Allowable flexplate to converter clearance?

After nearly two years I finally managed to finish the engine build and drop the stroker W2 motor into the engine bay.

It did NOT come easily...ran into clearance problems in that the crank would butt up against the converter hub (as the engine was being lowered into the bay) and that would cause the front engine mounts to missalign just enough that I couldn't literally even start them. Things were off by about 0.25-0.50", enough so that I ended up pulling the transmission to crossmember mount bolt out to move the whole assembly to the rear, which finally freed up enough room up front for me to seat the engine into the mounts. Converter registered with the crank pilot, all good there. This is Dynamic 9.5" 4K stall converter, which was previously bolted on to a factory crank in a 360 motor, whereas now this is an Eagle 4" stroker crank (forged), so I'm sure some measurements differ.

Alright...so that took nearly two days and countless tries (well, OK, maybe 5, but still, what a PITA to slug through this in your little DIY "piece of heaven" lol).

So, now comes the time to "tidy-up" with all the little things. First thing was to mate up the engine flexplate to the converter. The converter pulled up to the crank fine, no issues, and there is no clearance left between the converter body and the flexplate retaining bolts, it is pretty tight up there. However, that still left 0.065" clearance between the flexplate and the converter lugs.

Tossing a bolt in there nicely pulls up the plate to the converter but doing so distorts the flexplate a tiny amount. This is the core of my question, what is the max allowable amount that a flexplate can be expected to "take-up"?

The flexplate in question is a B&M 10230 part and the their install instructions only talk about engine to transmission alignment in terms of Total Indicator Reading (TIR).

In my case I can easily slide a 0.065" washer in between the flexplate and the converter lug, but oh boy, that just seems like such a HACK!!! However...it would certainly do the job...

What do you guys think / recommend?

This btw is a weekend toy of mine, expecting the car to pick up from the last leisurely pace of high 12s to maybe low 12s, but I don't anticipate it going any faster since the suspension is setup for road handling as opposed to drag racing, i.e. no traction to speak of!

I fought loosing converter bolts and even broke a flex plate years ago. Welded it and the bolts to get through the season. Turned out, it was the situation you describe. In that case, the snout was not going into the crank far enough. A slight dressing down of the snout along with a new flex plate and all was good.

Since then, I have had to dress down the snout of every converter I have bought when putting together a new combo. Now, I don't know if the converter or the crank (or both) is the real issue. But for me, dressing down the snout is the easiest 'fix'.

In my experience, the converter must slide right up to the flex plate. If it must be pulled up with the bolts, the flex plate will be forced to flex every revolution which, depending on the amount, can lead to loosing bolts and possibly a cracked flex plate.

Just looked at this the other day..I have a little less than 1/8 inch… has been running 5 years like that, no issues. Footbrake 904, 8 inch vert, good bit of racing.

Yes…. But you can probably pull the converter up to flex plate (flush) by hand. I know I can.

Yes. It needs pulled that far to bolt up. I put a post on another forum and a few guys all said the 1/8 was fine. My vert has anti ballooning, even though I don’t use the brake or spray, so that clearance “should” be OK.
I wasn’t sure hence the thread asking

Never use the bolts to take up any gap between the flexplate and the converter lugs nor install any shims to take up the gap. Find the cause and fix it. There are reports that some aftermarket crank registers are improperly machined and reports of the flexplate-to-crank bolts preventing proper pull up...especially with 8-bolt cranks.

This is precisely what I'm worried about: the flexplate-to-crank bolts are butting up against the converter body as I'm pulling the converter up against the flexplate.

I am about to head back out to the garage to troubleshoot further. Got a small snake camera and I'll see if I can get better close-up photos.

While I hear what you guys are saying, I've got to be honest: pulling all this apart to fix it correctly (assuming the crank may be the actual root cause here) is a non-starter. I might as well park things for another couple of years.

If I was to use machine shop shims to take up the gap, what is the potential downside to taking this approach? Assuming I can confirm that the converter pilot does correctly mate up to the crank (1/8" is the min I see in various docs), I think that would positively locate the transmission and is the most important thing to assure, is it not?

Regardless, I honestly appreciate all the feedback. I know doing things RIGHT is the right approach, but at this stage I either figure out a viable way of living with this, or like I said above: keep it parked for another day!

OK, got some pics!

I think I understand what's going on, not to say I know the root cause, but at least I see where the block is.

So the first picture shows the mated parts, the gray stuff is the crank the black stuff is the converter. Things match up pretty well here.

The second photo shows them unmated, you can tell how much of the converter is actually pulled out - so literally pushed back towards the transmission as far as I can - (I measured it to be 0.195"), so there is a decent engagement between the two when actually mated.

The third photo is of the crank pilot end, there are a few little notches you can see, and I think my snake camera just happend to capture one of these.

Alright, so this most likely means that the converter pilot is not machined down enough for it to engage the crank further.

It does look like the crank-to-flexplate bolts are also awfully tight, but there was just no way for me to actually measure this.

All Mopar rear wheel drive automatics in the 60’s were designed to bottom the converter pilot in the back of the crank and provide for a gap between the flex plate and the converters lugs. That’s is a fact, by design.

Assuming the crank, flex plate and converter are to print spec you will have a gap at the lug. The gap varies.
Tightening the converter to flex plate bolts will lightly bend the flex plate. That is by design.

You mention now that you have the crank to flex plate bolts hitting the converter.
That’s a different issue. Bolt head too thick, front cover bowed/ballooned, etc. These bolts should never touch the converter.

