Fait 124 Using Shims less Than 3.20mm

Road-race engines and ancillaries - general discussion
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mccamie
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Fait 124 Using Shims less Than 3.20mm

Post by mccamie » Sun May 12, 2013 4:03 pm

I had posted on the forum March 1, 2013 regarding a Fiat 1952cc twin cam bare head with an unidentifiable ID # that I got from an internet auction site. Never found what it came from!
I did find an excellent high performance machine shop in Millsboro DE, just a few miles from my home. They were able to machine the head with the oversize valves that I had purchased from Auto Ricambi along with (Isky) high performance valve spring set.
I am using a set of 1608 cams with original cam towers from a 197xs Fiat 124. I had them on my original 1800cc head and they fit fine. But I have found out that the oversize valves are longer than the original valves. The shim sizes I will need to get the proper clearances will range from 2.80mm to 3.30mm. Csaba @ Auto Ricambi contacted Guy Croft for me to get some info. Csaba let me know that the smallest shim I could use was 3.20mm. Csaba suggested I get the machine shop to cut the cam followers down.
This is what I have come up with. I need to cut the cam tower top current height of 3.0mm by 0.40mm leaving me a 2.6mm dept to hold the shim. I will need a clearance for the In/Ex from 0.41mm to 0.53mm. Using the 3to1 rule I should OK - the shims should not pop out.
I don’t want to have to buy a set of reground cams just to make it work.
I could use two sets of gaskets!!!

Your comments and thoughts are must welcome.
Camie

Urbancamo
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Re: Fait 124 Using Shims less Than 3.20mm

Post by Urbancamo » Sun May 12, 2013 7:28 pm

Maybe the most reasonable way is to grind down the valve tips to OE assembled height specs so that you could use shims in region of 4.xx millimeters. Just like factory have done. Of course if the valve tip is short, you can't grind it down infinitely - bucket must not sit over the retainer.
If this way works it also leaves you fair amount of clearance to minimum shim thickness too. Gasket thicknesses will vary and they might "settle" less or more. You never know and it's better have some clearance to live with.

Of course two cam box gaskets can be used too. Especially if you have adjustable cam pulleys to correct belt alignment issues caused by stacking two gaskets on top of each other.

Other comments are welcome.

T
GC_25

ace124
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Re: Fait 124 Using Shims less Than 3.20mm

Post by ace124 » Mon May 13, 2013 6:19 am

I had the opposite issue over the weekend when i fitted reduced base circle cams. I discovered that gaskets do vary in thickness substantially. I was lucky to get away with a thinner gasket which got my shim range 4-4.1mm. You can search for thicker gaskets or glue 2 together with a light coat of RTV. Personally id grind the valve tip taking into consideration what Tommy mentions about making sure the bucket sits on the tip and not retainers. Ive done this before and never experienced valve tip wear or bucket wear. Just make sure your cut is straight and polish the tips to mirror.
Fyi the gaskets i found were 0.6mm compressed compared to 1mm.

Guy Croft
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Re: Fait 124 Using Shims less Than 3.20mm

Post by Guy Croft » Mon May 13, 2013 12:48 pm

I have read the above threads.

My cambox gaskets, which I supply to Csaba at Autoricambi are substantially thicker than the old OE ones. What other types are on the market is of no great interest to me as some delaminate and leak oil!

You can use two of my gaskets stacked up but you should fit them dry not with RTV, they do in fact have an RTV bead on them anyway..

In my exp - even though the depth of the recess in the bucket is 3.0mm - if you run a shim of 3.25mm or less the camnose will strike the edge of the bucket. I don't feel disposed to check this (!) and yes, mayeb it depends on the cam profile...

In any event it is not good practice to go lower than about 3.35-3.40mm - or you won't be able to re-shim it safely. Allcambox lower gaskets settle so you have to leave them overnight and retorque and they also settle in service too and so clearances must be peridically checked & reset. That they settle (more so with 2 gaskets - 0.1mm or more overnight) is another good reason for keeping plenty in-hand when shimming...

G
Guy Croft, owner

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