Servicing CR125 (no headgasket?)

new things keep coming up so i’ll keep posting in this thread to not open new ones.

It’s the first time I service my engine (bough a used shifter) and when I took out the water pump I realized it didnt have gaskets but sealant instead, so it wasnt probably maintained properly, well then I took the engine head off and it also didnt have a head gasket, but some kind of O-rings or perhaps high-temp RTV silicon “gasket”.

What’s very strange is that the top of the block have these 2 “grooves” like O-ring slots that I’ve never seen in any manual or video, all I’ve seen are completely flat surfaces.

Are there some blocks like this? should I use a regular 99 head gasket? is this machined aftermarket to use RTV to increasee compression or something weird like that?

(my engine is a CR125 96-99)

Many of the mod motos were modified to use an oring head gasket. I think they are viton vs silicone. Unsure if you could use an OEM Honda gasket on it. If you did it would probably lose a little performance which might not be a bad thing if it’s heavily modded.

I’m still not sure if those are O-rings or if its simply gasket maker silicon because I removed some excess inside the water ports and if you look in the pic theres also material around the head bolts.

So no OEM block ever came with these grooves? they were machined to use a thinner gasket? so weird to not just simply mill the head or block for higher compression than to go these lenghts for it, were they playing around racing regulations or something?

I’m not that concerned about the performance with a headgasket, but about it not sealing properly having those grooves, perhaps just going with gasket maker is the way to go… :thinking:

Kart engines predominantly use o-ring heads. On the mod Hondas they would do this modification. You’d have the inner, outer and stud orings.

Are you sure they are O-rings and not gasket maker? any idea where would I get replacements?

If it’s gasket maker someone did it wrong… Replacements you might consult an engine builder with experience in mod motos. Probably could help you or sell you some.

I found what I think its gasket maker residue in the ports:

So I suspect its indeed gasket maker but I can’t tell, they feel solid and not very mushy like an O-ring would as you can see in the picture a few posts above.

Well anyway I measured the grooves in the block and the inner one is:

Inner groove:
-inside diameter: 60mm
-outside diameter: 65mm

Outer grove:
-inside diameter: 75mm
-outside diamter: 80mm

Are there any kart engine O-rings that match?

Could anyone point me to a specialist about these modded engines?

For a “mod” moto there’s few regulations.

That said, the oring is more for convenience in some ways. With how often the head gets removed over a weekend, using orings you don’t have to worry about keeping or finding head gaskets, nor worry about manufacturing variances.

As a class the mods are not very common, but folks who have experience with them are in abundance. Swedetech, DDR (Darcy Decoste Racing) off the top of my head would be able to help with parts I would say.

Given how common the CR125 and how its the same gasket for like 30 years it sounds like an strange mod for conveniance.

I’m looking for random O-rings and i’ve found ones tha match the size, any suggestions on material, there are many nitril ones that withstand 212F, would that be good? :thinking:

I’ve sent emails to swedetech and DDR to see if they can help.

iv seen in them some inner and outer o rings for modified honda cr 125 motor they write . maybe check here also

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look at the above, that’s what you need. You must use Viton only, which can cope with 600F. A regular rubber one will burn/melt guaranteed

It depends on the application. It’s standard in competition karting and other high performance two stroke engines engines.

Running them hard in competition you could be changing the head gasket 4/5 times over a weekend at times.

Or run a single set of orings, keeping another set as a spare for the entire season knowing your squish clearance is going to be the same, (provided nothing else come loose)

But for just turning laps, not necessarily needed.

Not to mention the stock head gasket can struggle with an engine that’s now close to double its power in stock form. (Thinking back to when I ran mod motos)

So, new update guys, I ordered the O-ring gaskets from swedetech and they fit, however they told me my piston was seriously damaged by detonation (I was running it on 93 octane gas), so I bought a Wiseco forged piston kit (are stock honda pistons forged) and upon inspection it seems to be shorter about 1.6mm.

Anyway I also found out that there’s an aluminum spacer under the cylinder sandnwiched between two cylinder gaskets, so I suppose this reduces compression.

I’m unsure how much compressoin/performance I would get from a 1.6mm reduction in the piston, I"m not racing so I’m not looking for max perfomance, so if this mean like a 5% power loss and I get to use pump gas (getting race gas or avgas is dificult in Mexico) that would be a win to me but I’m afraid it will feel like a lawnmower, any thoughts?

(I guess I could remove the spacer to regain compression right?)

Has this engine run for you yet? Yeah that piston is 100% toast. 93 pump is probably a little on the low side octane wise for a mod Honda, but it really depends on the build too. Some are mild, some are wild! If that engine has been run in Mexico without issues on pump gas then you’ll have to look elsewhere for the cause of the detonation. Aside from jetting ensure there are no obvious air leaks in the intake boot, reed cages and crankcase seals. The final one can be tricky to check without a vacuum pump however.

The spacer is more to correct port heights after other changes have been made, so you’re on the right path with the piston pin\crown distance.

I think you need to measure the squish (Piston\head clearance) before going any further. Although that should be part of a piston replacement anyway. Too much squish clearance can cause problems too.

Overall you have a few variables\uncertainties in your setup, so the more you can nail down and isolate, the better.

Yes this engine was running fine before I was just servicing it for the first time after a water leak in the water pump seal, and realized there was also a water leak coming from the block so i decided to service that too.

I emailed the guys at swedetech and they were the ones that told me that my piston was toast and that it was likely due to the pump gas and that modded Hondas run on race gas or avgas. If i had air leaks wouldn’t that be noticable on at idle?

I’m chaning all gaskets anyway so that shouldn’t be an issue, i’ll measure the squish tomorrow and i’ll post pics.

Ok so a summar of the measurments:

  • Squish with NEW piston 2.35mm
  • NEW piston is 1.6mm shorter than the old one.
  • The cylinder spacer is 1mm + the 2nd gasket
  • The old piston appears to be domed, (about 2mm), I’m unsure if it could be just wear or something.

Now if the older piston was 1.6mm higher and the spacer is 1mm+extra gasket my guess is that I can remove the spacer/gasket and assuming for arguments sake that the gasket is 0.6mm I would get the exact same compression and port clearance that I had before right?

Knowing these numbers would it be advisable to leave the spacer and run it on lower compression so maybe it can run on pump gas since I’m not in any competition and just driving it for fun?

Mexican kart racers typically use “avgas azul” which is 115 RON. It’s not the highest-performance fuel, but it is difficult to build an engine that it wouldn’t be safe for, and it’s consistent and cheap. If you care about toxicity, it is leaded, so you may need an alternative like a Sunoco or VP unleaded racing fuel.

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AvGas is regulated by the government probably to hinder cartels flying freely in cessnas so you gotta get a permit and get it in the airport etc, I’m im the process of looking if I can get it or not.