Topic: H/A 900 kit compression ratio and cam options

[Turbogrimace]

I'm piecing together a 67 mm Henry Abe kit to run with a ported head with oversized intake valves. The pistons are flat and the valve pockets are small. They basically look like oversized stock pistons.

Does anyone know what the compression ratio was on these old pistons? And does anyone have any suggestions for cams? I'd like to maintain torque down low but also make good use of the headwork.

I'm not married to the 900 kit. If I can get similar output from something else, then that works too.  If the HA 900 kit is around 8.5 to 1 CR, how does that compare to an 836 with 10.5 to 1 CR? I would think the greater displacement of the 900 would still make more torque down low. But the cam can change that quite a bit.

Input on any of this is greatly appreciated!

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[PeWe]

A higher lifting cam need deeper pockets. Both pistons and yours

More duration brings more overlap so higher compression is needed.

[Jerry Rxman Griffin aka MuthaF'er]

I had the Megacycle 125-75 in mine. No problem with valve interference. Both the head and jugs have been decked too. I "expect" the CR is more along the lines of 10.5 but have never seen a number.

[PeWe]

I had the Megacycle 125-75 in mine. No problem with valve interference. Both the head and jugs have been decked too. I "expect" the CR is more along the lines of 10.5 but have never seen a number.

Same type of HA pistons? There is another model with deeper pockets and crowns that look to bump the compression..

I have an old set of the flat pistons with almost no pockets as stock. 67.5mm.

[Jerry Rxman Griffin aka MuthaF'er]

I had the Megacycle 125-75 in mine. No problem with valve interference. Both the head and jugs have been decked too. I "expect" the CR is more along the lines of 10.5 but have never seen a number.

Same type of HA pistons? There is another model with deeper pockets and crowns that look to bump the compression..

I have an old set of the flat pistons with almost no pockets as stock. 67.5mm.

Yeah, mine are the later version (I think) without the full skirt. 67mm. Crown sounds the same.

[Turbogrimace]

I had the Megacycle 125-75 in mine. No problem with valve interference. Both the head and jugs have been decked too. I "expect" the CR is more along the lines of 10.5 but have never seen a number.

Same type of HA pistons? There is another model with deeper pockets and crowns that look to bump the compression..

I have an old set of the flat pistons with almost no pockets as stock. 67.5mm.

Yeah, mine are the later version (I think) without the full skirt. 67mm. Crown sounds the same.

I found some other info on here that suggested these flat top pistons had stock compression. Does the surface and pocket of your pistons look the same as the image I posted in the first post?

If this 900 kit has stock compression and is limited to mild cams, I'm wondering if a higher compression 836 and street cam would yield more of my intended results. Or would the 900 still create more torque toward the bottom end, even with a mild cam?

[Old Scrambler]

To remain pump-gas safe, I would stay with the lower compression 900 kit but consider deleting the base-gasket for cheap 5-thou compression boost. I would also consider running a smaller rear sprocket for current Interstate speeds at 4000-rpm.

[PeWe]

The limitation is no case bore for bigger sleeves?
970cc give good torque with a ported head + Megacycle 125-75 cam.
Now is the need of stronger rods in focus.

Stock rods and better power ends at 836cc, ported head + good cam.

[scottly]

How does the compression height of the 900 piston compare with a stock flat top piston? Mount both pistons on a piston pin and compare the heights; if they are the same, the compression will be higher due to a larger bore compressing into the same size chamber.

[Don R]

 Good point ^.

[Turbogrimace]

How does the compression height of the 900 piston compare with a stock flat top piston? Mount both pistons on a piston pin and compare the heights; if they are the same, the compression will be higher due to a larger bore compressing into the same size chamber.

Excellent point. With the pin in, they're the same height. So perhaps it's up around 10. I'd have to do some math to figure it out.

Now for the next issue. The valve reliefs are petite. Can I run a mild street cam without bumping in to things? Does anyone know what the max lift is with stock pistons? My cylinders will be decked and I'm pretty sure the head is as well. I'll need to do some mockup once I decide on a cam.

[Turbogrimace]

Actually I take back my statement on similarity. I went and checked again and noticed that the shoulder on the 90 is a tiny bit lower than the stock piston. Other than that, they're the same height. The 900 piston has more of a dome than the stockers. I wonder how much this difference affects any comparison of compression ratio. Now how to calculate.

[seanbarney41]

You just about have to mock up and cc the combination with a burette to determine compression ratio...but actual cranking compression numbers are what is really important and that all changes based off of amount of cam overlap...the more compression, the more overlap you need to be streetable but the more overlap the more you lose bottom end, big displacement makes for excessive bottom end so you can afford to lose some with the overlap of a big cam...it's just all a balancing act and everything is a trade-off.

