About Proper Break-In Procedure
We have all met him …. The racer who boldly says “I break em in the same way I race em … just ride em full throttle”. Very sadly, there are a small few sets of circumstances were this bold guy is right.
Short Term Setups for short Term Break-ins - There are a number of engine builders who (for their own reasons) will set up the top end clearances of a high-performance two-stroke (piston & ring end-gap) considerably looser than factory spec. The most common reason for doing this is that the engine is prepared in a way that will cause very rapid wear to the piston and rings. Engines like this, often run very wide exhaust ports with very little radius for the rings. Such an engine would seriously wear the piston and rings before any break-in period could be completed, and so they are clearanced to go into immediate service with no real break-in at all.
In such engines, the loose piston clearances not only accelerate piston collapse and ring wear, but they also induce very heavy wear on the cylinder walls (especially around the exhaust port). While everything about this setup sounds short sighted and perishable (it’s both) there is an abundance of builders setting up engines in this way. At Klemm Vintage, we do not (and will not) build such an engine platform. Our engine sets are built for good long term wear and performance … and as such our setups “do” require break-in….. here is why.
A Bit of History - In the early years of high performance two-strokes, most piston rings were a raw cast iron material that very literally had to “wear-in” to perfectly seal on the new cylinder bore it was fit to. With such rings, it often happened that “full ring sealing” didn’t take place until the ring had been in service for a good number of hours. Getting any racer to run the engine easy for that stretch of time just wasn’t going to happen. To help ease this problem, manufacturers applied a soft Teflon “skin” on the outside of the ring sealing surfaces. This soft Teflon skin “sealed” to the new bore in a much shorter amount of time, making for faster break-in and true “sealed” performance in a shorter amount of time. The down side was that when the Teflon wore completely away, the ring end-gap became excessive for “ideal” performance, and the ring tension against the bore was also not ideal. This was a particular performance problem on high-rpm small-bore racing setups.
Wise engine builders began setting up these top ends with “much closer than spec” piston clearances in an effort to get proper ring-end gaps when the Teflon skin wore completely off the rings…. And it worked great as long as you had an oil with a very high film strength. What didn’t work out great was that the break-in time required for such setups was much longer … and no pro-racer could be trusted to “go easy” on the engine for that long.
The solution was to simply let the engine run 60-90 minutes on a stand at a high idle, with a fan or breeze blowing across the fins. This initial run-in wears away the majority of the “skin” off the piston rings. In addition, it gives the cylinder bore and piston-skirts time to “get familiar” with each other in a “low load” atmosphere. This is a procedure that we still use (and strongly recommend) today. It bears noting that after this “static” break-in running, there is still some “loaded” break-in operation needed. However the risk of a piston-scoring event during that break-in ride is greatly reduced.
About Wiseco Piston Break-in - There are an abundance of two stroke enthusiasts that speak with very little enthusiasm about Wiseco pistons. The common response is “you have to give them lots of clearance … otherwise they seize”. We couldn’t disagree more … and here is why.
In the 1970s, we made many attempts to use Wiseco pistons in two-stroke race motors…with very poor results. The truth is that Wiseco (at that time) had numerous materials and design issues that needed resolving. However, Wiseco has done a great job of evolving their materials and designs since the 70’s, and today’s Wiseco Pistons are an excellent choice for most high-performance two-stroke platforms. All that said, Wiseco pistons do have one design issue that they have intentionally have not tried to resolve … longer than average piston-ring break-in times.
The forged material that Wiseco uses for their forged pistons does require “a little” more clearance and a little longer break-in that typical “cast” type pistons … but that is not the unresolved design issue…. It’s the rings. The piston ring material that Wiseco uses is a very tough material that takes much longer than average to “seal” to the bores. While the Wiseco pistons themselves are not a particularly close clearance fit to the bores, the Wiseco rings are. In truth, the Wiseco rings are made brand new with an end-gap that is much too small for full temperature high rpm operation. However if a Wiseco equipped engine gets the slightly longer than average break-in period it deserves, the outer-skin will wear off the rings to result in an ideal “running” end-gap that will give excellent long term wear and service. If however, you try to run Wiseco piston rings prematurely hard, those rings will quickly expand until the ring ends make contact and literally “bite” the full bore diameter in the ring path. Some engine builders incorrectly give Wiseco pistons excessive piston clearance in an effort to resolve this ring end-gap issue.
We respectfully submit that Wiseco pistons can be fit with very close clearances, and offer great service … as long as you let the rings have their break-in time. Our 8500rpm Kawasaki Bighorn road race bikes are equipped with 82mm Wiseco pistons fit at .0035” clearance. These pistons never showed the slightest sign of piston scoring, even after 25+ hours of racing, dyno-passes and high speed testing. You cannot abuse a piston harder than we abused these…. But we did give the rings a long gentle break-in.
About “Dry Top End Assembly” - One step that helps reduce break-in times is to assemble the top end with no oil at all on the piston skirt or cylinder walls. This step has a history worth telling.
In the middle 1960s and early 1970s American muscle-cars were a very popular item on automotive showroom floors. At that time, it was standard procedure for Detroit manufactures to assemble all automotive engines with a generous helping of oil on the cylinder bores and pistons. When these muscle-cars were first fired up, the assembly oil on the bores would burn onto the bores as a glaze that took a very long time for the rings to wear through before sealing.
Unfortunately, this assembly process caused problems for salesmen in the showroom. When prospective buyers took a muscle-car out for a test drive, it was a given that there would be at least one full throttle blast involved. During that full throttle blast, the poorly sealing rings allowed considerable oil to pass, and the brand new muscle-car would leave a big plume of unattractive blue oil smoke that could be seen in the rear view mirror…. Buyers were understandably put off by anything that appeared to be “already burning oil”.
This was a very real problem that Detroit had to address. In time, they learned that if they assembled the top ends dry, with no oil on the bores or pistons, they could avoid this initial burned on cylinder glaze. The result was much shorter ring sealing times (because the rings didn’t have to wear through that glaze), and the absence of blue smoke from those important test drive blasts.
This same rule applies to two-strokes … vintage or not. Some owners shy away from this assembly process because they fear scoring the piston … but it is an unfounded fear. On such assemblies, we do apply assembly oil to all the lower end bearings. In the first moments of initial combustion, that lower end assembly-oil disperses on the bore in even quantities that are easily enough to avert any kind of scoring. In addition, it bears noting that the lubrication needs of an unloaded engine spinning at very low rpms are very minimal. We have been assembling high performance two-strokes in this way since the early 1970’s. It works … and it results in greatly reduced ring sealing / break-in times.