® No Free Lunch

A very popular eBay big bore piston kit, this one for a Honda CB350F. A lot of parts there for very little money. What's the catch? Read on and find out.

We keep trying. Whenever some too-good-to-be-true deal comes along, we are drawn in. We don't believe in the impossible, we say. We're not gullible. Or are we? When something just looks so good we can't resist...?

That piece of shop equipment selling at a fraction of its normal value? It's parts are rusted together and will take herculean effort to make work again. That nice shirt at Walmart? Between buttons that fall off in the wash and impermanent colors, it too proves to not be such a bargain. And $200 carb rebuilds? Well, where do we start with that one?

The current "free lunch" in the vintage powersports world may be those sensational $200- $250 big bore piston kits on eBay. Folks without a background in the industry don't know what to look for. But believe me, here again, there is no free lunch. Think about it. Not only pistons, but also rings, pins, clips, and an oversize OEM reproduction head gasket? All at one-fifth the price of similar kits from industry leaders Wiseco and JE, i.e. roughly what just an OEM head gasket by itself costs? Can this be true?

Here are the facts.

Pistons. These kit pistons are actually surprisingly well made! More or less overseas knock-offs of the OEM parts, only 3mm larger of course, and best of all, the visual, measured (very consistently made!) and experiential evidence shows them to be excellent pieces and very dimensionally stable. In fact, indisputably the best part of these eBay kits. Good pistons. That's the piston pro. The piston cons? Starting with the least important and moving toward the more critical, the importers of these kits offer no freshening sizes. That is actually nomal and expected in the industry, so no harm and no foul there. Just be aware of it. The same is true of the rings, of course. Next on the con side is since these are cast pistons with virtually identical valve reliefs as OEM, these aren't pistons you will want to use in a high performance engine. There is no room in this lightweight piston for customizing the reliefs. So mild builds only. The third negative about these pistons is that in some models the deck height, i.e. the distance between the piston pin and the crown, is incorrect for the engine. So far, I have witnessed over a millimeter of negative deck, which can only be corrected by shortening the cylinder block that same amount. I would put this as the most serious of the piston's shortcomings if it weren't for the fact that it does not show up on all models these kits are offered for. Strangely enough.

Rings. But here is where things start to really go awry. There are no pros in connection with the piston rings. None. In a word, they're crap. First, they're soft. One bike I have seen these in showed a huge amount of wear -- the tin completely gone, the face looking scarred and torn, and the end gap double the assembled number, all in less than 500 miles! Speaking of end gaps, it appears they are manufactured too large to begin with, and with no optional rings, there is no solution to this. The rings are clearly where the scrimping is being done in these kits. What can you expect? Something has to give.

Big bore kits are cool, no doubt about it. This is what a 3mm increase looks like. That's a stock CB350F piston and an over-bored CB350F cylinder sleeve. Pretty substantial, isn't it? Don't expect a significant power increase though unless your engine is pretty sick. Low speed sharpness and pull is what this kind of bore increase delivers.

Pins. This is the other area in which there appears to be compromise. I have 45 years observing and examining (I was rebuilding Honda engines in my very first shop) factory Honda piston pins, and what I am seeing in these $200 kit pins is not encouraging. First, whereas a factory pin will get a just perceptible discoloration after thousands and thousands of miles, the eBay kit pins are starting to gall, not just discolor, almost immediately. Moreover, they appear to not be solid steel like OEM, but rather chrome plated! Hmm. Not for me, thank you!

Clips. One thing all these economy kits have in common is funky, Euro style piston pin circlips. You know, the kind with the curly-que built in. I don't like those. They interfere with the ability of the pin to self-lock its clips, and worse, they can actually do the opposite -- that is, encourage the clip to come loose. OEM clips are inexpensive, readily available and a no-brainer in my view.

Head gasket. A very pleasant surprise here. This is a quality gasket, fiber like OEM, with the requisite fire rings, and the gasket itself well cut and dimensioned. And the removal of used kit gaskets shows they seal just like OEM too. Good stuff.

The bottom line? You're paying for excellently made pistons and a very good head gasket in these kits, and still at a pretty good price, even if that was all there was. And really, that is in fact all. The rings, pins and clips are throwaways. OEM piston pins are easy to get and not expensive. The rings however are another matter, and the real challenge in using eBay big bore kits. No free lunch, as they say.

Suggested reading:
Big bore kits
Cylinders done right
Valve jobs done right
Engine assembly

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