overbore in order - Which pistons?

[Shadow24v]

Hotdog, I understand that custom pistons aren't the easiest way to go, nor the cheapest. However, the cost for OEM replacements is $75-100 per piston, and only 50% chance of including rings. Mikes is approx $91 per piston including rings. If the custom pistons are within $50-$60 or so of OEM/Mikes, to me it would make sense to fork over the extra cost of about ONE piston to have newer alloys, design, better wear and longevity etc than settling for 40 year old cast piston tech.

Hotdog, what drove you to choose custom pistons? was it a displacement increase or some other factor (parts avail. etc.)?

 

[hotdog]

I hope I have not misled you..? the custom pistons I have ordered are approx $300USD a set & are for use in a 750cc long rod engine - so not standard fitment + other significant other mods are required to install them.

If you are giving it the occasional spirited ride & not flogging it 24/7, the Mikes pistons will be fine. As I said, I'm running a set & am happy with the product.

Ask yourself - what riding do you intend on doing..? Don't get caught up in the whole "I need forged pistons BS". Understand once you start to go away from something other than OEM or a direct aftermarket replacement it IS going to cost you. How far you go & how much is your call....

 

[Shadow24v]

Hotdog, thanks for the input and concern, but you haven't misled me. If the cost point is too high, I won't be buying custom pistons. I am waiting to see what the various companies can offer and at what price before making the call. I am not looking for custom pistons just to say I have them, but for actual gains in reliability, wear characteristics, and lastly, if there is any performance gains from something like a CR bump, that is an added bonus.

The driving factor behind the inquiries is a "once burnt, twice shy" outlook. The burnt piston I suffered got me thinking, there have been a lot of advancements in metallurgical alloys in the 40+ years since the original cast pistons were made. As this bike is also 34 years old, I am looking to hedge my chances against a similar failure (why I have the motor apart in the first place) and to either prevent another like failure, or reduce the impact of a failure. If the extra cost of newer alloy pistons saves me from having to tear the motor apart again, that alone can outweigh the cost difference. Add onto it any improvements in wear characteristics both for the piston itself and the bores, it would also help increase the longevity of this rebuild, thus saving cost by less frequent teardowns.

Additionally, while people do have success with mikes pistons, I haven't been able to find any information on their alloy or construction method. In appearance, they seem to be a reproduction casting of the OEM pistons with probably a similar alloy as well. No where near the advances available from current piston companies.

 

[grizld1]

Shadow, there are a few things you may want to think about. First off, it's not the case that cast pistons burn because they're weak or metallurgically flawed. They burn because of combustion issues--timing, A/F mixture, or both. A forged piston will do the same; there's no bulletproof magic alloy out there.

Forged pistons are stronger than cast, but that's only going to signify in very high-stress conditions. On the downside, forged pistons are significantly heavier and require more clearance because they expand more with heat than do cast pistons.

I'm not bashing forged pistons. I have two sets of them, high compression Venolias and lower compression JE's, and they've served me well. But if 77.5 mm. cast pistons had been available, I'd have opted for them; they'll stand up to as much punishment as aggressive street use will inflict.

 

[Jack]

The area around the rings is not coated. Just the skirts and the dome.

So, you're saying all coatings are a waste of time and money, but you're having yours coated by Swain, is that it?

Swain looks like the real deal, no question, and $40 per piston for their recommended skirt and top coatings seems reasonable. Just want to clarify your assertion that coatings is a waste, except for Swain.

The dry film product by "Tech line" for the skirts is a"waste of time and money" been there and done that! as you'll soon discover. Swain developes their own specialty products that are far superior than Techline and are used by countless racing teams around the world. I've used tech line coatings in the past and the ceramic thermal top coating when applied with the correct thickness,showed good results and they have also have two versions of the ceramic. This time around I'll be using Swain to get my money's worth

 

[Shadow24v]

Shadow, there are a few things you may want to think about. First off, it's not the case that cast pistons burn because they're weak or metallurgically flawed. They burn because of combustion issues--timing, A/F mixture, or both. A forged piston will do the same; there's no bulletproof magic alloy out there.

Forged pistons are stronger than cast, but that's only going to signify in very high-stress conditions. On the downside, forged pistons are significantly heavier and require more clearance because they expand more with heat than do cast pistons.

I'm not bashing forged pistons. I have two sets of them, high compression Venolias and lower compression JE's, and they've served me well. But if 77.5 mm. cast pistons had been available, I'd have opted for them; they'll stand up to as much punishment as aggressive street use will inflict.

Grizld1 - indeed, even forged pistons will succumb to such things as excessive advance or lean mixtures and still "burn" in those conditions, but possibly not as quickly as the OEM cast alloys. I realize that there isn't an "indestructible" piston alloy, but there probably a "better" piston than OEM that can be had.

