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T120 Cylinder Head Thickness Unit Engines
#789466 11/07/19 11:35 pm
Joined: Nov 2012
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Denis J Offline OP
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I searched and found the thickness specifications for a T140 head, but nothing regarding the T120 head. I vaguely remember they are the same..please correct me if I am wrong.

I am specifically looking for the cylinder head thickness (rocker surface to cylinder) and a minimum thickness if there is one.

I have a head here that has seen a number of services. My measurements are 0.20mm different cylinder 1 to 2. The head has apparently been cut/skimmed without regard to being square to the rocker surfaces. Cylinder one side 70.8mm.....Cylinder 2 side 70.6mm

I am hoping to cut it square if I have enough material, and then follow up with head and base gaskets of the appropriate thicknesses.

Thank you for your help!!


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Re: T120 Cylinder Head Thickness Unit Engines
Denis J #789480 11/08/19 4:41 am
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Hi Dennis, In my experience than non parallel is not out of line. Triumph didn’t seem to worry about it. With push rods it’s not critical. I’m away from my notes until Sunday. I can let you know the measurements on the original 69 Bonnie head, never skimmed. Both it & my ‘73 Tiger head is tapered. Yet both top & bottom surfaces are flat.

If both are flat I probably would leave it.

What will cause problems is a dip in head surface between bolt holes.
Copper gasket has very little crush. Maybe only .001” so it cannot fill dips.

Some gentle bowing can be pulled flat sometimes, but for perfect results a truly flat surface is needed.

Extra thick head gaskets are made & can help compensate for skimming heads. They do not tend to leak worse than normal thickness. PRT seal crush is effected by skimming & head gasket thickness so keep that in mind. Squish band of piston is effected also, as is rocket are geometry.

Seems nothing is simple on a Triumph one you leave factory new parts.
Don

Last edited by TR7RVMan; 11/08/19 4:44 am. Reason: Changed sentence

1973 Tiger 750
Re: T120 Cylinder Head Thickness Unit Engines
Denis J #789489 11/08/19 11:40 am
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I bought a brand new TR7 head recently and measured it thoroughly. It was about ad out of parallel as yours. No idea why they came out like this - you'd think that they'd have done the head gasket & rocker box gasket facing on the same machine, just turning the workpiece over, in which case they must have routinely not bothered brushing the swarf away. Otherwise, they should have come out parallel automatically.


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Re: T120 Cylinder Head Thickness Unit Engines
Denis J #789505 11/08/19 4:12 pm
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Somewhere close to 71 mm is normal, it's also normal to have a lightly wedge shape head.
On a TR7 I had, the timing chest wasn't parallell with the center line. I haven't checked any other timing chests, and I don't care.
Nothing surprises me with a Triumph anymore, but they usually go fast anyway. That's the beauty of them.

Re: T120 Cylinder Head Thickness Unit Engines
Stein Roger #789518 11/08/19 6:40 pm
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I've never thought to check the timing cover / crankcase centreline truth. Amazing that this was out, but as long as it works, it doesn't matter I suppose!


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Re: T120 Cylinder Head Thickness Unit Engines
Denis J #789520 11/08/19 6:46 pm
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It is the crankcase mouth not parallel with the center line of the crankshaft which is most bothersome.

Re: T120 Cylinder Head Thickness Unit Engines
John Healy #789579 11/09/19 12:07 pm
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Originally Posted by John Healy
It is the crankcase mouth not parallel with the center line of the crankshaft which is most bothersome.

Yes and it's a very real problem to rectify for the average enthusiast. If anyone has a DIY trick that doesn't involve expensive tooling, I'd be happy to learn about it...

SR

Re: T120 Cylinder Head Thickness Unit Engines
Denis J #789854 11/11/19 8:03 pm
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Hi Denis, On the '69 Bonnie head the thick point is 2.799", the thin is 2.790".

Thick in front by .009". Side to side taper is .003". This all the inner 4 large bolts. Center bolt is 2.318".

The outside bolts don't really count. Outside bolts are thickest 2.786", thinnest 2.780". outside bolts are thinner in front. Just the opposite of the rocker box surface bolts.

This is about how my '73 Tiger is also. I'll have to dig deeper in log to find it though. Both my Tiger & the '69 work perfectly in real life. No leaks or rocker, valve issues.
Don


1973 Tiger 750

Moderated by  John Healy 

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