CR480 broken case solution

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9/19/2018 6:06 AM

I'm in the process of refurbing an 82 cr480, broken left center case, which seems to be extremely common. I finally found a good set of cases, but I am trying to avoid having the problem with these. Is it mainly a matter of installing a compression release, or has anyone else re-inforced the case before it breaks? What are the best options for a compression release? I have tried Eric Gorr, but I haven't heard back from him yet. I had an 83 new back in the day, I don't remember this being an issue then. Thanks in advance.

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9/19/2018 7:47 AM

I had Eric Gorr install his decomp button on my CR480 head. Worked great. From my experience the best way to communicate with Eric is via text.

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“Anything that gets your blood pumping is probably worth doing”

Hunter S Thompson

9/19/2018 8:09 AM
Edited Date/Time: 9/19/2018 7:34 PM

Your dealing with an old, old engine. With how many kicks put into it, who knows? And, how many 'kicks' have been put into your replacement cases - assuming they are second hand?

The combination of a big old engine with a reasonable amount of compression, with a tendency to 'kick back', Plus, that lever and fulcrum made by the KS lever hitting the foot peg with a vicious metallic Bang, is a killer of cases.

Broken cases at the KS lever area, happens on all sorts of bikes though, not just big Bikes. That lever and fulcrum and metal shock effect is pretty bloody severe in many bikes.

A Decompressor will certainly help, with you not having to hammer the lever ( my decomp on my 500 - and a short KS lever- made it so I could start the bike by hand, in situations where my non bending knees made it damned near impossible to use my leg), thus reducing the massive metallic shock to components when the lever hits the peg.

Making a 'bump stop' for the lever to peg interface is a good thing, too. Even with my shorty lever, I run one.

Then also, as I mentioned earlier, you can fit / make to fit a shorter KS lever. I've used a Gas Gas Trials bike KS lever ( it's pivot 'knuckle' welded to a 500 splined mount) for well over a decade now, and with my lowered pegs ( KTM 450RR pegs), means there's virtually no leverage on the case when the lever contacts the peg. An added benefit of the short lever is the extra 'speed' you get to the rotation of the KS shaft and gear. But, of course, you Definitely have to have a Decomp set up with a short KS lever.

Then if course, you could probably fit an E Start kit to the bike. I'd be surprised if the one available wouldn't fit, but it's a POS, that I had to correct many f**k ups by the manufacturer. Of course, the manufacturer said that the f**k ups were no problem...... I've detailed them in the early pages of the ever continuing KX500 /KTM build on this site. I also put up video of how dreadfully made was the interim KS drive gear in that thread. Severely 'out' both radially and axially - but hey, it's perfectly acceptable by the manufacturers standards...........Suffice to say, I'd not trust their other products, and will be getting another manufacturers cases for a further 500 project.

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9/19/2018 10:50 AM

jeffro667 wrote:

I'm in the process of refurbing an 82 cr480, broken left center case, which seems to be extremely common. I finally found a ...more

had an 83 that had cracked cases couple inches behind cylinder cross both halves . they are awful thin , was told buy dealer when I was looking for new set that they were not available , and he said 82 , 83 ,84 all different ...

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9/19/2018 8:12 PM
Edited Date/Time: 9/19/2018 9:05 PM

As for a decomp unit:

The little Harley / VTwin 'Pop Out' types have become Very popular. They are nice little units, and some can be very high quality, in Stainless steel.

The smallest ones I've found have a 10mm x 1mm thread. I'd use these in stainless - don't be a cheapskate (hey Jeff Cw00t ) and buy cruddy ones, and don't get 'miffed' that they seem to be sold only in pairs......

I don't know the thickness of the 480 head. Neither do I know of the best place to fit one - for accessibility. You'd have to determine that - to what suits you / and the bikes componentry layout.

I would Strongly recommend an adaptor be made, to go into the head, that then the Decomp valve can fit into. To Not have the unit directly in the head, as I doubt that it would be thick enough to go in, whilst still having enough thickness for the valve to open, whilst only having a small decomp hole that - Key issue - would not Ever allow a failed valve, to drop into the combustion chamber. I've seen it happen, and hell, do people want to have their 2t 'drop a valve' and destroy the engine like a 4t can????