I did read up on the common 2618 alloy used my the majority of the piston manufacturers, and yes, the different alloy does have a higher expansion rate than most OEM pistons, necessitating an extra thousandth or two clearance when cold. It also tends to have better wear characteristics (skirt wear being one) than the OEMs due to the type of alloy used. Part of the reason I am investigating pistons is not just a burnt piston, but the fact that both pistons and bores were worn out of spec.

The real question is, do the pros and cons of forged pistons have sufficient benefits over OEM cast, especially when looking at the different characteristics (wear, strength, etc) and cost. I am also trying to avoid the hog-wild overbores and re-sleeving needed by the 750cc kits that appear to be the usual choice over a stock re-fresh.

Weight-wise, are cast SIGNIFICANTLY lighter than forged, or are we talking something like a <10% increase in mass? While stock pistons may be "adequate" is the quality difference of forged pistons a significant enough factor to help offset the 50%+ cost difference?

 

[GreasyC]

As hotdog said, it's not really a question of "can it be better", it can always be better. It's "what do I need?" that's important. You can build with oem pistons and go 50,000 miles between overhauls if the rest of the build is good. If your engine crapped out before that it's not because the pistons weren't expensive enough. What you won't get with stock pistons is more than 45 horsepower.

You started out saying you were on a budget and interested in reliability, but it sounds like you're interested in increasing performance too. I don't blame you, we all want it all, but reliability and performance are almost always a "choose one" situation. You mentioned increasing compression, that's definitely harder on the engine. These performance pistons are heavier than the already overweight stock ones. That's more stress and wear on the pins, small ends, bearings, and crank, so again a decrease in reliability there too.

A 250cc motocross bike has some of the most high-tech, up to date, performance pistons you can find. They allow a 250cc single cylinder to make almost as much power as our 650 twins (about 35 RWHP). They last for about 100 hours. The serious racers replace them after only 25 hours of racing.

 

[Jack]

Grizld1 - indeed, even forged pistons will succumb to such things as excessive advance or lean mixtures and still "burn" in those conditions, but possibly not as quickly as the OEM cast alloys. I realize that there isn't an "indestructible" piston alloy, but there probably a "better" piston than OEM that can be had.

I did read up on the common 2618 alloy used my the majority of the piston manufacturers, and yes, the different alloy does have a higher expansion rate than most OEM pistons, necessitating an extra thousandth or two clearance when cold. It also tends to have better wear characteristics (skirt wear being one) than the OEMs due to the type of alloy used. Part of the reason I am investigating pistons is not just a burnt piston, but the fact that both pistons and bores were worn out of spec.

The real question is, do the pros and cons of forged pistons have sufficient benefits over OEM cast, especially when looking at the different characteristics (wear, strength, etc) and cost. I am also trying to avoid the hog-wild overbores and re-sleeving needed by the 750cc kits that appear to be the usual choice over a stock re-fresh.

Weight-wise, are cast SIGNIFICANTLY lighter than forged, or are we talking something like a <10% increase in mass? While stock pistons may be "adequate" is the quality difference of forged pistons a significant enough factor to help offset the 50%+ cost difference?

On cast vs forged pistons weight, a forged piston of 2618 due to it's higher density can be produced lighter than a cast pistons cuz of the advancement in design to relieve those higher stress points thus allowing the minimal structural thickness to lessen reciprocating masses. How much of a % reduction will depend on how far you or the manufacture push the envelop. The 2618 forging VS a Wisceo or cast piston won't have the tight ring seal properties due to the require loser wall clearances and over time will shorten the bore life expectancy. If it were me,the JE's would be the preferable choice and can be further lightened between the pin bosses from the underside,you'll want a crown thickness of .200 or +.

 

[Jack]

I hope I have not misled you..? the custom pistons I have ordered are approx $300USD a set & are for use in a 750cc long rod engine - so not standard fitment + other significant other mods are required to install them.

If you are giving it the occasional spirited ride & not flogging it 24/7, the Mikes pistons will be fine. As I said, I'm running a set & am happy with the product.

Ask yourself - what riding do you intend on doing..? Don't get caught up in the whole "I need forged pistons BS". Understand once you start to go away from something other than OEM or a direct aftermarket replacement it IS going to cost you. How far you go & how much is your call....

How did you manage paying only $300 for a set of custom L/R pistons and some photos would be nice

 

[grizld1]

Shadow, if it's longevity you're after, send your cylinders with whatever pistons you choose to Bill Moeller at Bore Tech in Ohio (Google for the URL) and have him do a bore job and treat the cylinders with silicon carbide. The silicon carbide impregnates the pores in the cast iron, providing faster and better break in and reducing friction. The result is cooler operation and extended life for piston, bore, and rings. I'll never do another engine without it.

