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Valve to deck clearance

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Valve to deck clearance
#1
  This topic is about my 1954 MG TF
Ctwidle Chris Twidle
Reesville, Queensland, Australia   AUS
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1954 MG TF
When XPAG 31921 was taken apart it was found that the head had ‘lost’ a lot of height sometime in the past and was now slightly thinner than the recommended height for a stage two tune. The big worry was that the exhaust valves might now hit the deck. The cam came back yesterday after being reground to standard spec and I couldn’t wait to install it and do a dummy rebuild with the old head gasket which was about .040 thick. The images show an exhaust valve biting into a piece of solder as it opened. The calipers show the resulting neck being measured at just over .9mm across all 4 valves. The opinion I got from a machinist is that .5mm clearance should be sufficient. Anybody disagree?
I realise I may still need to sort out the rocker geometry and work out what has happened to the CR (its +.030 on the bore) but it looks like I might not have to recess the head or have a thick gasket made.
Comments appreciated.
Chris


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crankjournal Avatar
crankjournal Platinum AdvertiserAdvertiser Len Fanelli
Yonkers, NY, USA   USA
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1953 MG TD
Chris, you have enough clearance from the exhaust valve to the block, however your compression ratio will be too high for a stock duration cam.This will result in detonation> broken pistons & broken piston rings, & cylinder wall damage. on XPAG engines detonation is inaudible IMO. Leave the rocker geometry alone. Sell the reground cam and buy a state of the art longer duration, high lift cam. Ask the cam grinder for the duration at .050" lifter rise.This will require fly cutting the block, the deck height is more than .220" thick so fly cutting for more clearance is not a problem. What is the height of the head from the head gasket surface to the valve cover surface?
Len Fanelli
Abingdon Performance LTD.
Static Compression Ratio
(SCR) Intake Valve Duration
(degrees @ .050" lift) Power Range
(RPM)
8.00:1 185º Idle-4,000
8.25:1 189º Idle-4300
8.50:1 194º 800-4,500
8.75:1 200º 900-4,600
9.00:1 204º 1,000-4,600
9.25:1 208º 1,200-5,200
9.50:1 212º 1,600-5,400
9.75:1 216º 1,800-5,600
10.00:1 221º 2,000-5,800
10.25:1 227º 2,400-6,200
10.50:1 233º 2,800-6,400
10.75:1 236º 3,000-6,800
11.00:1 240º 3,200-7,000
11.50:1 244º 3,400-7,200
12.00:1 248º 3,600-7,400


Member Services:
MG XPAG Crane camshafts, roller lifter & flat lifter CSI ignition distributors, Roller rocker arms Swirl polished MG T valves, Positive stop Teflon valve stem seals. Safety Faster! Len Fanelli Abingdon Performance Ltd. laf48@aol.com 914 420 8699

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hurst280 Avatar
hurst280 Sean D
Plano, TX, USA   USA
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1954 MG TF "Yellow Bird"
I ran into similar problems after the machine shop accidentally removed >125mil from my head that was already shaved. I also planned to use a crane fast road cam with +0.080 overbore the cylinders.

To keep 8.5CR and 0.060" valve binding clearance I had to do the following:
1. But new head & send it to Sean brown for p&p
2. Made a Excel spreadsheet to exactly calculate how much to shave the head to get 8.55CR
3. Notch the head so exhaust valves have 0.060 clearance (you can do this yourself very easily with fantastic results with a simple homemade tool)
4. XPAG valve springs could not support the extra travel with the cam and P&p head, so I switched to MGB springs and had a friend mill custom made spacers to get the right spring tension.
5.buy a 123ignition Bluetooth programmable distributor to customize advance curve for the modified engine to get the most performance and efficiency I could.

Len is right, you can't hear detonation in XPAG, it is at 10kHz, but you can detect detonation as vibrations in gearshift, hood frame, etc, if the detonation is really bad. Ive built a prototype knock detector to find exactly where the engine begins to knock at different RPM to find the most aggressive advance curve I could.

I haven't had time to get it Dyno tested yet, but I am happy with the car's new performance. I can keep up with Dallas traffic (which I could not do before), and have sufficient torque pull away from traffic in 4th gear. I don't know what performance was like on an original car to make a comparison, but I suspect the engine output is likely to be much higher now. The engine revs easily now, kind of like a Honda ,and this had lead to smooth gear shifting without grinding in second. ...if this makes any sense.



Patience gets you nowhere fast!

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AZTD Avatar
AZTD Mike Grogan
Glendale, AZ, USA   USA
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1953 MG TD
1953 MG TD
2020 Jeep Grand Cherokee "Big Black"
Sean,

"Ive built a prototype knock detector to find exactly where the engine begins to knock at different RPM to find the most aggressive advance curve I could"

Could you explain a bit more? Any pics?



1953 MG TD TD23816 Sold to insurance

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