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Gary
Gary, ASE Certified Technician
Category: Car
Satisfied Customers: 743
Experience:  Senior Technician
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1975 308 Gt4 Dino with 46,000 miles on her and am doing a

Customer Question

1975 308 Gt4 Dino with 46,000 miles on her and am doing a valve adjustment because my wife told me there were flames coming out of the exhaust when I had the car opened up (High revs with peddle to the metal between gears) on a stretch of road. Does the valve specification for intake 0.25mm-0.30mm and exhaust 0.30mm-0.35mm valve clearance increase in gap (making it looser) or reduce in gap (making the gap closer to the tight side) when becoming out of spec? Should I wish to be on the on the tighter or looser side of the clearance specification? Everything is within range, except 1 and 4 exhaust on the front side that are 0.36mm. Also, If I had a 4.05 shim in and it reads with a micrometer a 0.05mm wear when measure in the middle of the shim where the cam makes contact for a 4.00mm reading, shouldn't I just replace it with a new 4.05mm shim to reduce the gap from 0.36mm to 0.31mm? Lastly, does the valve clearance explain the flames from the exhaust or is it a timing issue?
Submitted: 5 months ago.
Category: Car
Expert:  Gary replied 5 months ago.

Hi Patrick,

The valves you have mentioned are only just outside spec.

You can do as you have mentioned.

The valve clearances are not your cause though.

I would be inspecting the ignition system and the carburettors.

Multi Webered engines are very prone to these phenomena.

Gary

Customer: replied 5 months ago.
Ok, so is the gap between the cam and shim supposed to get smaller or larger? I've added a pic of all the dims
Customer: replied 5 months ago.
Also, my cam lift calls for 8.24 mm, but when I put a digital on it, I only get 7.92mm. Thoughts?
Customer: replied 5 months ago.
on the exhaust side
Expert:  Gary replied 5 months ago.

A thicker shim makes the clearance smaller.

So, if you measure .35, and you want .30, you add .05 to the current measured shim thickness.

Then I would measure them all again.

Your cam of 8.24 - .32 would give you 7.92 valve lift.

Any cam lift specs for shim/bucket DOHC engines will have to take valve clearance away for true valve lift.

Did you measure this with a dial indicator at the shim/bucket?

Gary

Customer: replied 5 months ago.
Gary, I measured the cam lift from a dial/digital gauge directly on the valve shim as it was being depressed by the cam while I rotated the engine manually.
My main question was about the range of gap specification. Do I want to be on the larger gap or thinner gap side of the 0.30mm-0.35mm side. I've read that in some cases the gap is decreased due to valve wear, so the shim gets closer to the cam and the gap is reduced and becomes tighter. However, in my case, I'm seeing the gap increasing, so I'm not sure if I should want to be on the larger or smaller end of the spectrum. Meaning, closer to 3.0mm or closer to the 3.5mm end of the range. If closer to the 3.5mm and that the gap will reduce due to valve wear, then I shouldn't wish to do anything woth these two exhaust reading that are 3.6mm
Expert:  Gary replied 5 months ago.

I would shoot for the middle of the range for a customers car.

If it was my own I would shoot for close to minimum.

I would check my own more often though.

Valve faces and seats don't wear as bad these days, compared to the 50s to early 70s.

Technically the shim should wear at the same rate as the valve seat/face.

If the shim wears faster then the clearance gets larger.

I rarely see engines that have clearances that tighten up.

Gary

Customer: replied 5 months ago.
Given that this is a 1975 Ferrari 308 GT4 Dino, making it a 70's car. So are you suggesting that these valve seats may wear faster? if so, shouldn't that tighten the gap and indicate staying on the larger side? Not trying to be complicated, just want to be sure I understand you correctly.
Expert:  Gary replied 5 months ago.

That's why I would go for the middle of spec.

I run two of own cars at near minimum spec, since I'm chasing power.

I don't mind checking them every 1000kms.

I know it is hard to get say .33 as a target, just get them as close as you can to mid spec.

You can't go wrong doing it that way.

Too tight burns valve seats, too loose wears seats and shims, shims more than seats.

Rattling valvetrains also wear cam lobes, all things to think about.

That's why mid spec is safe.

Yours has DCNF Webers?

Gary

Expert:  Gary replied 5 months ago.

Hi Patrick,

How are going with your Ferrari?

I hope my advice has been of assistance to you.

Kind regards

Gary

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