Honda GX620 modifications

Bigjohn1980 · 12-05-2014, 04:15 PM

Hi, this is my first post on here so be gentle if I'm repeating other questions.

I have a honda Gx620 and I'm looking to up the revs. My latest plan is to fit a GX670 carb and inlet manifold and some 1.3:1 rockers and maybe do a bit of work on the ports. My worry is, how much will the standard aluminium con rods take? I have done a lot of research and have now got all the part numbers for some steel con rods from the older model GX620 with bearing Shells but I can't find out if these will fit my Pistons and pins. It's an expensive option so I'm quite keen to know if it's a worthwhile upgrade or should I tune the engine up and replace the rods if or when they go?

Any advice would be great!

slonomo · 12-05-2014, 07:07 PM

I don't know anything about the GX620, but the GX390's the connecting rod is aluminum and doesn't have bearings. They are used for racing. The weak link in these engines is the pushrods. You'll have to upgrade the pushrods and maybe the springs and remove the governor, if it has an internal type. If this engine has the electronic governor, good luck, no way to bypass those. But the rods are usually ok if you don't abuse the engine and if they are in good condition now. Revs should prob stay below 5000.

LVCJ · 12-05-2014, 07:27 PM

Quote:

Originally Posted by slonomo

I don't know anything about the GX620, but the GX390's the connecting rod is aluminum and doesn't have bearings. They are used for racing. The weak link in these engines is the pushrods. You'll have to upgrade the pushrods and maybe the springs and remove the governor, if it has an internal type. If this engine has the electronic governor, good luck, no way to bypass those. But the rods are usually ok if you don't abuse the engine and if they are in good condition now. Revs should prob stay below 5000.

. Also, above 5,000, the stock flywheel may become an issue???

Bigjohn1980 · 12-05-2014, 07:36 PM

Thanks Slonomo, I should have said that the governor has been removed and the valve springs have been upgraded already.

I've been told that the aluminium con rods are good for 5000-5500rpm but the older steel rods are good for upto 7500rpm. This could be rubbish?

My only real experience is racing a mower with a honda GXV530 with the over head cam......... Aluminium con rods put a hole in the crank case at around 5000rpm.

Bigjohn1980 · 12-05-2014, 07:39 PM

Does anybody make a performance flywheel for these or do I need to buy a bigger lathe?

TheGDFP · 12-05-2014, 09:47 PM

Quote:

Originally Posted by Bigjohn1980

Does anybody make a performance flywheel for these or do I need to buy a bigger lathe?

Maybe re-taper the crank for the small-block PVL? $100 is attractive.

Bigjohn1980 · 12-06-2014, 11:37 AM

Good plan! Thanks

12-05-2014, 04:15 PM	#1
Bigjohn1980 Not Yet Wild Join Date: Dec 2014 Location: Worcestershire UK Posts: 4	Honda GX620 modifications Hi, this is my first post on here so be gentle if I'm repeating other questions. I have a honda Gx620 and I'm looking to up the revs. My latest plan is to fit a GX670 carb and inlet manifold and some 1.3:1 rockers and maybe do a bit of work on the ports. My worry is, how much will the standard aluminium con rods take? I have done a lot of research and have now got all the part numbers for some steel con rods from the older model GX620 with bearing Shells but I can't find out if these will fit my Pistons and pins. It's an expensive option so I'm quite keen to know if it's a worthwhile upgrade or should I tune the engine up and replace the rods if or when they go? Any advice would be great!

12-05-2014, 07:07 PM	#2
slonomo Searching for The Way Join Date: Oct 2009 Location: Medina, Ohio (NEOHIO) Posts: 11,441	Re: Honda GX620 modifications I don't know anything about the GX620, but the GX390's the connecting rod is aluminum and doesn't have bearings. They are used for racing. The weak link in these engines is the pushrods. You'll have to upgrade the pushrods and maybe the springs and remove the governor, if it has an internal type. If this engine has the electronic governor, good luck, no way to bypass those. But the rods are usually ok if you don't abuse the engine and if they are in good condition now. Revs should prob stay below 5000.

12-05-2014, 07:36 PM	#4
Bigjohn1980 Not Yet Wild Join Date: Dec 2014 Location: Worcestershire UK Posts: 4	Re: Honda GX620 modifications Thanks Slonomo, I should have said that the governor has been removed and the valve springs have been upgraded already. I've been told that the aluminium con rods are good for 5000-5500rpm but the older steel rods are good for upto 7500rpm. This could be rubbish? My only real experience is racing a mower with a honda GXV530 with the over head cam......... Aluminium con rods put a hole in the crank case at around 5000rpm.

12-05-2014, 07:39 PM	#5
Bigjohn1980 Not Yet Wild Join Date: Dec 2014 Location: Worcestershire UK Posts: 4	Re: Honda GX620 modifications Does anybody make a performance flywheel for these or do I need to buy a bigger lathe?

12-06-2014, 11:37 AM	#7
Bigjohn1980 Not Yet Wild Join Date: Dec 2014 Location: Worcestershire UK Posts: 4	Re: Honda GX620 modifications Good plan! Thanks

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