Author Topic: SF750 valves and seats  (Read 603 times)

Offline Fitz

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SF750 valves and seats
« on: September 17, 2020, 14:25 »
Hello all, just started to rebuild teh motor of my recently aquired SF750 (1971)

I may have mentioned that I found quite a lot of carbon build up on the back-face of the inlet valves which I thought was a bit odd. Before cleaning, with the head on the bench, I re-fitted the plugs and poured petrol into the chambers, one side was not sealing very well

When I cleaned things up in my vapour blaster, I found the exhaust valves and seats are in pretty good order but the inlet valves need attention. The valve sealing face is concave, the actual sealing portion is very narrow, I know this is only supposed ot be 1.5mm or so, but the weird under-cut shape can't be good, I don't think I can use the valves, after re-facing they may be too far gone

The bike was one of a number that were imported from Italy recently, my letter states it belonged to Giovanni Laverda others allegedly belonged to Pierro Laverda. I had considered fitting a battery, see what lights up, get it running and go from there - I am so glad I decided to refurbish everything and drop the motor, it is in very good original condition but did need overhauling to get it back in shape

my questions are:-
1) Does the SF need to have new valve seats suitable for modern fuels, my apologies if I have asked this previously, I have a lot on at the minute. I think the last opinion I got was that they do not need attention

2) Have a look at the images of my inlet valves, you can see the concave faces, I think these need to be replaced but you may know better
Cheers Dave Fitz'

Offline Paul Marx

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Re: SF750 valves and seats
« Reply #1 on: September 17, 2020, 16:21 »
Seats don't need changing for lead free, however, your seats and valves may need changing because of wear.
Not being an expert, no doubt expert advice will be along.

Paul

Offline sfcpiet

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Re: SF750 valves and seats
« Reply #2 on: September 17, 2020, 19:37 »
All depends if you want max flow and power, or if you're willing to sacrifice just a little performance.

If the seats are freshly cut into the cast iron skull to specs for max performance, ie, narrow seating area >1.2mm, these will quickly recede and valve clearances will close up.  This will continue until the seats attain a certain work hardness, but they will be considerably wider by then...  If the seats are cut to 1.8mm width or more, specific pressure is a lot less and they will cope nicely with modern fuels with very little initial recession taking place.  Wide seats are detrimental to flow, but do aid heat transfer from the valve.  Pros and cons in both directions.

Valve seat inserts of course manage to get around all this, seats can be cut as narrow as you'd like, with nothing much changing over the course of a few 10000 kms.

Valves manage quite OK, it's just the cast iron skull that's a bit soft.

Your valves look like they'd come out nice with a re-grind.

piet
180s feel quick, 120s are...      If it ain't broke, fix it till it is.

"A motorcycle is a bicycle with a pandemonium attachment and is designed for the especial use of mechanical geniuses, daredevils and lunatics"  Georg Fitch 1916

Offline BOETJE

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Re: SF750 valves and seats
« Reply #3 on: September 17, 2020, 21:45 »
Has anybody ever bored out the seat area and shrunk in new seats of a different
material in those 750 heads ?

Offline sfcpiet

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Re: SF750 valves and seats
« Reply #4 on: September 17, 2020, 22:01 »
Has anybody ever bored out the seat area and shrunk in new seats of a different
material in those 750 heads ?

Yes.

piet
180s feel quick, 120s are...      If it ain't broke, fix it till it is.

"A motorcycle is a bicycle with a pandemonium attachment and is designed for the especial use of mechanical geniuses, daredevils and lunatics"  Georg Fitch 1916

Offline Piranha Brother 2

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Re: SF750 valves and seats
« Reply #5 on: September 18, 2020, 05:38 »
Has anybody ever bored out the seat area and shrunk in new seats of a different
material in those 750 heads ?

Me too - years ago - never had a prob with them.
What ... leave it standard??!!

Offline Dellortoman

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Re: SF750 valves and seats
« Reply #6 on: September 18, 2020, 06:11 »
Has anybody ever bored out the seat area and shrunk in new seats of a different
material in those 750 heads ?

No  ;)

Is this a survey?
Location: Tasmania, Approx 4253S 14723E

Offline BOETJE

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Re: SF750 valves and seats
« Reply #7 on: September 18, 2020, 08:50 »
No , no survey.   ;)
Just wondered if it is possible , rather than throwing the whole head away.
Wondered how thick the cast steel  wall thickness is, and, if the seat recess is bored , if it
then sits in the steel ,or partly in the Aluminium.
This has repercussions for the interference fit I could imagine .
Yeaaaaars ago I stuck 45 mm valves  a 750 motor ,and while cutting away, hoping that I wouldn't break trough the
steel....
« Last Edit: September 18, 2020, 08:57 by BOETJE »

Offline phil37

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Re: SF750 valves and seats
« Reply #8 on: September 18, 2020, 09:13 »
Quote
Wondered how thick the cast steel  wall thickness is, and, if the seat recess is bored , if it

See photo - Regards - Phil

Offline Fitz

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Re: SF750 valves and seats
« Reply #9 on: September 18, 2020, 13:22 »
Thanks all, I will spend a little more time with my lapping stick and see what I can acheive, only Phil would cut a section through a head, but I can see the marks where you were looking to improve gas flow, very helpful as usual thanks gents, have a good weekend
Cheers Dave Fitz'

Offline Tippie

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Re: SF750 valves and seats
« Reply #10 on: September 18, 2020, 16:01 »
On my street motor which had ended up with a very wide seat contact area I just trimmed the outer and inner cuts to produce a desired width contact area where I wanted it on the valve face, but did not touch the 45 degree face cut, which only needed a light lap. I don't have access to a valve facer and don't think the slight concave is going to rob much power. It has done around ten thousand km since and the tappets haven't gone out of adjustment, bike goes great.
SF2 17483 (race)
SF2 17424 (road)
An Australian living near Oslo in Norway

Offline BOETJE

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Re: SF750 valves and seats
« Reply #11 on: September 18, 2020, 22:05 »
COOL !
Thanks for that picture !

Offline Fitz

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Re: SF750 valves and seats
« Reply #12 on: September 28, 2020, 10:20 »
Thanks all, very helpful as usual
Cheers Dave Fitz'