Frequently Asked Questions - Pistons - CP-Carrillo

Both CP-CARRILLO and BULLET are high quality forged pistons, made from -T61 Alloy. CP-Carrillo pistons are designed for high performance racing engines and are available for many "race specific" applications. Additionally, CP-Carrillo can be custom made in as little as 3 days to your specification. BULLET Series Pistons are large production runs on centralized manufacturing cells made from dedicated “Bullet” forgings. An application specific forging, as well as manufacturing in larger quantities reduces machining time. We pass the savings on to you, offering a more economically priced part, made on the same machines as a traditional CP!

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In addition, the BULLET Series features a quality 1.2mm, 1.5mm, 3.0mm ring package and includes Chromoloy wrist pins with wirelocks. The 1.2mm carbon steel nitrided top ring is extremely durable and offers reduced friction over 1.5mm and 1/16" top rings. A 1.5mm Napier (hooked face) 2nd ring is used to improve oil control over conventional taper-faced rings.

If any form of model is provided (old piston, mold, cylinder head or 3D CAD model) for manufacturing new pistons, please make sure you take the following steps to ensure a proper sample is received:

If sending a piston sample, make sure it is clean and piston sample needs to be the least damaged out of the set, remove all rings, locks and wristpins.

If you are sending a Mold from cylinder head this mold needs to be made out of a hard material (e.g. Bondo ®) and should include “X” and “Y” centerlines (representing the bore CL) and backside of mold needs to be completely flat.

If sending a Cylinder head include gasket, valves, spark plug, dowel pins (if possible) and please remove valve springs. CAD 3D models are also an excellent source of information and often times more convenient, talk to your CP-Carrillo salesman for the appropriate file type. For any other types of models/samples please talk to your CP-Carrillo salesman for proper instructions before sending samples.

See figure 1. Assuming your top and second rings are of different thicknesses you can simply measureyour rings and compare them to the groove dimensions. (SEE DRAWING) In instances where the top and second rings are of the same thickness the top ring will generally be of a shiny silver/copper color whereas the second ring will be of a dark grey or black coloring. Both rings will normally be marked with a dot(.), number, or letter that will aid in determining which side of the ring will face up toward the dome of the piston.

If the rings don’t have a dot or letter to mark “up”, check the ID of the ring and they should be beveled. The inner beveled edge of the top ring will face up towards the cylinder head, and the inner beveled edge of the second ring will face down towards the crank.

If you are looking for more details, kindly visit Piston Wear Ring.

Additionally, some second rings will have a tapered hook groove on the OD. This hooked edge will face downward as its function is to scrape oil back down into the crank case. See Figure 4

In the rare instance that you receive a ring with no designation, though it could mean the ring manufacturer has determined that it will work effectively either way, we recommend that you contact us for clarification.

All of our oil rings for 4-stroke applications utilize a 3-piece design which consists of two thin oil rings separated by an expander (wavy) ring.

PLEASE NOTE: DO NOT ALTER YOUR EXPANDER. Oil ring gap specifications are for the top and bottom rails only! Oil ring expanders are designed to butt against each other and when installed along with the rails allow the ring to have the correct tension when seated in the cylinder.

You may also have a piston that requires the use of a rail support, that rail will have a dimple to prevent rotation.

Yes all of our parts all are forged. All CP-CARRILLO and BULLET Series pistons are forged for maximum strength and durability. We predominantly use -T61 and some .

is a high silica (glass sand) content material that has good scuff resistance and allows tighter piston to wall clearances, however this is at the expense of strength. The greater the silica content, the more brittle the material, so does not lend itself well to high cyl. pressure applications such as those found in Super charged, Turbo charged, or Nitrous Oxide applications, with relatively light piston weights. has been used in some low boost applications in OEM and in some high boost diesel applications, but the pistons are much thicker and heavier than there counter parts, in an effort to keep them from failing. is also more expensive to machine, because it wears out tooling quicker. -T61 on the other hand is a low silica content material, so it is more ductile/forgiving, so it is the material predominately used in high cylinder pressure applications. However, does require more cylinder to wall clearance because of the lack of silica and it reduces anti-scuffing qualities.

