being a Hepolite distributor for more years than I can remember. The problem starts with the pistons made by Triumph were made from a lo-ex (low expansion) aluminum and used .0035" to .004" clearance. The Hepolite design used a different aluminum requiring .0043 to .007" clearance. There is a lot of technical information that lists the piston clearance as .0035" to .004". I still find dealers who try to fit the 650 piston to .0035" clearance and wonder why they get that polish spot. It doesn't take a lot of detonation to get the piston bigger than the bore at that clearance.

For oil dilution due to a rich mixture I would look for other clues such as exhaust valve stem galling, excess carbon collected on the top of the piston and plug fouling.

There have not been any "new" Hepolites in the market place for nearly 10 years. The current pistons sold under the Hepolite brand name are JCC.

Snakeoil - the computer you wrote those words on, and nearly everything else you have come to rely on, is made somewhere in Asia. Most of the rest of the stuff was made in Asia and assembled in Canada or Mexico. The JCC piston manufacturing tolerances are far superior to anything Hepolite ever sold!!!!!!

We find a few tenths variation between runs of JCC pistons where it was often thousandths with Hepolite. The pistons varied so much that Triumph had to grade their cylinders to match the pistons.

The Taiwanese pistons I've used are lighter than old Hepolite ones.

Well spotted, yes I did mean JP.

John, I'm sure you will understand that I was reffering to them as New Hepolites as they come branded as hepolite and they are of new manufacture ( not NOS)

Have additional info to share. First, my cylinders cleaned up nicely with a 3/10's hone using 180 grit stones. The porosity in the left jug almost went away completely.

Based on using bore gages at the machine shop, I had 8 mils of clearance on both jugs. I would think that would preclude a tight fit causing seizure. So, detonation would be the next suspect. No indication of any kind on the piston tops. Plus, I know that the mixture was rich based on black smoke out the exhaust.

With the new honing, I'll have 8.3 mils of clearance on each piston. Yeah, it will be a tad noisy, but sure won't hurt anything. I have to take responsibility for the 8 mils of initial clearance because I did not give him the specs, assuming he had them. He used auto racing clearances, which he said are typically 8 mils in a water cooled engine.

Lastly, armed with a straight edge, there is no radius that I can see on the top skirt of the pistons. But, this time using a mic instead of a dial caliper, I did find there to be about 2 mils of taper, top to bottom. Not what I would expect from the OEM specs, but to John's point about material changes and better accuracy out of Taiwan, I am not informed enough to make an assessment of the adequacy of that taper on that set of pistons.

I will polish those spots, John.

I'll be going back together with Hastings rings. Will also have Cometic make a custom thickness cylinder base gasket to make up for my milled head. I assembled the engine today with stock gaskets and clay in all 4 valve pockets. Both intake vavles are way too close. One actually sheared the clay and the other had maybe 10 mils of clearance. Exhausts are okay, but not great. I know understand why the head had the valves sunk so deep into the seats. It was to provide sufficient clearance.

regards,
Rob

There are thicker head gaskets already available which work with milled heads.

Another example of over thinking these old turds. Just stick a couple of extra base gaskets on. There is almost no pressure there.

One could only wish to have Healy's knowledge and tech knowhow.

In no way distracting from John's comments (my brain has gone to custard anyway) I seriously identify with Desco's comments about "over thinking these old turds"

Damnation and hell to the guys who took away our leaded high octane fuel, causing Dumbo's like me to pay at least some attention to John's technical comments. RR

I'm not a big fan of stacking gaskets. I clayed the engine and have zero clearance at the intake valve relief pockets with standard base and head gaskets. So, I'm thinking I need to raise the jugs 0.060. So that would be 4 base gaskets stacked together. I suppose that would work, but not my preferred solution.

I asked Lance to find a thicker base gasket and he came up empty. If Coventry has them, then I can only assume he forgot to ask. I can try my local shop back home and see what he has. John, if you have them, please tell me what thicknesses you offer and I'll have Lance order them. If you would be comfortable with 4 base gaskets stacked up, just say so and I'll take that as the voice of experience and use that fix.