What might be possible in a no-expense-spared custom run is one thing, but I can tell you that all off-the-shelf forged pistons for the XS650 are significantly heavier than OE and OE replacement cast pistons.

 

[Shadow24v]

Shadow, if it's longevity you're after, send your cylinders with whatever pistons you choose to Bill Moeller at Bore Tech in Ohio (Google for the URL) and have him do a bore job and treat the cylinders with silicon carbide. The silicon carbide impregnates the pores in the cast iron, providing faster and better break and reducing friction. The result is cooler operation and extended life for piston, bore, and rings. I'll never do another engine without it.

What might be possible in a no-expense-spared custom run is one thing, but I can tell you that all off-the-shelf forged pistons for the XS650 are significantly heavier than OE and OE replacement cast pistons.

Haven't heard of the silicon carbide treatment before. I'll have to investigate...

Regarding forged weight vs stock cast...Have the current forged pistons been evaluated for reduced material where not needed? or are they mimics of originals just in different materials and sizes? If you could adjust the design a bit below the rings, accounting for the stronger alloy, I'm sure the forged pistons can be a lot closer, if not lighter than OEM.

 

[Jack]

Shadow, if it's longevity you're after, send your cylinders with whatever pistons you choose to Bill Moeller at Bore Tech in Ohio (Google for the URL) and have him do a bore job and treat the cylinders with silicon carbide. The silicon carbide impregnates the pores in the cast iron, providing faster and better break and reducing friction. The result is cooler operation and extended life for piston, bore, and rings. I'll never do another engine without it.

What might be possible in a no-expense-spared custom run is one thing, but I can tell you that all off-the-shelf forged pistons for the XS650 are significantly heavier than OE and OE replacement cast pistons.

JE doesn't go to the trouble of lighting the pistons to match or come close to stock weight and could be easily lightened by hollowing out the dome,pin boss area or you could purchase GSXR 1100-Falicon H-11 high strength tooling steel for wrist pins in straight wall .100 thick and proceed to cutting them to XS stock pin length dimensions for even further reduction over stock pin weight or the pins supplied with JE's. Lighter pistons equals less stress overall as you know and there are means to lighten those JE's.

 

[grizld1]

Right, Jack, it's the weight of the whole assembly that signifies. Both Venolia and JE supply wrist pins that are quite a bit lighter than OE, but those modded Falicon pins should be quite an improvement--thanks for the tip.

How far a guy can go in lightening pistons is going to depend a lot on his skill level. I dug up some of my old weight comparisons. The figures are for complete piston assemblies--piston, rings, and wrist pin. Stock piston (75 mm.): 377 grams. Shell JE (unmodified, 77.5 mm., domed for 8.6:1 CR at standard deck height): 397 grams. Venolia (77.5 mm., the dome cut for 10.2:1 CR at standard deck height, pistons lightened and beam balanced): 405 grams.

 

[Shadow24v]

so while there is a weight gain in unmodified JE and Venolias, we are talking about less than 10% (5.3% more for JE and 7.4% for Venolia). I can definitely see if a piston manufacturer actually went to do lightening, that they could match or surpass the stock weight.

Additionally, If sticking around the 75.xx diameter, the weight would be even closer to OEM

 

[grizld1]

Weights given above are for complete assemblies, and the OE wrist pins are quite a bit heavier than the Wiseco pins that I weighed in the Venolia assembly and the pins used in the JE assembly (yeah, I know--shoulda taken the weights of the parts separately, kicking myself for the oversight).

 

[Shadow24v]

even so, we are still looking at a <10% weight difference, pre-lightening on forged pistons.

 

[Marvelicious]

Has anyone actually tried those super cheapies out of Japan? Even if you tossed the rings, you'd still be money ahead, but my BS detector says they're still jet-lagged from China.

Jack: that silicon carbide treatment you're talking about, I'm familiar with Nikasil which is a silicon carbide plating (a bit out of my price range) but I get the impression maybe you're talking about something else. What is it and is it less painful to pay for?

 

[grizld1]

You called me Jack but that's a compliment, so I'll answer to it. Nikasil is, as you note, a plating process. The Bore Tech process impregnates the pores in the liner with silicon carbide without building up the surface. The treatment is around $75 per hole if I remember right, but it's been a few years.

 

[Jack]

You called me Jack but that's a compliment, so I'll answer to it. Nikasil is, as you note, a plating process. The Bore Tech process impregnates the pores in the liner with silicon carbide without building up the surface. The treatment is around $75 per hole if I remember right, but it's been a few years.

..... Griz,on stock piston weight,PJ weigh one at 301.1 grams without pins or rings and complete came out to about 390+ grams. Don't know how accurate his scales were at the time.

 

[racerdave]

What is the overbore size for JE 700cc pistons? How many passes does it take to bore to their required clearance? Does the actual cc size come out to 700cc or is it +-?