How it all began - It was in my mind for 2 years. Didnt got the spare time and budget to do it until now. I home ported the spare block i had months ago for this. For starters its called a bigbore mod. But for the enthusiast who want the long answer, its called shogun piston modding a RX100 block. the shogun piston is ligter but larger(52.50mm vs the stock 50.00mm). but the block needs to be heavily modified to accommodate the (about)10 mm shorter shogun piston. Compression height is vastly off. All the work except for reboring and cylinder block milling is done by me.

I'm new to the tuning front. But i always felt tuning is more than just a performance air filter and an iridium spark plug. so sourced a bored out block from a friend for free, that is 3 years ago. everything was preplanned except for some vital data.
1) I had no idea about the specs of a stock shogun piston(asked for help here, and as usual, no one replied!).
2) I don't know the difference in specification b/w the Shogun engine and RX100 engine.
3) I had no previous experience. (also called a noob)

so, putting my engine at stake, i decided to work on it one day. Stripped the whole engine in about 1/2 hours and started to examine the parts for wear. I wasnt a slow rider. i never properly runs in an engine even though i know the risks. so was expecting some mild seizure marks on the piston.

All parts removed and ready to examine.


Small end had a little play. so concluded that piston pin and/or bearing is shot. Big end was fine as i did my recent bottom end rebuild a year or so ago.
Now, the surprise was that the piston didnt showed any abnormalities. It was of the brand Goetze. This brand of piston is not a favorite among enthusiasts. but it handled the power from a polished and piped block. Looks like Indian brands are getting better!


Exhaust side. Some normal scoring marks.


Intake side. it was also looking good.


Piston dome. Its a first oversize piston.


Later that day, i went out to look for the shogun piston. after searching for a while, i found a 52.75mm(1st oversize) SAM piston. Now before you all bash me for not going for an ART piston, let me remind you that in Kerala(not just Kerala, almost in every part of india), not much people are aware of performance parts or tuning. so availability sucks. And also, i didn't knew how the build will turn out to be. so didn't wanted to spent a lot on performance parts and later blow it all up. Piston kit included piston, rings, piston pin and circlips.

I went home, and it was measuring time. the first thing you'll notice is how light the shogun piston is comparing the 100's. placing them both on a level surface brought out new challenges for me. Shogun piston deck height is about 6-7mm shorter than the 100. 100's stock piston came with ported skirts to increase intake duration but shogun piston is not ported. The "feel good" part was when i found out that the skirt of both the pistons where about the same in length(Shogun piston had just about an mm shorter skirt than the RX), Shogun and RX piston has a same diameter piston pin and that shogun piston has a taller dome.


After getting those crucial numbers i needed for this build to happen, i decided to mill 7mm from the base of the block and an mm from the top of the block. That will even out the piston deck height to cylinder height when at TDC, gives me better compression and also increased intake duration. Went to the lathe shop, and after lecturing the lathe guy in every possible way i could on how important "Precision" is crucial in this job, he said okay. He told me to drop by 2 days later. And so I went home.

Later that day i decided to map the stock cylinder block. after a lot of time and paper sheets away i got it just about right. Now, you cant do much with a divider and a scale. can we?


The next problem i came across was with the small end bearing. Shogun piston had wider pin supports than that of the 100's. So when using the stock bearing along with the shogun piston caused excessive play b/w the pin supports and bearing, which can result in bearing failure as now bearing is running around instead of staying in a place! Refer the below pic.


So again, i was puzzled with questions. After a while, i decided to go for Shogun's small end bearing. It also posed a slight problem. As the shogun piston small end bearing is longer, it will protrude out of the small end of the RX conrod 2-3 mm. Decided that it is much better than a bearing which runs around in its place. The Shogun bearing on the left in the below pic is a new bearing . it cost only 50Rs. And out of the box it was rusted and after a while handling it for measuring a roller came off. I guess i'm contradicting myself, but some Indians quality still suck!


So, decided to look for an original TVS small end bearing. after searching for a while i found one. Now, that was quality. the roller supports where made so that it will lock the rollers in their place. It was also machined much better and it also cost 100+rs.


The next difference is b/w the piston rings. Shogun uses thinner rings in comparison to the RX100 rings. in the below pic it is not very evident, but in person it is.


After 2 days, i went to the lathe shop to get my "Gunned" block back. at first inspection i thought he did a great job. machining looked fine, he chamfered the sleeve. everything looked fine except for 2 new ports opening up on the intake side of the block and a slight difference in thickness b/w the left and right side mating surface of the block. i told him to clean the mating surface up. he did that in a jiffy and i left home with my block. Now, what the hell am i going to do with the "New" ports? that was in my mind.


After placing the block on the bottom end i decided that the "New" intake ports wont be a problem after fixing the cylinder. the problem was with the four fastening holes. After all that machining it had became too thin. Too much tightening torque and i will end up breaking it.

Here is the original block on the left and "Gunned" block on the right.