[Turbogrimace]

I follow you that dynamic compression is what really matters, but know the static compression helps determine the specs for everything else.

I ran some numbers while my son had me hang nearby while he did his soccer drills. With the assumption that the HA pistons are the same as stock aside from diameter ,the CR ends up around 11 to 1. This also assumes that the chamber volume is the same. But that cant be the case because the squish band gets bigger with the increased diameter of the piston. The center of the HA piston is the same height of the stock piston but the shoulder is lower. This has to create a larger chamber. Considering the added squish and the slight differences in the pistons, if this adds up a 4 cc larger chamber, then it's right back down to 9 to 1. So that didnt help hahaha. I'm still a a loss on what the compression might be.

I added a couple images of the pistons side by side. The HA is on the left. For grins, I also slapped a cb350 64 mm piston on a pin.

[scottly]

I'm piecing together a 67 mm Henry Abe kit to run with a ported head with oversized intake valves. The pistons are flat and the valve pockets are small. They basically look like oversized stock pistons.

As long as you have the parts, you might as well run them.
The bigger valves mean you MUST check the piston-valve clearance with the motor mocked up, usually by placing modeling clay on the pistons, turning the motor over 2 times, and then measuring the thickness of the clay at the thinnest point.
A cam that would be considered "hot" in a 736 would be milder than in a larger engine like a 900, regardless of static compression.

[Turbogrimace]

I'm piecing together a 67 mm Henry Abe kit to run with a ported head with oversized intake valves. The pistons are flat and the valve pockets are small. They basically look like oversized stock pistons.

As long as you have the parts, you might as well run them.
The bigger valves mean you MUST check the piston-valve clearance with the motor mocked up, usually by placing modeling clay on the pistons, turning the motor over 2 times, and then measuring the thickness of the clay at the thinnest point.
A cam that would be considered "hot" in a 736 would be milder than in a larger engine like a 900, regardless of static compression.

If I run it, I'll definitely be checking clearance. My goal is to increase low end torque through to midrange rpm. So knowing static compression let's me get a better feel for how big I can go on a cam and still maintain decent dynamic compression, especially at low rpm. With the minimal valve pockets and both block and head being decked, I doubt I can run any cam thats remotely substantial. But I'll still get good power with a mild cam. The head is nicely ported and 900 cc is a good bump in displacement. I just have a hard time not jamming a fat cam in there. My 836 is built to the 9s and it's tough to tell myself not to do the same here

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[PeWe]

I have verified piston-valve clearance by tighten the adjuster screw so the valve will open  more. This  on a few spots between ca 15-20 degrees before and after TDC.
Rotate engine, tighten without stop, release back and move to next. (Count how much you tighten.)
If valve can be opened +2mm without stop when tighten, fine for me. In and Ex.

Valve to valve clearance needs head off.
For cams like RC/DP315 with much overlap, 34/28mm valves. One head was rather tight, think it was 1mm. Better power with the 295, 125-75.

[Jerry Rxman Griffin aka MuthaF'er]

I had the Megacycle 125-75 in mine. No problem with valve interference. Both the head and jugs have been decked too. I "expect" the CR is more along the lines of 10.5 but have never seen a number.

Same type of HA pistons? There is another model with deeper pockets and crowns that look to bump the compression..

I have an old set of the flat pistons with almost no pockets as stock. 67.5mm.

Yeah, mine are the later version (I think) without the full skirt. 67mm. Crown sounds the same.

I found some other info on here that suggested these flat top pistons had stock compression. Does the surface and pocket of your pistons look the same as the image I posted in the first post?

If this 900 kit has stock compression and is limited to mild cams, I'm wondering if a higher compression 836 and street cam would yield more of my intended results. Or would the 900 still create more torque toward the bottom end, even with a mild cam?

Looks the same

[Turbogrimace]

I had the Megacycle 125-75 in mine. No problem with valve interference. Both the head and jugs have been decked too. I "expect" the CR is more along the lines of 10.5 but have never seen a number.

Same type of HA pistons? There is another model with deeper pockets and crowns that look to bump the compression..

I have an old set of the flat pistons with almost no pockets as stock. 67.5mm.

Yeah, mine are the later version (I think) without the full skirt. 67mm. Crown sounds the same.

I found some other info on here that suggested these flat top pistons had stock compression. Does the surface and pocket of your pistons look the same as the image I posted in the first post?

