Titanium pushrods

[pete roper]

The reason why different pushrod materials require different clearances is that the differences in the coefficients of expansion of the two different materials will alter the requirements for a cold clearance.

 

The whole point of having a clearance is to ensure that when the engine reaches operating temperature there is only a minimal clearance in the valvetrain to a.) prevent hammering of the cam flannks and assorted valvetrain components and b.) to ensure that there is still sufficient room when the fallower is on the cam's base circle to allow the ingress of cooling and lubricating oil in sufficient quantities for it to form a protective wedge when the follower starts to climb the opening flank.

 

Standard, early Guzzi big twin clearances were 8 thou and 8 thou. This was very, very conservative but the bikes were designed to sit idling in heavy traffic for long periods so a healthy safety margin was built in. I know from my own experiences of sitting in gridlocked Milan traffic in summer that the valves on an older Tonti *will* go frighteningly quiet when the motor gets real hot! With the later models with the *hotter* B10 cam the clearances were upped to 9 and 10 thou. Why? I'm not sure but I can only imagine that the more aggresive nature of the cam would cause more heat losses due to friction and energy transformation, (kinetic to heat) so exspansion of components would be greater so more clearance would be required?

 

Later models the clearances have been closed substantially by the factory. This is to a great degree dictated by noise regs but also I'd guess that the factory just doesn't need to be so conservative when it suspects that few of it's engines will ever get as hot as they used to when they had fat coppers sittingh on them in traffic! The fact remains that the materials and construction of the entire valvetrain has changed very, very little in 40 years, the motor still uses alloy pushrods running forged rockers on one end and chilled cast iron followers on the other. The newer cam profiles are not enormously more aggressive than the older ones, although they are a bit less *lawnmowery* .

 

On my *near stock* engines in the Convert and the SP I run 8 and 8 thou, why? Because it's easy, it doesn't damage anything and I coudn't give a fat rats arse if they sound like two cheesegraters fornicating in an iron tank. Using the ChroMo rods in first the hot-rod and now the racer I used 2 thou inlet and exhaust because this was what I was told to by Amadeo who I bought 'em off. ChroMo Steel has a much lower C of E than the alloy munt of the barrels and head etc so the clearances have to be smaller, (Shrug.) S'obvious really. Incidentally one of the *best* things about the ChroMo rods was the noise they made!~ When you got the engine over about 7,750RPM up to *When you stopped wringing it's neck* the pushrods would make a sort of unholy shrieking like a bandsaw cutting through mild steel Yum, Yum!!! Dunno if they really helped all that much but the sound made it worth it, like a Merlin with the charger on high boost on a strafing run! On the hot-rod I've reverted to standard Guzzi pushrods, (Albeit hand picked from my selection of pre '83 stock and I'm now running the clearances on that at 5 and 7 thou, with a P3 cam, (very similar to the V11 cam BTW.) with no ill effects, a bit more duration and slightly less noise. Although that is hard to judge as the hot-rod is a noise abatement society's worst nighmare made metal, it is a rather antisocial motorbike .

 

As to the merits of titanium? Well, it's got toss-pot factor to burn but it isn't as durable and generally components made out of it have to be bulkier so the weight savings aren't as great as they might be. We were thinking about some Ti Arrow rods for the racer but the costs and the service intervals soon put paid to that, they were an expensive joke! To be honest I'd suggest that there are probably a zillion and one other things you could more sensibly spend your money on, lets face it, if Ti pushrods were the dog's bollocks then all the people who used to race Guzzis seriously 15 to 20 years ago would of used them. To the best of my knowledge nobody did, at least nobody successfull.

 

If you want real toss-pot factor get some carbon fibre pushrods made up! These expand much less than the alloy and you actually have to set them with negative clearance when cold so you start the motor and warm it up trying to burn your valves out! Well smart!!!! They are also horribly prone to greenstick fractures and like anything else made of carbon fiber are no good for anything apart from being used as expensive firewood, just MHO of course.

 

Pete.

 

(PS. If you want to set the clearances hot you can. get the motor stinkin' hot then whip the covers off and set everything at 2 thou as quick as you can. Personally I wouldn't bother but you can if you like.)

[Steve G.]

My brother got titanium rods for his Norton, seems to me they were $200us for 4. I got chrome moli, can't really notice the difference. The main advantage would be in extreme cam profiles, where lightening the valve train for quicker actuation is the goal.

Ciao, Steve G.

[Guest Bruce]

When I was in school I worked for the local automotive guru. His approach was to set valves hot, raging hot. He looked for just a barely perceptable amount of movement and he's theory was you want the minimum amount of clearance but so the valves fully closed. I've always wondered how well the hot, static method worked at higher RPMs.

 

I've also seen how quickly less than fully closed valves can burn so I run the "world settings" on mine and the engine seems happy across the range.