Author Topic: Valve clearances - how critical is the tolerance?  (Read 416 times)

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fazersix

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Valve clearances - how critical is the tolerance?
« on: August 24, 2019, 10:13:03 PM »
Hi, I have finally installed 10548 cams on a 1981 2.0. The manual says specs for valve clearances are .40 - .45 inlet and .45 - .50 exhaust. I have not started the engine yet!
After installing the cams I have
       Inlet.      Exh
1.     .381.      .406
2.      .360.     .457
3.      .381.      .457
4.      .360.      .457

So how critical are the tolerances since my inlet are .02 and .04mm under spec and one exh is .05 under spec?
Do I really need to adjust these?

Thanks in advance

bonno

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Re: Valve clearances - how critical is the tolerance?
« Reply #1 on: August 24, 2019, 11:29:39 PM »
Too little valve clearance can result in poor performance (loss of compression) or a rough idle because the engine can't “breathe” normally and operate at peak efficiency. The designers have taken into account the different rate of expansion of the dissimilar metals and as such, the specified valve clearances  are critical in achieving this. 
now
83 Alfetta GTV
05 156 JTS manual
past
00 156 selespeed
72 1750 GTV

LukeC

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Re: Valve clearances - how critical is the tolerance?
« Reply #2 on: August 25, 2019, 09:06:58 AM »
Those clearances, while not within the Factory spec will be fine. OE Alfa cams have significant difference in diameter between the base circle and opening ramps. You can feel this with you finger... Not sure why Alfa did it this way, but I suspect it was to allow for non-maintenance and a very large safety margin in running too little clearance.

Most real performance cams for Alfas (10548 is not a performance cam but is better than the stock Alfetta cam Note: 10548 was used on late 116 cars) run clearances around .2 - .25 mm. I have run valve clearance on Alfas as low as .15 mm to get a quieter valve train with aftermarket performance cams. Some engines with similar design/material heads run this sort of clearance from factory.
Luke Clayton

qvae.com.au

fazersix

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Re: Valve clearances - how critical is the tolerance?
« Reply #3 on: August 25, 2019, 06:02:32 PM »
Thanks guys for your advice. I took it for a good run today and noticed it more responsive under 4000 whilst 4000 to 5000 it appears to rev more freely! A vast improvement over stock cams. I also change the valve stem seals, the black ones were hard and had partly disintegrated and now now big ball of smoke when I change over 4000!

NSharpley

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Re: Valve clearances - how critical is the tolerance?
« Reply #4 on: November 11, 2019, 01:29:34 PM »
You can make another adjusment with the 10548 cams ... I cannot remember exactly but I think you advance the inlet cam by 5 degrees for an earlier opening.

This also increases the overlap.
Current Alfa:
1975 Alfetta GT 2.0

Past Alfas:
1989 Alfa75 2.5ltr
1982 Alfetta Sedan 2.0
1976 Alfetta GT

sportiva

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Re: Valve clearances - how critical is the tolerance?
« Reply #5 on: November 11, 2019, 02:15:06 PM »
10548 cams were the standard cams before the 1975 ADR27a.

 https://www.infrastructure.gov.au/vehicles/design/files/27A.pdf

1053200's were fitted to Alfetta's and 116 Giulietta's so they could pass the 1975 ADR27a rules
10548 cams are not a performance cam but are a much better cam than the 1053200.
You can advance or retard each cam by 1.5-degree increments. If you advance or retard them to far you can bend the valves so always rotate the engine a least twice before starting it.
« Last Edit: November 11, 2019, 02:40:02 PM by sportiva »
3.0 GTV6 156JTS

75 2.5
2.0 Alfetta Sportiva #22
1.8 Alfetta
1.2 alfasud
2 75 parted out
15 alfettas parted
10 gtv parted
5 alfasuds parted
156 sele