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Had a persistent oil leak from the head gasket, and rather high oil consumption, so I dismantled the top end of my Commando and noticed signs of piston blow by as you can see in the pics,there is some carbon build up below the scraper rings. I measured the bores, and noticed minimal wear in all directions (less than 0.015mm) although the cilinder walls do show glazing.
My question is : what causes this . Bent conrods seem unlikely since the bottom end has never been apart. The pistons are sill STD size. bike has run approx 90.000km from new., and oil changes and filter replacements were always done after 2-km.
Both pistons show this pattern, could this be caused by crankshaft flex ?
Opinions please.


Measurements were done with a dial type bore gauge and inside micrometer, The RH piston shows the same pattern, although hidden from view on the pics. It could be that the ring lands have worn out as Jim suggests, could it also be that the cilinder bores are out of parallel ?, although this is not easy to check. (inaccuracy in the manufacturing process ? )
Small chance that the rods are bent, although I will do the check with the straight edge.
Thanks for the suggestions so far.
btw, I have good quality measuring equipment,(dial bore gauge and inside micrometers) and do know how to use it. Peter R, I think your assessment is pretty much on the mark. Out of alignment bores is really an obscure one but thought it worth mentioning. Get a feeler gauge and check clearance between the rings and ring lands. Take care not to nacker up the soft aluminum ring lands while doing this. From recollection acceptable clearance is 0." to 0.002". I pulled some race pistons once with 0.004" after many races and seem to recall my homework telling me that 0.004" is too much and my recollection is they started at 0.002". Get with a manufacture or maybe the manual to confirm acceptable clearances.

As for bores, if you do not have access to quality measuring equipment, strip the the pistons and use a feeler gauge between the bore and piston skirt throughout the bore to check for clearances.

You may be able to install new rings, glaze bust or light hone the barrels and go. If the pistons fit are too loose, look to see if new standard pistons make the grade with proper clearance.

Yes, I would also thoroughly check and service the head.

As stated above, unless you are set up for all of this, it is best to get the barrels and pistons to a reputable and preferably Norton experienced shop to sort things out. The pistons and barrel owe you nothing at this point. You had a decent run with them. That is great life from your engine Peter. I am totally impressed with the even wear on the cylinders as you stated:
"I measured the bores, and noticed minimal wear in all directions (less than 0.015mm) although the cilinder walls do show glazing".

I am surprised at the 1/2 thou wear, I believe that is the typical tolerance for a freshly honed cylinder;
0.015mm = 0.". We know that 1mm = .", or as we are more familiar; 1 mil is 40 thou.
Have you compared your cylinder micrometer measurement to the standard bore size? You didn't mention the piston to cylinder clearance; how much is that? I am also wondering if you did a compression test or a leak down test before pulling the head - if so, what was the result? Good luck with your rebuild, and share your progress.

Some more observations: I did the check as suggested by Jim C, cleaned the tops of the pistons, and checked with an accurate straigh edge, this check showed that the conrods are absolutely straight.
I also measured the amount of play of the piston rings in their grooves, I found 7-8 thou of play (or 0.2mm for the metric world). this seems out of spec to me.

I think you found the problem. Not unusual for the mileage. Jim Mr splatt, I have owned this bike since , and although old age and alcohol may have caused some kind of lapse in my memory, (LOL) I do know for certain that the barrel has never been off before. The cylider head has been off a couple of times due to a stripped thread of the exhaust port (around ). And once again due to a leaking head gasket, approx. 8years ago.
The barrels have never been re-sleeved. Thermal distortion between the bores sounds plausible, although this did not concur with my measurements. Time to shop for new pistons. As stated in my earlier post, I got rid of my pistons with around 0.004" ring land clearance. When you think about it, the scraper and compression rings act like an integral pump/check valve. With minor factory recommended ring land clearances, just enough lubrication passes, too much clearance then too much migrates up past the rings.

Hopefully the barrels allow you to drop in new standard OEM pistons with a glaze bust or light hone; be mindful of any ridge at the ends of the ring travel in the barrel. A good shop can sort this out for you and make recommendations on overbore or not. Well there you go , that's what an engine looks like that hasn't been molested by 25 different owners. As has been recommended, new piston and rings and carry on, and hopfully it will be another 50k before you ever see those marks again
Unfortunately new pistons will probably be on minimum size so clearance may not be perfect but it seems a shame to wear it out quicker by reboring