410/450/477/510cc Big Bore Kit comparison

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We have made only two 450cc for ourselves for testing and evaluation so some of our data is from the Italian producer who has not only sold many but also used EFI while we are limited to carburated Himas.

The very first conversion we made ended shortly after break-in in a burned piston because we had failed to inform the boring company that this piston is FORGED and not CAST as almost all OEM pistons are. Forged pistons  have their advantages, among them solidity, endurance and the option to make small numbers at reasonable costs. They are used in race engines and for high performance tasks. Their downside is a relatively high coefficient of expansion under heat. Cast pistons have two advantages, they are cheaper when made in great numbers and they have a small coefficient of expansion when heated.

Herein lies the crunch: The Indian company doing the boring and honing had never seen a forged piston, did not recognize ours as such and made the cylinder too narrow for the forged piston,  under load it expanded too much and hence the partial destruction.

The second piston, now with the correct measurements in the cylinder, runs great with no problems, our learning curve was steep one can say.


To have any real scientific data on this the minimum requirement would be a Dyno chart, however due to Covid we are not able to reach a facility where such is available. So the subjective impressions of our main rider (45 years experience on many different models, 3 years and 40 tkm on the Himalayan) will have to suffice at this time.

Update: one of our customers was nice enough to provide a Dyno chart for the 450cc piston, it shows an approx. 30% increase over the OEM engine:

450cc vs. 462cc:
I have driven the 462cc for over 2000km before we replace the cylinder/piston with our own 450cc. The following 1000 or so km where then done with the 450cc.
Initially my impression was that the 450 lacked a bit when compared to the 462 and I wrote this off as the result of 12cc less displacement – however the Italian producer insisted it should be equal, and he had done dyno tests to prove it (but left the prints in Spain). When it was revealed that our cylinder was bored too narrow it became clear why the engine was so “reluctant” – and the second piston then behaved as expected – at par with the 462cc, due to slightly higher compression and a slightly different design.

It should be remembered at this point that the 450cc was an attempt to have a solid cylinder wall by just boring the OEM cylinder to 450cc specs without the expensive need to have a new sleeve made. This target was fully reached (3.2mm vs. 1.8mm).

450cc vs. 477cc:
We have had two 477cc test-beds and at this point it should be noted that the 477 project was just that: a project, not fit for public release. It was made to find the maximum acceptable wall thinness of a sleeve made for the Hima engine body without having to machine the body (or the head), in other words, the biggest possible plug-and-play solution. We also wanted to find out if the gains would be worth the efforts when compared to the 462 or 450.

It turns out the 477 is quite a different animal. The torque again is way stronger from 3000rpm upwards while running temperature remains in the green and fuel efficiency is even BETTER that the OEM 411cc engine.

With the 477 engine it is possible to reach red-line in seconds on a straight-and-level highway even with 120kg load on the bike. Of course there is no difference in top speed unless one also changes the chain sprocket to allow for the additional power to translate into speed – but that was never our goal, we wanted a quick reaction engine for fast take-over in steep and high terrain (aka the Himalayas) – and this the 477 does a lot better than the 450 or 462 – and there is no comparison to the OEM 411cc.

All these tests where done with carb bikes and mainly unaltered jetting. We had a go at a 130 main jet (no changes) and a 150 main jet (engine runs horrible) – so we watched the oil thermometer like a hawk and checked the spark plug for signs of excessive lean burn – but found nothing.

Now we have a 477cc piston professionally designed for us that has alleviated the problem with high compression and the need for high octane fuel. The 477cc is in the price range of the Hitchcock set as it also needs a custom made sleeve.

410cc vs. 510cc:
There really is no point in comparing those two. The 510cc LS410 engine will be fully reworked, with engine housing and head modified, different throttle body as well as injector. Output will be in the 35 BHP range and that will likely take the Hima frame to its limits. Again the end goal is only a slightly higher top speed of maybe 135/140 km/h sustained but the main goal is a lot of torque where it counts. Our test-bed with a home made piston (adapted from the 535cc cafe racer) shows promise but there is nothing definitive to talk about before 2021 when the real deal will hit the road.

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