If this 900 kit has stock compression and is limited to mild cams, I'm wondering if a higher compression 836 and street cam would yield more of my intended results. Or would the 900 still create more torque toward the bottom end, even with a mild cam?

Looks the same

Just triple checking. I added a couple photos to the last post I made here. They didn't get attached when I originally made that post.

So are your pistons as flat as the one on the left in the 1st two pics and have factory valve reliefs? The pistons have the slightest dome to them, but are essentially flat.

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[Jerry Rxman Griffin aka MuthaF'er]

I can not find my 3 good HA 67mm pistons. Not sure what I did with them. Sigh. Damn near new too. Found the one piece oil rings though that I swapped for 3 piece.

My 900 story: I sent my cylinders and crank to Big Jay at APE. RIP Big Jay. He fit the liners and bored them. He cut lightened balanced the crank but did not remove the oil passage stopper balls which tells me the passages were not cleaned out or properly cleaned out. One stopper ball was also cut down. It came out and I lost #3. I sent it back and he "redid" the crank. Polished the journals and staked the ball in place. Still no grub screws installed. Don sent him a new HA 900 kit in trade for some tranny work (I think). Jay replaced and re-bored cylinder 3 liner and returned everything. I put it back together only to have #3 burn up again. I then sent it to Kenny at CycleX for his 915 kit and boring plus a replacement crank. All is well now. Out of respect for Big Jay and all he had done I deferred repeating my story. RIP Jay.     

I dug through my pics and came up with one I "think" is the HA. I do remember that those pistons had "HAIND" cast inside the shirts. I have seen 2 different HA 67mm piston styles.

Pretty sure this is the HA piston. As you can see #3 is different which is why I feel this is the HA. 

My HA pistons were not the full skirt set. Plenty of room for 34mm intakes with the 125/75 cam or enough anyway. No strike marks but not many miles either.

[Turbogrimace]

I can not find my 3 good HA 67mm pistons. Not sure what I did with them. Sigh. Damn near new too. Found the one piece oil rings though that I swapped for 3 piece.

My 900 story: I sent my cylinders and crank to Big Jay at APE. RIP Big Jay. He fit the liners and bored them. He cut lightened balanced the crank but did not remove the oil passage stopper balls which tells me the passages were not cleaned out or properly cleaned out. One stopper ball was also cut down. It came out and I lost #3. I sent it back and he "redid" the crank. Polished the journals and staked the ball in place. Still no grub screws installed. Don sent him a new HA 900 kit in trade for some tranny work (I think). Jay replaced and re-bored cylinder 3 liner and returned everything. I put it back together only to have #3 burn up again. I then sent it to Kenny at CycleX for his 915 kit and boring plus a replacement crank. All is well now. Out of respect for Big Jay and all he had done I deferred repeating my story. RIP Jay.     

I dug through my pics and came up with one I "think" is the HA. I do remember that those pistons had "HAIND" cast inside the shirts. I have seen 2 different HA 67mm piston styles.

Pretty sure this is the HA piston. As you can see #3 is different which is why I feel this is the HA. 

My HA pistons were not the full skirt set. Plenty of room for 34mm intakes with the 125/75 cam or enough anyway. No strike marks but not many miles either.

Thanks Jerry. Mine definitely don't have pockets that big. A 125 75 punch holes in my pistons.

[Laids]

Here’s some AH 900 pistons that I ran from 2009 to 2020, I’m sure already had some history. Broke a ring and replaced with a 836. From personal experience, there is definitely a difference in torque, with or without a cam.

[Turbogrimace]

Here’s some AH 900 pistons that I ran from 2009 to 2020, I’m sure already had some history. Broke a ring and replaced with a 836. From personal experience, there is definitely a difference in torque, with or without a cam.

Thanks for the insight. What cam did you run with the 900? Did you have any bottom end work done?

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[Jerry Rxman Griffin aka MuthaF'er]

Mine were the "other" set than what Laids has shown

[Laids]

Here’s some AH 900 pistons that I ran from 2009 to 2020, I’m sure already had some history. Broke a ring and replaced with a 836. From personal experience, there is definitely a difference in torque, with or without a cam.

Thanks for the insight. What cam did you run with the 900? Did you have any bottom end work done?

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In the 70s I had HA 900 with KH d grind cam, stock rods, used to rev the crap out it without issue. More recently the the HA 900 had a KH f grind cam, stock rods, but rarely went to red line. In the 70s I bought a K1 with the HA900 already in it, stock cam and exhaust, noticeable power difference when the bigger cam was added.

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