Twin cam valve clearance issues

Road-race engines and ancillaries - general discussion
Post Reply
Lee67
Posts: 30
Joined: January 8th, 2015, 9:47 pm
Contact:

Twin cam valve clearance issues

Post by Lee67 » May 18th, 2019, 11:14 am

Good morning all,

I've got an issue with setting valve clearances on my Strada 105TC head which is fitted with high lift cams.

The manufacturers figure for valve clearances is 0.014" (0.35mm) for inlet and exhaust.

The problem I have is that I cannot get anything less than around 0.70mm without the cam locking up on full lift presumably due to the valve spring becoming fully compressed and binding. I'm using shims in the region of 4.65/4.7/4.75mm so right up towards the thicker end of the scale, if I use thinner shims then the problem cures itself but obviously then the clearances are even larger. The best I can manage is a clearance of 0.7mm before getting binding issues, therefore the valves are opening fully but the clearance is double the figure that is recommended.

The head is off the car on the bench, the problem is not being caused by valves touching each other as I've checked several times.

Am I missing something here or is it just a case of putting up with large clearances to enable the cams to give full lift and not lock up?

Thanks in advance for any info here
Lee
GC_106

Guy Croft
Site Admin
Posts: 5026
Joined: June 18th, 2006, 9:31 am
Location: Lincoln, UK
Contact:

Re: Twin cam valve clearance issues

Post by Guy Croft » May 19th, 2019, 2:37 pm

and what is the quoted cam or valve lift with this new cam?

G
Guy Croft, owner

Lee67
Posts: 30
Joined: January 8th, 2015, 9:47 pm
Contact:

Re: Twin cam valve clearance issues

Post by Lee67 » May 19th, 2019, 4:13 pm

Hello Guy,

Spec for the cams is as follows...

Valve timing - 42/82 82/42
Degrees Duration - 304
Lift MM - 10.6
Degrees Overlap - 84
Valve Clearance Inches - .014
GC_106

Guy Croft
Site Admin
Posts: 5026
Joined: June 18th, 2006, 9:31 am
Location: Lincoln, UK
Contact:

Re: Twin cam valve clearance issues

Post by Guy Croft » May 20th, 2019, 10:43 am

I revisited some measurements

- standard Fiat TC springs coil bind ('box') at around 11.6mm lift based on standard valve tip height of around 40.5mm (tip to the alloy platform on the head) so I don't see how yours could be binding with those cams..

coils must not close up up so you should allow at least 1mm between closest coils at full lift which means that the max lift on std springs should not be more than 10.6mm. I'm not necessarily saying that std springs are the right choice for those cams however...

something a bit odd here that I cannot quite put my finger on. I assume you did NOT try to shim it on-engine?

Let me know..

G
Guy Croft, owner

Lee67
Posts: 30
Joined: January 8th, 2015, 9:47 pm
Contact:

Re: Twin cam valve clearance issues

Post by Lee67 » May 20th, 2019, 7:48 pm

Hi Guy

No, all set up is being done with the head off.

I'm thinking now that maybe I should refit the standard cam and see whether I can get within tolerance or failing that I do have several spare cam boxes so maybe I should try this set up with another cam box and see whether I can get within tolerances.

I'm wondering whether the cam box has possibly been skimmed at some point therefore throwing everything out?? I can't see why this may have been done but anything is possible!

I'm using cam boxes that came with the engine that I'm building but the head is from stock and has not been used on the engine so far.

I'm certain that all the common cam boxes are interchangeable across the twin cam engine range - or aren't they?

Lee
GC_106

Guy Croft
Site Admin
Posts: 5026
Joined: June 18th, 2006, 9:31 am
Location: Lincoln, UK
Contact:

Re: Twin cam valve clearance issues

Post by Guy Croft » May 21st, 2019, 6:19 pm

machined camboxes would make the clearances tighter not looser, I doubt that as cause, tbh you'd best bring it to me at LN63QT and let me see it..

G
Guy Croft, owner

Lee67
Posts: 30
Joined: January 8th, 2015, 9:47 pm
Contact:

Re: Twin cam valve clearance issues

Post by Lee67 » May 22nd, 2019, 9:43 pm

Hello Guy,

Today I refitted the original standard cam and was easily able to set the valve clearances to the standard factory settings - no problem.

Before I do anything else I am going to make contact with the cam supplier and ensure that the figures supplied are indeed correct.

Once I've established that they are I'll be in touch .

Thanks for your help on this
Lee
GC_106

Guy Croft
Site Admin
Posts: 5026
Joined: June 18th, 2006, 9:31 am
Location: Lincoln, UK
Contact:

Re: Twin cam valve clearance issues

Post by Guy Croft » May 23rd, 2019, 11:48 am

Have a closer look at one of those competition cams:

1. Measure the distance from the base-circle to the lift radius (nose) with a calliper
2. Measure the cam lift with a dial gauge and subtract one from the other - that will give you the base-circle diameter
3. Do the same with a standard cam and compare.

Let me know what you see!

G
Guy Croft, owner

Lee67
Posts: 30
Joined: January 8th, 2015, 9:47 pm
Contact:

Re: Twin cam valve clearance issues

Post by Lee67 » May 24th, 2019, 1:26 pm

Hello Guy

Requested measurements are as follows:

Original used standard cam:
1: 35.5mm base circle to lobe nose
2: Lift measured with DTI 8.65mm, base circle dia 26.85mm

New unused performance cam:
1: 35.00mm base circle to lobe nose.
2: Lift measured with DTI 10.55mm, base circle dia 24.45mm

I then took a different approach to calculate and got a different result - using a digital caliper I measured the base circle diameter of each cam as well as the base circle to lobe measurement I'd already got and used these figures to calculate lift.

Original used cam:
1: 35.5mm base circle to lobe nose, 26.35mm base circle dia, lift 9.15mm

New performance cam:
1: 35.00mm base circle to lobe nose, 24.90mm base circle dia, lift 10.10mm

I look forward to your thoughts whenever convenient...
Lee
GC_106

Guy Croft
Site Admin
Posts: 5026
Joined: June 18th, 2006, 9:31 am
Location: Lincoln, UK
Contact:

Re: Twin cam valve clearance issues

Post by Guy Croft » May 24th, 2019, 1:52 pm

Original used standard cam:
1: 35.5mm base circle to lobe nose
2: Lift measured with DTI 8.65mm, base circle dia 26.85mm

New unused performance cam:
1: 35.00mm base circle to lobe nose.
2: Lift measured with DTI 10.55mm, base circle dia 24.45mm

the above is correct, your later method is wrong, you cannot measure the base circle diameter directly by any other way.

We can see that the base circle on the comp cam is smaller than standard and should not inhibit shimming-up.

G
Guy Croft, owner

Lee67
Posts: 30
Joined: January 8th, 2015, 9:47 pm
Contact:

Re: Twin cam valve clearance issues

Post by Lee67 » May 27th, 2019, 7:49 pm

Thanks Guy,

I shall go over everything again in the next couple of days as soon as I get chance and see where I get to.
Thanks as always for your input

Lee
GC_106

Post Reply

Who is online

Users browsing this forum: No registered users and 0 guests