477cc Upgrade FAQ – Royal Enfield Himalayan Big Bore Kit

This FAQ last updated: 01.07.2022

Please ask questions in the FORUM where issues are discussed in greater detail.

Warning: we had one case where our liner did not fit the engine body. This is a RE QC issue, the body had a small protuberance that had not been machined properly in the factory. The liner can then be fitted to the body by using a lathe. Or using a dremel you can carefully remove the protuberance (make sure no metal residue falls into the engine housing e.g. by stuffing it with an oily cloth).

Q: When can I expect the 477cc forged set to ship?
A: Now

Q: What power increase can I realistically expect?
A: We have researched this issue in depth and verified it with Dyno runs. The naked 477 (all stock except cylinder/piston) will give you 27 RWHP, that is 6 RWHP (not BHP as RE uses) over stock. You can further enhance this with:

  • K&N air filter plus open exhaust: gains: additionally 1 RWHP.
  • A polished head with bigger inlet etc.: gains: 1 RWHP. Now you are at 29 RWHP (or about 34 BHP).
  • Further ca. 3 RWHP are available with our HT-b cam.
  • Our earlier cam design (HT-cam) gives you only extra 1 RWHP but enhances torque in the low rpm which is more practical in daily use.

For a more detailed discussion please visit the forum.

Q: What is the difference between the forged full kit and the cast liner kit?
A: The complete kit has a FORGED piston of 83.5mm size. This ready-to-install cylinder requires no work (other than replacing it with the old cylinder of course).
The kit with the CAST piston, liner and gaskets requires removal of your cylinder, then fitting of the liner and drilling and honing by a professional engine rebuild shop, and insertion of the new cylinder into the engine.

Q: How much does the kit cost?
A: Please check the main 477 page for this information.

Q: Do I need special fuel or octane booster for this piston?
A: The compression we get allows for running it with 95RON or better. If your fuel has lower quality please use both the foot gaskets we provide to reduce the compression.

Q: What are the advantages of a bigger piston/increase in displacement?
A: Mainly these are:

  • More displacement means more mixture can fill the larger cylinder and be burned.
  • Larger bore size means larger area on the top of the piston, so the combustion pressure acts on a larger area, providing more “push”, resulting in increased torque.
  • Larger piston area creates a harder “pull” on the intake system, increasing gas velocity in the intake port, making better use of it.
  • Larger displacement, with the same cams and ports lowers the peak torque rpm and the peak hp rpm, approximately equal to the displacement increase, this giving more power at a lower range on the rpm band. 15% more displacement lowers the working power band by about 15%, or about 700 rpm.
  • More power in the useful mid range, you don’t have to rev as high for power. You ride lower rpm before lugging.
  • Larger bore size increases compression. Without being a higher compression piston by design, just by virtue that you are compressing a larger volume into the same size combustion chamber, you are getting a compression increase.
  • Although larger size pistons could be heavier, this one is lighter thereby decreasing the reciprocating mass, and causing less stress on the rod and bearings which will add to engine life expectancy.

Q: Will the 477cc sleeve fit into the engine body without widening it?
A: YES, that was the idea behind this upgrade, to keep it plug & play.
A bigger piston would require you to change the engine body as well as the head which would make it way more expensive.

Q: Will overall reliability suffer from this upgrade?
A: Reliability in terms of life expectancy is very much a function of maintenance and correct use. The 411cc engine is very robust. Well maintained it can last well over 80.000km. The 477cc upgrade is a performance upgrade and requires handling as such. The correct calibration of a fuel injection control device (e.g. Powertronic) plus exact timing are essential for this upgrade to run properly and not degrade due to knocking or overheating.

Q: What total cost must I expect for a 477cc conversion?
A: That depends on you doing the work of changing cylinder and piston or if you outsource the work – then add ca. 3 hours worth of a local technician’s pay to the set price. If you have an EFI version you will need to purchase some fuel flow regulating device like the Powertronic or Power Commander.

Q: Do you provide a map for the 477cc and the Powertronic?
A: We do. Please consult the forum where these maps are made available.

Q: I have a BS3, do I need to change the carburetor configuration or the carb itself?
A: The standard issue carb of the Himalayan is identical to the one on the 500cc Bullet. So the upgrading to 477cc is no issue as such. We have re-jetted to 130 main and one up the pilot as well to be able to get a low idle and reasonable cold run. But the bike will run with the 125 stock jets as well. If you go over 2000m elevation we suggest a change to a 110 main jet.

Q: Will the 477cc bike use more fuel than the 410cc?
A: No, it will use less, given comparable driving conditions.

Q: Do you publish measurements, Dyno charts etc. of your setup?
A: There are plenty Dyno charts done by our Australian partner available on the forum with all kinds of different configurations. Please check the forum.

Q: Are you using the factory liner bored to 477cc or are you providing the cylinder with a new liner when upgrading to 477cc?
A: For this upgrade it is necessary to use a new liner that will however still fit into the engine body without any changes to the latter.

Q: What is the difference between a cast and a forged piston?
A:  A good explanation of the difference can be found here.
And a video explanation here.

Q: In your upgrade set do you provide everything I need for the upgrade or do I need special tools/other stuff to do it?
A: We provide everything except the tools. But the tools you need are very common and are all listed in the “How To Upgrade” video. We did it without even a torque wrench (if this is your first engine upgrade it may be a good idea to use one). There is one special tool for valve adjustment, you can order it from various online sources if you can’t get it locally.

Q: Will this piston give me a greater top speed?
A: Yes and no. If you are heavier than most or ride with pillion and luggage it will because you likely never red-lined the 411 engine in such a configuration. However, if you are a small lone driver – then no. The red-line RPM is the same. To change that you would have to change the number of teeth on either sprocket – You would want to add one tooth to the front or subtract three from the rear. Make sure the chain fits after that change. Hitchcock and others offer such sprockets (and we add a free 16 teeth sprocket to each full kit). As of our latest test a 16 teeth sprocket does a lot for a much smoother an quieter run especially at highway speeds of around 100km/h.

Q: Are there specific break-in rules I must follow?
A: Yes, there are. The engine should not be moved beyond 3000rpm for the first 500km. Then an oil change is needed and for the next 1500km 4000rpm should only be surpassed for take over. It can take up to 2000 km to reach full free running, up to this point top speed can be used for short moments but not sustained. After the initial 500km break-in full synthetic oil should be used exclusively.

Q: My engine has run for over 30.000km, is there anything I should exchange/renew with the upgrade?
A: It is advisable to change the timing chain which is suggested at 30.000km for the older versions and at 50.000km for the newer ones.

Q: Does your piston come with any kind of warranty?
A: The warranty is limited to delivery failures or damaged delivery goods see here. This is a high performance upgrade, it requires you to handle your bike accordingly. E.g. we have no influence on how the break-in, maintenance and riding is done. For that reason we do not warranty the upgrade set as such. As unlikely as they are, failures could have any number of reasons outside our responsibility. BUT we do offer to sell anyone who bought an upgrade set from us and experiences a problem a factory new 410cc cylinder+piston set for 100.- € plus shipping.

Please ask questions in the FORUM