Tuning GSR 285 cam confusion

[jamiewilson]

I know i'm just being stupid but wanted to check just to reassure myself...

I'm fitting a GSR 285 cam in a 1242, its a regrind so i've got top hat shims as i'll prob need to make up the clearances. I thought i knew what i was talking about until i read a thread about fitting a P75 cam where everyone was talking about using P75 clearances,

so just to confirm, i want inlet = 0.4mm and exhaust = 0.5mm +- 0.05mm ???

i measured the clearances of my current cam, starting from the pulley end:

1: 0.47mm
2: 0.45mm
3: 0.45mm
4: 0.4mm
5: 0.35mm
6: 0.5mm
7: 0.42mm
8: 0.45mm

I understand that as it wears the clearances get smaller, which makes some of these a bit odd unless they were incorrectly shimmed at some point?

Thanks

Jamie

 

[arc]

i would ask peter what clearances to use

 

[randomizered]

I've asked for cam specs, with no answer

 

[arc]

but if he's sold the cam, then surely he supply the info needed to fit the thing !!

 

[jamie86]

but if he's sold the cam, then surely he supply the info needed to fit the thing !!

Yeah you would imagine so

Is it a similar spec to a 75 cam

 

[Oldschool]

I've asked for cam specs, with no answer

285 degrees

Lobe centres identical

 

[balidey]

I understand that as it wears the clearances get smaller, which makes some of these a bit odd unless they were incorrectly shimmed at some point?

Nope, as a shim wears the gap will get BIGGER

 

[jamiewilson]

285 degrees

Lobe centres identical

Does that mean i use the standard clearances???

Nope, as a shim wears the gap will get BIGGER

I thought the gap would get bigger as the shims wear, it makes sense, but then i read in some thread that it gets smaller! But i have no idea how, and cant find the tread now!

Cheers

Jamie

 

[craigb244]

depends what wears. its usually the seat that wears causing the clearance to shrink. a gap getting bigger might be from the cam itself wearing but this is not as quick

 

[fingers99]

Nope, as a shim wears the gap will get BIGGER

Nope. In practice wear on the shim itself is minimal unless something is extremely wrong. Wear takes place mainly at the valve seats.

You're unlikely -- in the extreme -- to need top hats, unless you're going for a shim under bucket configuration (a la Alfa), which is superior for high performance engines, but a PITA for maintenance.

 

[Oldschool]

Does that mean i use the standard clearances???

0.3mm +- 0.05mm for inlet
0.4mm +- 0.05mm for exhaust.

 

[balidey]

Nope. In practice wear on the shim itself is minimal unless something is extremely wrong. Wear takes place mainly at the valve seats.

You're unlikely -- in the extreme -- to need top hats, unless you're going for a shim under bucket configuration (a la Alfa), which is superior for high performance engines, but a PITA for maintenance.

OK, i didn't word that well, what I meant was any wear on the valve train will mean gaps increase, not decrease. And the last set of cams I took out of an RV8 were almost round god knows how little surface hardening Rover put on them

 

[Pep0]

Every thing can wear, however if the clearance is too small the valve faces i.e. the exhaust valve faces a,d head inserts will be eroded.

If the clearances are too large then the can will hit the shim too hard and drop the valve from too large an opening.

If the oil is neglected or insufficient the lobes will vear and the shaft bearings.

If there is lots of grit in the engine the cam bearings and bushes will wear.

Check every time the cover is off, replace the oil and filter regularly.

Noel