Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Has Anyone Turned down 80 Pistons?

Has Anyone Turned down 80 Pistons?

Moderators: Curt!, Pa

Post Mon Jan 09, 2012 7:17 pm

Posts: 1642
Location: Interlaken, NY USA

So Here's the question. I was playing around with the lathe the other day and chucked up an old HD 80 piston. Drilled a hole in the crown and measured its thickness. Roughly 3/8" inch. I was trying to see how light you can make one of these pistons, so I turned off 1/16" from the crown, which cut down 40gms of weight.
So, does anyone have any opinions as to whether this would dangerously weaken one of these things to the point of failure in use? By the way, the repop pistons from Ted are of similar construction, which is what I would do the actual surgery on to use in my new 88" stroker. I'm going to turn down the skirts some to shorten them to clear the flywheels for sure, so while I've got the lathe going, I'm wondering the feasibility in cutting the crown as well.
Thanks in advance,

Post Mon Jan 09, 2012 10:18 pm

Posts: 83
Panic says (80 performance book) you can remove this amount 1/16 from the top of the piston dome. Considering your measurements you'll be removing only 16% of the total thickness.
If you do so, for a 4 3/4 stroker you still have 0.1725 to compensate with stroker plate or pop up, or both.
What's you opinion on automotive pistons? Don't call me nutz but I think you can use toyota 2.0 car/trucks pistons from the seventies. They are flat topped and the std outside diamenter is 3.46mm or bit less than ULH + .030".
The compression distance is shorter at 1.53 and exactly you need to compensate the extra stroke. Again, don't call me nutz! :)
If you don't like toyota you can find many automotive pistons in the 87, 87.5, 88mm range, and they all have shorter compression distance and shorter skirts.

Post Tue Jan 10, 2012 6:33 am

Posts: 1642
Location: Interlaken, NY USA

You're not Nutz! I have no problem using whatever works to get the job done. I think highly of Toyota engineering. My Xbox has 180K miles and doesn't use a drop of oil.
However, I have the repop pistons already, plus I've already relieved the heads .040 when I stroked it to 84". I'm going for a little more popup and some stroker plate. If the stock manifold won't work, I'll make up a new one. I'm also intrigued about having custom pistons made. In another thread it was mentioned that Venolia will make pistons for 200.00 a pair. Just what the repops cost. So, I may go down that road next time I need pistons.
Thanks for the reply,

Post Tue Jan 10, 2012 8:15 pm

Posts: 3011
Location: Central Illinois, USA

I confess to milling out from the underside of many auto and import bike piston "domes" in the late Eighties.
Its what I was told to do and I did it.

The pistons have overkill designed into them, but if you start removing any of it, you shorten its fuse in the event of a temperature spike from other causes.

We now know how to avoid the traditional hole in the top of the piston with airtight manifolds.

But I am confused as why you would want to drop your compression for the sake of weight,
or did milling the heads down bring them too close?


Post Wed Jan 11, 2012 5:27 am

Posts: 1642
Location: Interlaken, NY USA

By increasing the stroke from 4 1/2 to 4 3/4, I'm going to have to deal with the tops of the pistons coming up further. I've machined .040 popup clearance pocket into the heads right now, with the pistons just coming about .010 proud of the top of the jugs at TDC. With the extra 1/8" to deal with, I'm looking at my options to deal with the extra protrusion, ie, stroker plates or more popup and a deeper pocket in the head to accommodate. My current thought is to take a little off the top of the pistons, and making use of those James steel/silicone? gaskets at the base. They're way thicker than traditional paper types. Might act as a mini stroker plate. I'm going to piece the engine together later this week to take some accurate measurements. I'm thinking you can probably have too much "popup", but don't know what that quantity might be.

Post Wed Jan 11, 2012 9:27 am

Posts: 766
Location: CA USA
I'm not sure what too much pop up would measure out as, I'm sure HD or Branch did lots of trial and error development on that subject. I just measured several sets of late WR heads and the average pop up is .088" deep. I no longer have any KH or KR stuff to measure, maybe one of the other folks out there could provide that measurement. Custom pistons might be the best bet. I don't know how much relief you have between your valves and the bore. My biggest concern would be where the top ring ends up in relation to that relief valley at TDC. Venolia will put the rings where you want them in relationship to the piston top and the wrist pin.

Post Wed Jan 11, 2012 11:40 am

Posts: 2677
Location: Los Angeles, CA
Don't forget that using stroker plates increases the distance between your intake ports. You could have manifold problems.

Post Wed Jan 11, 2012 7:52 pm

Posts: 1642
Location: Interlaken, NY USA

Right, it would be nice to know what the great ones ended up doing with popup. As for relief, I've got a fresh pair of cylinders to work with, with no relief at all. I did some preliminary measurements months ago with my ulh style relief cylinders, and it appeared that the rings weren't exposed, but it would be nice to know what a safe margin would be. This may well be an interim build, with the prospect of a set of new "indian" cylinders, reworked by Paul Freibus, down the road in a year or so. In which case, I would definitely look into custom pistons to try and get it all!
On the manifold, if I do run into problems, I'm prepared to weld up one to fit out of tubing for the time being. After building the individual runner one a few years ago, it should be a no brainer. :lol:

Post Wed Jan 11, 2012 10:40 pm

Posts: 83
Some info I found on pop up (most from this forum):
Late WR .0625" (corresponds to beachdog's measurements, considering gasket and squish)
Early KR .0625"
Later KR .125"
Mr. Jerry Branch's modified KR .1875"
80 performance book states up to .125" is OK for a popup. Also top ring must be at least .030 below the top cylinder surface or relief.

one more thing i just read in the same ring must be at least .030" below the top cylinder BUT preferably .060"
Last edited by helsingborg on Thu Jan 12, 2012 7:25 pm, edited 1 time in total.

Post Thu Jan 12, 2012 6:09 am

Posts: 1642
Location: Interlaken, NY USA

Thanks for the info, much appreciated! I'll try to do some accurate measurements over the weekend if I get a chance to do a mockup. The .030" is particularly important to know, as that looms as the most likely problem to crop up first.

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