Topic: Piston deck height ponderings - bent rod?

[RottenRalph]

Hello and thanks in advance for keeping things PG in regards to my possible bent rod problem!

I've tried using the search function to no avail (the answer is probably out there so apologies) but this is my 2nd motor build and coming to buttoning everything up now. I decided to utilise my nice new dial gauge to measure piston to deck height with base gasket installed and all are coming up around the edges of the piston to be below the deck height so that's fine. However -measured above the wrist pins - one piston seems to not be square, with one side lower than the other and vice versa. Hopefully the pictures are showing to better describe what I'm seeing (1000 words and all that).

2 questions;
- is this indicative of a bent rod and is it 'acceptable tolerance' or sourcing a straight rod and replacing?
- is the fact that the deck heights aren't all exactly equal going to pose a problem in an of itself?

Thanks again!

The motor is a k8 with all new bearings on the crank+rods and all new chains and ball bearings etc, no work done on the crankshaft and I never had it checked for warpage, although the middle crank journal did require a set of brown bearings whereas the rest were all greens.
Motor was from a 20k mile basket case from the states - so actual mileage may be different.

[C317414]

This could also be caused by a defective piston, with the top not perpendicular to the sides or parallel to the wrist pin bosses. 

[enwri]

If you type sohc4 after your search term, google brings up posts from here. I found a bent rod in mine as well. Was much softer than I thought it would be. Took almost nothing to tweak the little end straight again.  Much to the horror of a few on here. Was also about 60000klms ago.

[Tracksnblades1]

Where are your cylinder studs..?  Might be a challenge to keep that cylinder stable with new sharp rings turning it over.?

What does your indicator set up look like ? Repeatable ?

What brand of pistons ?

What is your piston to cylinder wall clearance? 
Surely a bent small end rod would show some piston skirt contact with OEM new bore clearances

[enwri]

I thought it looked like it was bent by someone using a big screwdriver through No.2 little end to undo the alternator bolt, but with the handle of the screwdriver keeping the shaft from laying flat across the crankcase. Was almost the perfect angle.  Had a stainless shaft that fit the little end perfectly and long enough to thread through all 4 at once to check when it was straight.
Carbon deposits on the sides of the piston one side the skirt, the other the ring lands. No actual damaging contact though, It was not a nice tight assembly even back then.

[RottenRalph]

This could also be caused by a defective piston, with the top not perpendicular to the sides or parallel to the wrist pin bosses.

Very good point, my assumption being that it is a brand new piston that this wouldn't be an issue but I will disassemble and check this when I get home. Cheers!

[RottenRalph]

Where are your cylinder studs..?  Might be a challenge to keep that cylinder stable with new sharp rings turning it over.?

- I'm using a new type of stud that's coated with a special invisi-dye, seems to be working! (S) I kid - APE HD studs are sitting on the shelf awaiting their turn to go in!

What does your indicator set up look like ? Repeatable ?

- It's a relatively cheap dial gauge but it has a solid magnetic base and I was getting the same readings on all pistons within .01mm after multiple revolutions so I'm confident in the #s.

What brand of pistons ?

- Cruzinimage - heard good things all round so went for it. Can't beat that price!

What is your piston to cylinder wall clearance? 

- I forget exactly as the boring was done over a year ago but just slightly over whatever it was HondaMan suggested in one of his threads a while back! - not sure how that would affect the piston/deck height though?

Surely a bent small end rod would show some piston skirt contact with OEM new bore clearances

- Nothing obvious so far, bores look good still after multiple revolutions. (Rings are not installed- still need to gap them)

[Tracksnblades1]

Sounds like you’ve already homed in on the bent little end cause…

Our local machine shop has a rod fixture that can check the rod holes for perpendicular and lying in the same plane (parallel).
Similar to what 414 indicated about your piston. I’m not touching straightening your rod. Besides, it’s not for me…🤫 I would compare big end, little end, and total rod weight if you go the new/good used route.

