Over bore or not? By hoser..At some time or another in our machines life we will come to the point where we run out of bores and need to bore beyond the OEM piston sizes and move to a after market piston but in doing so we are reducing the reliability and performance of our engine. Most have it in their head that Honda and other ATV companies only offer 2-3 over sizes so they can sell you a new cylinder, not the case (more on this later) other after market companies cover this area by offering replacement sleeves to take your cylinder back to the stock bore if done correctly and be warned if not done correctly it can turn into a real nightmare, I have see quite a few engines fitted with a new sleeve and so far the results have not been all that favorable so if you choose to go that route I would pick a company that specializes in replacing sleeves like LA Sleeve or Kustom Kraft, at this time I can not recommend anybody else because all the sleeves I have seen done by others have been a real botch job and ended up costing the owners a bunch of money and more importantly additional down time, the ones I have seen done by LA Sleeve and Kustom Kraft were high quality and you had to look close to see that the cylinder had in fact a sleeve installed. Back to the over bore, as I mentioned before the factory limits it over bore sizes for several reasons but for the most part it is for reliability, boring to the maximum OEM over bore will usually retain all of the performance and reliability going beyond that size and you start reducing the performance and reliability. What happens when you bore a cylinder first you change the size and location of the port openings, some ports just get smaller, transfer ports that are angled up are now positioned higher (change the port timing) ports such as the exhaust are lowered, most porting will include raising the exhaust port, every time you bore the cylinder your lowering it (considerable power loss at higher RPM's, your increasing the trapped volume of the cylinder so bottom end power will be slightly higher). As the cylinder gets thinner it will loose it stability and ability to maintain its shape and is more prone to warping. The factory head gaskets are already sized for the maximum over bore which is usually 2 over bores when you go to the 3rd over bore you end up with part of the head gasket hanging into the cylinder, after market companies such as Wiseco offer oversize gaskets but that too comes with a price, the gaskets usually cost less than OEM but that is not the price I am talking about the price I am referring to is performance, the after market gaskets are usually large enough to accommodate the largest oversize piston they offer so if you bore to .040 over bore their gasket is good to .080 over bore, with the gasket being .040 too large you would not think that is very much and it isn't but it does reduce the trapped volume in the cylinder and creates a ledge on the top of the cylinder that affects the combustion process they also are usually .020 or more thicker lowering the compression even further. (draw and insert head gasket/ledge illustrations here) Head As you probably already figured out by now after reading and thinking about the head gasket problem it probably dawned on you that the head comes from the factory with a outside diameter ready to accommodate a 2nd over bore piston just like the factory head gasket, now what does a guy do, well probably nothing since the after market company has probably already thought of that and gave you a thicker head gasket raising your head (lowering the compression) far enough away from the outside edge of the piston so it will not hit the head but think about that setup for a minute you now have the smaller diameter combustion chamber hanging over into the cylinder see the problems your creating by going past the factory limits? Piston weight By going over bore your adding weight to the piston, on engines equipped with a counter balance like the FL350 and the Pilot the balance shaft is weighted at the factory to accommodate a maximum over bore of .20 over size go larger and you end up with more vibration, more vibration is easy enough to live with in a engine that has rubber engine mounts in good shape if yours are slightly worn I suggest you install new ones since the Pilot engine is hard mounted to the transmission you will need to replace the transmission mounts also, enough about mounts lets get back to the added weight, the added weight also adds more stress to the rod, crankshaft, crankshaft bearings and crankcases not a good combination in a well worn engine, when I say well worn engine I assume the reason your going beyond the 2nd over size piston is because you wore out all the bore sizes it is possible to take a low hour engine this far when their has been damage to the cylinder and it was necessary to bore the cylinder more than one size to remove the damage. Big bore kits. You have probably seen a bunch of these being offered in the motorcycle racing scene and from what I have heard have been done with good success, to make theses big bore kits they are not simply boring the cylinder oversize and installing a larger piston they are however redesigning the whole engine, port and head modifications are made to accept the new bore size, these engines do not use a counter balance shaft so modifications to the shaft are not required, these engines are also require a lot more maintenance after being modified. You can successfully make a big bore Pilot or go beyond the second bore, ATVR used to offer a 465 kit but wiseco/Honda no longer makes the pistons for that bore size, CW Racing makes the 435cc-440cc kits that use a motorcycle piston and last I heard is still available but all the engines require porting changes and head modifications to work, I don't recommend these modifications for the recreational rider from what I understand they were developed for racing, were not all that reliable of a setup and required a lot of maintenance. Options So when we reach our last bore what are the options, one would be to replace the cylinder with a new one from Honda (hillside Honda $245.00) expensive but you retain the Honda reliability and suffer no loss of power, you can sell your old cylinder to someone else willing to run the less reliable more seizure sensitive less durable after market pistons. Another option would be to have your cylinder sleeved back to the stock bore, I am told the new sleeve material will wear about 3 times longer than the factory sleeve (LA sleeve and Wear master brands) but their is a risk of getting a botch job, again I recommend LA Sleeve and Kustom Kraft. Their is a 3rd option I am looking at but have not tried, they plate the cylinder back to the stock bore (.040 thick max) this is done by Millennium Technologies My bore is currently on its second over bore so at the end of the 2002 season I will be sending my cylinder to them for plating (160.00 + shipping) you can run the stock Honda piston and rings using this setup, the guy I talked to said you can wear out 3-5 pistons in the cylinder before needing it honed to the next over bore how ever if you have a catastrophic piston failure where the plating is damaged you will need to have it stripped off and plating replaced where is on a cast bore you would simply bore to the next size. Their is also some side benefits to using the plating other than longer wear than cast you automatically l will see a 4-6% power increase from the friction decrease, visit their web page for more information. Bottom line is going over bore worth the risk of reduced reliability over the cost of replacing the cylinder, installing a sleeve or plating the cylinder back to the stock bore, no matter what route you take you still need to buy a piston, rings, wrist pin, wrist pin bearing, cir clips and gaskets so you really can not factor in these cost. Since my engine is modified (porting, pipe, carb) and I already am inducing more than normal stresses to my whole engine I rather spend the extra money now to get the added reliability/ stability of OEM pistons, my current cylinder is 13 years old and has seen 6 pistons in 7 years, if I take the most expensive route and buy a new cylinder I can expect at least 7 more years of the Honda reliability and probably 10 years if I don't port or modify the new cylinder.