Lash cap leaving the exhaust valve

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Stefan Ingi Jonsson
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Lash cap leaving the exhaust valve

Inlägg av Stefan Ingi Jonsson »

I was at the first drift practise this season and when the motor got to the rev limiter the first time the lash cap jumped of the exhaust valve and got lodged between the valve and lifter.

Do you have any idea other then valve float that would cause this to happen?

Im using KLR-S/T5 and double valve springs from kl racing, unfortunetely I dont remember how much the springs are compressed.
quattro
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Re: Lash cap leaving the exhaust valve

Inlägg av quattro »

I would say there are several possible reasons.
If you're using any kind of ALS to keep the turbo spinning you will get pressure spikes in the exhaust manifold that might make exhaust valves bounce slightly off their seats.
If your valve springs are on the marginal side hitting the rev limiter might be enough. It depends on how abruptly the rev limiter cuts in and out.
It might even be enough with a few misfires.
Another possibility could be if you have relatively high exhaust back pressure. It will negatively affect the proper seating of the exhaust valve and could create valve float at lower revs then expected.

I prefer to stay away from lash cap on a competition engine.
bmw2101
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Re: Lash cap leaving the exhaust valve

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quat, if you dont use lash caps, what do you use?


thanks
quattro
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Re: Lash cap leaving the exhaust valve

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Basically I make sure all my camshafts have the same base diameter and build my engine's valvetrain to that spec. When buying a camshaft I always order it to be machined to my base circle diameter spec.
When building an engine I use ordinary bucket shims if possible and if not I adjust the thickness of the buckets (the internal tap) or the length of the valve shafts by removing material from them.
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Stefan Ingi Jonsson
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Re: Lash cap leaving the exhaust valve

Inlägg av Stefan Ingi Jonsson »

I found the problem I think,
The valve guides are severly worn and there was guide material on the valve, I guess it is my mistake because I used iron guides for stainless valves.

is it possible that the valve got seized in the guide?

I found another strange thing, I measured the lift of my camshaft and it was 12.25 on all lobes but the KLR t5 cam should be 12.9 or maybe I measured it wrong somehow.
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Re: Lash cap leaving the exhaust valve

Inlägg av bmw2101 »

is the lack of lift on the valve because you need to have the clearance between cam and tappet when cold? I recently found in my engine a guide to be very sloppy after maybe 300 miles. We think its because i had a sticking injector and its washed oil from the guide.



Quat, oh i see, you use the standard bucket if you can, but if you are using a more wild cam you use the bucket and shim type set up? I have the t5 cam in my engine, i used the bucket and shim type.
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Re: Lash cap leaving the exhaust valve

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At the moment I have 3 different cams with the same base diameter for the same engine - a 10 valve 5 cyl Audi. They are ground to 250, 260 and 290 degrees duration respectively, at actual zero lift 0.30 mm running clearance. They all share exactly the same base circle diameter.

Having set the base circle starting with relatively mild cams you will probably end up with a smaller base circle if you would go over 300 degrees. In that situation I would get new after market steel bucket lifters that you can machine internally to achieve the desired clearance without any need for a shim. Then, if you would test a different cam in the same region I would order it with the same base circle and not change the buckets.
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Re: Lash cap leaving the exhaust valve

Inlägg av bmw2101 »

right ok! [red wine area uk time for me atm ]


a friend of mine has a bmw s14 engine, it spat the shim off the top of the buckets and did the engine in. He thought it was of the top of hat shims.

The T5 cam i use, and Stefan, i read somewhere, again red wine non making memory work well, someone measures the degs of lift etc and angle separation, and im sure they said the point of lift its measured at and the normal measuring point makes the cam actually like 280-290 degs, so not actually that wild.

in my car, a n/a engine, for rallying, this is a good camshaft. Pulls well low down, and goes wild up top.
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Re: Lash cap leaving the exhaust valve

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OK.

Standardizing a good design makes life easier. With steel bucket lifters with no shims, there's nothing that can come lose.
Cams of the same base diameter enables you to change cams easily, safely and quickly for testing. Even for different events if there's a need for it. No need to adjust anything. Once you've found out where the engine prefers a specific cam and the corresponding ignition/fuel tables to be at, with a programmable ECU and standardized cam base circle and no shims involved you can switch cams and the programming to suit in less than an hour. Depending on the engine, of course.
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Re: Lash cap leaving the exhaust valve

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right thanks! makes sense!
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