With all due respect to those here, it is easy to sit at a computer screen and say folks overthink things if you have a better experience base than they do. I've often thought that about some posts I've read. But I would suggest they walk a mile in the other person's shoes before they start to preach. I've had a head that had been butchered by someone else and it was not cheap to repair. That was tear it apart again No. 1. Then the rings failed to seat on one side and partially on the other. Take it apart again No. 2. Add to that new carbs direct from AMAL that appear to be over-jetted for the application and no apparent insight from others here,on a good starting point for the rejetting process. Oh, yeah, I forgot to mention porosity that showed up when we initially bored the cylinders to remove rust rings from the machine sitting so long.

Although I may dig harder than some into why things are what they are, beat a point harder than others, look for experience that I don't have, to use as guidance as I work thru issues and always strive to come up with the best solution and not just a band-aid fix, I do it so that when I'm done, I don't have to tear into it again. This bike has had more issues than any other bike I've ever worked on. I refuse to let it beat me and actually have no concern that it will.

And how would you like to have dealt with all of this, sorted it all out and had the bike running like a champ, only to have it pissing oil out the base gasket?

regards,
Rob

Try, try again. It took more than 5 years and over $10,000 to get the 72 sorted. Every time I called my parts guy, the late Keith Moore, with a new problem he would say ,"I never heard of that before", and he had heard it all. Bill Getty would just shake his head at each new abomination I discovered. Bill finally solved my longest running problem. The left cylinder had been bored at a slant, like a Moto-Guzzi.
The 68 only took 3 years and about four grand, but look at all I learned on the 72.
Both PO's swore on a stack of bibles that every nut, bolt and washer had been replaced.
The three things you need to own an old Triumph;
Patience
Money
Mechanical aptitude.
If you are lacking in any one area, you better have gobs of the others to make up for it.
I no longer have the money.

The answer to that is in your own hands, pay for some main jets and tune the bike yourself, you know it 8 strokes so you just go down the way in 20 main jet jumps. Todsys fuel is not the same as 40 odd years ago so you will have to tune anyway and doing it is good for experience.

On the base gaskets again why go there, the head gasket is available off the shelf in a thickness that suits the milled heads. My 65 head is milled and I will fit one of these. Single piece and made from copper.

You are making mountains out of molehills and blaming others.

Okay.

Thanks for your candor.

regards,
Rob

Rob I try very hard to not use this forum to promote my business and what I offer. Check your PM. - OK tried to send PM but you are over your limit. Clear out the file and I will send you list of what we offer for your problem.

Desco Why do you think no one offers .010" oversize pistons for Triumphs. It is the norm that the factory bored the cylinder at a "slant." If you are lucky the crankcase mouth is slanted the other way and the piston goes up and down a bore that is right angles to the crankshaft. I mentioned years ago what one could expect when "blueprinting" one of these engines and forum member with a well equipped machine shop thought I was exaggerating until he took measurements of his own. His next post said that I understated the problems.

Of course you check that your rods are straight before you install them. You should.

"I never heard that before" is common in the British vintage parts suppliers vocabulary and is used in the same vane as we use "have a nice day."

PMs cleaned out, John. Rarely use them so never thought to check the limit.

But I think we are in good shape. Stopped at M&S on the way home and talked to Steve. He called Tom and found out you have an 0.080 head gasket and a 0.040 base shim. So I asked him to get both.

When I clayed the engine, I had an 0.021 base gasket and a 0.045 head gasket. Bolts were snugged up but not torqued. I had zero clearance on the left intake valve and maybe 0.010 on the right.

So, here is where I need some guidance from an experienced builder. The 0.080 head gasket will give me 0.035" of clearance in a vertical direction to the piston relief pockets. Steve has a steel sandwich base gasket that measures 0.030. So if it crushes 3 mils, that's 6 more mils of clearance for a total of 0.041". Is that enough for an otherwise stock engine? Or do I need to insert the base shim, and two stock gaskets, which should raise the clearance to about. 0.098", maybe a couple of mils less with gasket crush? Gut tells me 0.098 is a lot, maybe too much.

I understand and can appreciate you not wanting to use the forum to foster business. I only asked because I figured you'd tell me what you had and I could take it from there with Lance.

Thanks,
Rob