Deck and quench clearances can be a debate. Some call it squish. I like the minimum quench clearances. The sohc cylinders get longer as they’re heated. HondaMan and others have recently indicated some aftermarket head gaskets have gotten thicker. Which would further increase the desirable quench clearances. (As well as causing  proper oil oring thickness, oil leak problems)

I don’t know any thing about cruising images. It looks like the cast finish on the piston tops are different on 3 and 1.

[PeWe]

Maybe the upper cut for valve pocket that differs.  If rod, there are rods for not much. But that need to open the engine.

There is time before spring.
Maybe easier to find a complete crank with rods. If one rod is bent like that, what else might have been done...

I should remove cylinder and check pistons.
There is a chance that pistons are a little bit wrong cut, pocket not matching the valve, possible to grind the groove a little bit longer.

My K2 got those pistons, but 61.50mm K7 (392). I did not notice anything strange then.

I have a new extra set I can check.

[Tracksnblades1]

Ralph,

Is the cast finish as rough as it looks on 1&3 or is it just the photo…

Do you think the casting may have shifted on 2..?

I don’t see a good machined top around the edge for a really good measurement..?

What special tool has the 🦇 bats on the label..?  We may not have that on this side of the puddle…

[RottenRalph]

UPDATE! HOW EXCITING 

So I took it all apart again (no bother really it was only a dry assembly to check all this stuff - good thing I did!) So it seems on closer inspection that Tracksnblades was right, the casting may have shifted resulting in the wrist pin having an angle from side to side, roughly .15mm which would tie up riughly with what I was seeing on the top deck heights.

Rods are straight but I may still spring for cycle x's super rods... they look so purdy  but maybe after the next paycheck!

Still awaiting a reply from cruzinimage re piston replacement - i bought them in 2020 - so will see about that, word on the street is they arent all that quick on the replies and its been a week already...

On that note, has anyone here ever used IMDs offering of pistons from the UK? They also do kz650 pistons which a user here has mentioned having some luck with using in our sohc 4s with a nice compression dome to them, has anyone done this? What are valve clearances like?

[Don R]

 Don't be shocked if the Cruzin guy just blocks you. We've heard of that before. If I recall correctly, someone actually had to get a friend to buy a set of rings to replace a broken one.

[ekpent]

I remember for awhile he was on a blocking binge if you had questions or concerns. Kind of like the Soup Nazi.

[Jerry Rxman Griffin aka MuthaF'er]

Don't be shocked if the Cruzin guy just blocks you. We've heard of that before. If I recall correctly, someone actually had to get a friend to buy a set of rings to replace a broken one.

Yes, he did that to me for questioning the carb intake boots I acquired from him

[PeWe]

The guy at Cruzinimage work really well on their website
www.cruzinimage.net

You need the old order references for mention a refund or new pistons today when over 2 years have passed.

[calj737]

If you are considering using Super Rods then for Pete’s sake buy some quality forged pistons and ditch those crap Chinese casts. 

[denward17]

Thread on IMD pistons, I have not used them.

http://forums.sohc4.net/index.php/topic,191607.msg2230732.html#msg2230732

Note post #10.................

[RottenRalph]

The bats are from an old halloween gingerbread tub I acquired from a relative... they quickly disappeared out of it and Ive reused the tub as a bearing box!

Thanks for all the help everyone, I ducking love this forum.

I managed to swindle a forged set of wiseco 65mm kz650 pistons off feebay but looking at the dome height and Im wondering how close to 20:1 CR they would be  Ive seen strippers with smaller domes than that! - apparently people ran these?! Not sure how ...
- they were sold as an 836 big bore but after poring over the various forums and topics on these pistons it seems I may have just bought myself some pretty looking bookends.

Calj737 this was always meant to be a budget build - whatever that means! Cases have been apart for over 3 years as I acquire the know-how and finish my other rebuilds first. This has been a sort of 'hospital job' in the background - Ive been busy practising porting junk heads and doing my own valve jobs with neway and ape and cycle X stuff, fun to do but by god has it been a learning curve.


Also: please enjoy a picture of my K6 out in the stix.