GK73a sp rocker arms/valvetrain

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moforockband
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Bike owned: GK73a SP CRM250 mk3
GK73a sp rocker arms/valvetrain

Post by moforockband » Sun Apr 12, 2020 1:24 pm

Just started a new thread on valvetrain, but I also want to know what if any difference the sp valve gear is. Part no's for the valve springs and rocker arms are different from generic gk73a stuff.

I have just stripped down a supposedly 'sp' head and found that the only difference between the rocker arms is that they are darker in colour, and have a '63' cast mark in stead of a '13' from my standard 73 ones.
They both weigh exactly the same, and as far as I can tell the same dimensions etc.
https://photos.app.goo.gl/TNsP9SdVVpJDSYMw5
https://photos.app.goo.gl/hEQrb5g53d3JgH867
https://photos.app.goo.gl/uMZra3F7zadgwFL7A



I have been told that the sp springs can take more valve lift.
If anyone has a cylinder head in bits, I'd be interested to see what rocker arms you have, ie cast mark.
Last edited by moforockband on Sun Apr 19, 2020 10:16 pm, edited 3 times in total.

SevenThreeSeven
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Re: GK73a sp rocker arms/valvetrain

Post by SevenThreeSeven » Sat Apr 18, 2020 5:15 pm

I'm currently in the process of removing/replacing the rubber valve stem seals on my '93 GSF400. Haven't dug into it yet but planning to do the work in the next few days (not working a lot right now, so there's time available to dink around in the garage).

I also have a complete cylinder head assembly from a '92 year model GSF400 that I purchased off of Ebay. When I bought it I was thinking about "upgrading" the exhaust camshaft on my '93 by swapping in the exhaust camshaft from the '92 cylinder head, along with the exhaust valve lower seat shim that was used on the '91 and '92 B4s due to the higher lift of the cam lobes.

Image

If you want any pictures or other documentation of the various parts that are in the fearsome GSF400 K707 engine (for comparison purposes) I'm willing to help.

Also, just like the other new thread you put up, I'm unable to view the pictures on this thread.

moforockband
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Re: GK73a sp rocker arms/valvetrain

Post by moforockband » Sat Apr 18, 2020 10:42 pm

Hi, that valve shim is to increase the valve spring pressure, not sure why it is on the exhaust side, as they inlet/ex valves weigh the same and use the same springs. (Maybe the smaller exhaust valve is more likely to bounce at 15000rpm!) The exhaust cam has less lift than the inlet, like most bikes.
Prob a good idea to keep your inlet and exhaust cams matched, so to not lose power.
If you could weigh a rocker arm, I'd appreciate it from your spare head ;)
I'm using Google photo's atm, and it only seems to work for a day, and then goes off :(

SevenThreeSeven
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Re: GK73a sp rocker arms/valvetrain

Post by SevenThreeSeven » Sun Apr 19, 2020 5:48 pm

Image

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SevenThreeSeven
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Re: GK73a sp rocker arms/valvetrain

Post by SevenThreeSeven » Sun Apr 19, 2020 6:35 pm

moforockband wrote:
Sat Apr 18, 2020 10:42 pm
(Maybe the smaller exhaust valve is more likely to bounce at 15000rpm!) The exhaust cam has less lift than the inlet, like most bikes.
Yes, you're right, normally motorcycle engines have less lift on the Intake Valve. But on the '93 year model of the GSF400 Suzuki used 6.2mm of lift on both Intake and Exhaust.

In the U.S.A. we only got '91,'92 and '93 year models. On the '91 and '92 models Suzuki used 7.6mm Intake Lift and 7.0mm Exhaust Lift.

I've wondered about why Suzuki only put the "lower seat shim" under the exhaust springs for the "bigger camshaft" '91 and '92 year models. Maybe the extra heat that the Exhaust Valves are subject to makes a difference in how well the Valve Springs perform.

Across all production year models there is no difference in the Camshaft Duration of the valve opening.
Last edited by SevenThreeSeven on Sun Apr 19, 2020 7:10 pm, edited 1 time in total.

SevenThreeSeven
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Re: GK73a sp rocker arms/valvetrain

Post by SevenThreeSeven » Sun Apr 19, 2020 7:09 pm

moforockband wrote:
Sat Apr 18, 2020 10:42 pm
Prob a good idea to keep your inlet and exhaust cams matched, so to not lose power.
In this case I don't see how that could happen. The only variable at play is the amount of lift for the exhaust valve changing from 6.2mm to 7.0mm

The GSF400 Maintenance Manual shows that Suzuki used the exact same camshaft lobe duration across all year models and versions.
Image

moforockband
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Re: GK73a sp rocker arms/valvetrain

Post by moforockband » Sun Apr 19, 2020 10:14 pm

Thx for the rocker arm info, so far looking like they're all pretty much the same weightwise....
That spring washer is shown on the 91/92 and 93 UK parts diagram I'm looking at. Valve springs are the same across all models, 53 and 59hp.
When I said matched cams, I meant the inlet and exhaust from the same bike, so it's easy to set up.

moforockband
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Re: GK73a sp rocker arms/valvetrain

Post by moforockband » Sat Nov 14, 2020 10:27 am

Finally received a 73SP head. I've pulled it apart and found the reason the valve gear is different, is that the springs are physically smaller, but have more tension, hence the need for different retainers and seat washers.
The rockers have a '13' marking, and weigh exactly the same as std counterparts. These differ in that they have a slight cutout in the middle of the the rocker, not sure if they have the same ratio..


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