An adaptor enables you to say, put in a smaller threaded hole (bugger trying to get in and weld the adaptor in among the fins) into the head - probably a 8mmm x 1.25mm. That then can have around a 2mm / 2.5mm hole in it - you don't need a massive hole / port in the head to get a quite noticeable Decomp effect. The last one I checked for volume around the valve stem and the units 'port', equated to the volume given by a 3mm hole, but you don't need a hole that big. A smaller hole also lessens the tendency for the unit to 'pop closed' when kicking, but they can still be a bit frustrating with that. Better quality units, resist early closing far better than cheap ones.

Then, on the Decomp unit side, you have the 10mm x 1mm threaded section, machined with enough depth to that pocket to give around 2mm clearance when the valve is 'open'. The gas can easily flow around the valve. I've generally made the adaptors out of stainless steel, but also done them in aluminium.

The adaptor set up also brings out the Decomp to a more reachable point - Fins, boys, on a 480 Air Cooler - and, of course, you probably will have to machine a bit of 'finning' away. Though, a bit of careful thinking about the Decomp landing may mean No Fin trimming may be needed - again, I'm not that familiar with the 480 / 500 Air Cooled heads.

As I said, I can't offer any specific area to place it - it's down to the points I mentioned at the start. But I Do Not favour placing a decomp hole / port in the Squish Zone , though I've done it when necessary. In the 'bath' of the head, and , generally following the 'arc' of the combustion chamber is best.

To those that think an 'in head' Decomp port will lower working / firing compression - no, it's not an issue. It only comes into play when the valve is open. I've fitted Decomps for years, and have Never had anyone say their bike did not run the same, after it.

All Long winded, I know, but I try to impart information as well as I can.

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9/20/2018 3:37 AM

Thanks everyone for the replys, Eric got back to me, so I'm sending him the head for squish mods and compression release. I'll post up the results when I get it done.

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9/20/2018 4:52 AM

Please post some pictures when its done, think i install one

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3/20/2019 4:21 PM

jeffro667 wrote:

Thanks everyone for the replys, Eric got back to me, so I'm sending him the head for squish mods and compression release. I'll ...more

Did you get the decomp fitted?

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3/20/2019 6:04 PM

jeffro667 wrote:

Thanks everyone for the replys, Eric got back to me, so I'm sending him the head for squish mods and compression release. I'll ...more

TTperra wrote:

Did you get the decomp fitted?

I did get the decompression valve installed. I'm still trying to source a few parts, silencer, intake boot, and pipe hanger, but I should be ready to try it out soon.

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3/21/2019 12:37 AM

jeffro667 wrote:

I did get the decompression valve installed. I'm still trying to source a few parts, silencer, intake boot, and pipe hanger, ...more

Can you post a pic of the head with decomp?
i am curios how/where he placed it.
this is how i did yesterday, have not tried it yet..

Photo
Photo
Photo

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3/21/2019 8:10 AM

TTperra wrote:

Did you get the decomp fitted?

jeffro667 wrote:

I did get the decompression valve installed. I'm still trying to source a few parts, silencer, intake boot, and pipe hanger, ...more

TTperra wrote:

Can you post a pic of the head with decomp?
i am curios how/where he placed it.
this is how i did yesterday, have not tried it ...more

Here's a picture of the decompression valve. Photo

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3/21/2019 8:32 AM

Thanks
Same location, different angle, i tried to keep it 90deg to combustion chamber radius

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3/22/2019 12:07 PM

Can´t remember which years but the big Honda had two shallow grooves in the cylinder above the exhaust port. They let some compression out while kicking the bike so it was easy to start. When the engine fires everything happens at a much faster pace so the grooves don´t cost any power. On the first overbore the grooves were gone and the kickstarting became a lot worse. Most of the engine builders thought that the shallow grooves were worn into the cylinder and did not remachine them back into the surface and the ignorant customer was happy that his engine "has a lot more compression than before the rebuild" - never checking that power output was exactly the same......
cheers!

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3/22/2019 1:28 PM

WEAL wrote:

Cant remember which years but the big Honda had two shallow grooves in the cylinder above the exhaust port. They let some ...more

Interesting, i have not seen it in 480 cylinders, i have cylinder thats not bored, going to take a closer look

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