Hitchcocks Forum

I just blew the top of my piston, clean off at the oil ring. The bike is an Indian iron barrel 2006 Classic. I have been told this is quite common at around 20,000 km which was about the mileage mine had done. The timing was spot on and the mixture was a little rich if anything so I think it is just planned obsolescence. I had to strip the motor to clean out all the metal etc and have re-assembled it with another similar piston, second hand from a mate which has only done 5000km. The piston not my mate. The bike runs fine now so if it keeps going I may change that piston for a better quality one in the next year or so before I have to strip the motor again when this one breaks. What other experiences have you blokes had like this and what pistons do you recommend. Not interested in big bore kits or forged pistons either. Nothing too expensive as this bike already owes me a small fortune.

I think it's at this point some smart ask comes along and says 'They all do that Sir!'

... pishton broke? ... hic ... sho'm I, mate ...

Ed, I was gonna say that but I thought this might be read by small children.(An oldie but a goodie eh!)

Hi CREAKY45---- oh dear, bummer aint it ??......Been there myself a few years ago.....I'm assuming you changed the oil and filter during the re-build ??.....Did you find any silvery, or cloudy Aluminium particles in the old oil ??---- If so I'd suggest you strip the rocker assemblies and prove the oilways clear and unblocked.....Remember, unlike the Triumph / BSA types etc..ALL the scavenged oil from the cranckcase passes up to the rockers via that outside pipe....( The others usually have a T-joint in the return pipe. It's this t-return that takes a small proportion of scavenged oil to the rockers.. )...The rockers run in a plain Aluminium journal bearing so any foreign matter could end up scoring the bearing faces....mine certainly did !!...The OE Indian piston is I believe, made in CAST ALUMINIUM !! Not the best material in that given situation...I would re-consider the use of a forged piston but at the same CR of 6:1 ....Have a read of this}-https://www.carsdirect.com/aftermarket- ... ed-pistons ---------------- Here's the oil flow diagramme}---https://image.slidesharecdn.com/bulletm ... cb=1214007

oops---that last one went haywire !!--------- Maybe this will do it ? ----

Was it a 350 or a 500. A friend of mine pulled the top off his 500 piston.

The 350 doesn't (often) do this.

It is a common failing of the OEM piston, (Cast T Slot Split skirt design) and it is directly related to the pistons top land clearance relative to the actual bore size.


Most stock pistons that I have measured run a top-land to bore clearance of 0.024" others as high as 0.028" in conjunction with 0.002: Skirt taper. In service the taper soon increases and the top land are of the piston starts to make contact with the bore: Piston Slap.


This can be seen as a small area clear of carbon deposits, fore & aft, on the pistons top land.
When the pistons travel changes direction at BDC and TDC the top-land area makes contact with the bore (Slap) and over time this Slapping compacts the alloy of the piston, trapping the rings Fore & Aft. The Compression rings wear, the rings lands continue to compact, and pressure is in turn brought to bare on the oil ring.


Evidence of all this occurring can be found at as low as 2000m and manifests itself with the rings being no longer free to rotate around the piston. The wear is most noticeable if the rings are removed from the piston and their thickness measured. Inserting a new ring into the grove will also prove to be tight to impossible. The Oil ring top & bottom surface will also exhibit Bright and Dark areas that correspond to the 'bridges' between the rails.


Eventually the Oil ring shatters under the load, then the crown of the pistons pulls of when the piston is at TDC. Frequently bending the valves and pushrods. In extreme cases (high speed) the motor will explode!!!




The cure: Eliminate piston slap! Turn the piston top-land diameter down by no less than 0.002" - 0.003" (I use 0.004") so that the top land is AT LEAST 0.028" smaller than the actual bore diameter. (I like 0.030" smaller than bore size)


The CheepO cure: Carefully draw file the Front and Rear edges of the the piston Top-land for about 1/2" either side of the piston centre line. DONT cut into the 2nd ring land!!! Just the top land.




Once the piston top-land has commenced to compact down on the rings the piston is typically beyond redemption. I tried several times to remachine/clean up afflicted pistons and checks found that once the Taper of the piston (Skirt distortion) had been compromised ring Blow-by increases due to the excessive piston rocking.


Or you can fit a forged piston.

Hi Tim NZ your explination of the piston problem is what happens. The problem is that most people do not have access to lathes and things to do the piston work. I am wondering if a piston can be purchased off the shelf that will solve the problem.
You mentioned a forged piston. What is the difference of a forged piston, and can forged pistons be bought. My friend's 500 iron barrel
broke the piston top off after only doing 11,000 miles. Do 350 motors with ordinary Indian made pistons suffer from this problem. Thank you for helping with this troubling problem.