As you stated the converter is about 1/2" from lugs to flex plate . Would this keep the ring gear away from the starter ? I have never seen this much distance , all of my converters have been right up to the flex plate . When I worked at Chry dealer stock converters never had a distance of 1/2" , they always slid right up close to flex plate . I have heard of the pilot not being machined correctly . Never seen this . I would vote for pulling the converter & relieve the snout . Gary

...but is there a min/max type of a spec to this?


Well, I cannot tell this for sure. Visually it looks awfully close in there, but at the same time when I push the converter towards the transmission first and than pull it back towards the crank I get a very crisp "thunk" sound...feels and sounds like a solid metal-on-metal hit in a single spot, as opposed to multiple smaller "thunks" that one might expect if it were the crank-to-flexplate bolts hitting instead.

This "thunk" sound is something I spent about 10 mins on trying to figure out what the heck was producing it. Visually it matched to the crank-converter_pilot mating up and stopping at the very end.

Therefore, I am more and more inclined now to conclude that that converter pilot could probably use a little more machining (towards the face), and in my case given your above comment that could be as little as 0.050".

Hi Gary,

No no...when the converter is pulled OUT of the transmission and brought towards the flexplate there appears to be only about 0.065" room left. I re-read my posts and honestly could not find the reference to "...converter is about 1/2" from the lugs to flex plate...", if I missed it please tell me where because that is absolutely incorrect and it would be a mistake on my part, which I would like to fix.

FYI - for what it's worth, as I deep-dived into troubleshooting this issue I reviewed numerous "Trans / Converter Installation" write-ups. I've attached a Turbo_Action PDF because it basically touched on nearly everything that has been described in this thread so far (including the specific recommendations you guys have provided).

In particular, take a look at P4 which highlights the recommended crank-to-flexplate bolt head thickness and provides a good explanation for why this needs to be checked.

In hindsight, I would have covered this had I actually pulled the converter out fully and mated it up against the installed flexplate to check for any such binding prior to installing the motor back in the car.

OK, I stand corrected...never seen a gap between lugs and flexplate after pulling forward.

Hopefully your converter to flex plate bolts are 7/16 and not the stock OEM low performance smaller 5/16 bolts. On the larger bolts you can use a 7/16 harden head bolt washer as a spacer between the flex plate and converter bosses so when you tighten up the converter bolls it doesn't bend or tweak the flex plate

Hello , I guess I misinterpret the distance , but I am still thinking there is an issue . It should come closer to flex plate . I have done a lot of these & never had a converter not come out to flex plate . Maybe place a smudge of grease on snout of converter to show where its contacting crank , adjust as needed . Gary

The 3.79 crank I have, requires buffing the paint off the converter hub. If it’s painted, or has any oil on it, it acts like a sealed piston.

Make sure your flexplate to crank bolt heads aren't too thick causing the front of the converter to hit the heads of the bolts before allow the converter to go fully into the crank register

What bolts are using? Should have a real thin head on it.

I've got a set of brand new 7/16" ARP converter bolts if you need them. I can bring them to work tomorrow if you do.

If someone wants an affordable alternative to the ARP bolts; just get some short grade 8 bolts from the local hardware store. Chuck them up in a drill and grind the head down on a bench grinder. Takes a few minutes, but is a fraction of the price of the ARPs and does the same job.

After pricing the ARPs, I have done this many times.

What about loosing the heat treatment from getting them hot while grinding them down

Mark,
Appreciate the offer but I'm thinking these are exactly the same bolts I am using.

1) crank to flexplate - ARP 200-2903, 7/16"

2) flexplate to converter - ARP 240-7302, 7/16"

when I got an ATI treemaster years ago that's all that came with it, except done in a lathe. Changing heat treat would take getting it to a dull cherry red. that ain't happening on a bench grinder. Well I guess it could but you know what I mean.

Yes, hopefully common sense comes into play.

This seems to be more common with 8-bolt cranks...bolt heads will actually leave noticeable witness marks in the converter.

how do the ARP converter bolt heads, the 12 point style, work out on the 8 bolt cranks ?

The bigger (7/16) converter bolts won't work on the 8 bolt cranks that take the 1/2 inch fine thread flywheel, flex plate bolts on those cranks

Moparx , When I built my new combo I wanted to use ARP on flexplate , so I bought ARP , for whatever reason they only came in 6pc packages . Yep , had to buy 2 to get the job done with a couple extra in the box . No wonder they are expensive , you need to buy more than you need . Gary

PS:They do work without issues .

Every time I've seen a change of colors on bolts being ground on did make a change on the heat treat, I'm not sure it that made them softer or harder
Hard bolts break easier before twisting a tiny bit than the softer bolts due

Just a light mist of water and taking your time grinding will prevent any discoloring. Not at all hard to do. Besides, any discoloring would be limited to the surface of the bolt head unless common sense was absent. The vast majority of the bolt wouldn't be affected. Especially if water was used to prevent the heat build up in the first place.

Honestly, Cab, this isn't that hard or risky.

And if prefer softer bolts, just buy Grade 5 rather than 8. Of course, you don't have that choice with ARP.

On 440 Source, 6 crank to flex plate bolts are about $19 plus shipping. If you need 8,that would be over $5 a bolt. Five dollars would buy just about all 8 at your local hardware store. In either Grade 8 or 5.

Just putting all choices into perspective.

i know exactly what you mean, as i did the same thing.
as i haven't put this combo together yet, i was wondering if there would be any issues with the ARP bolts.
thanks for the info. just one more thing i don't have to worry about.

So here are a couple more pics that (at least for this particular bolt I checked - and I did NOT check all six of them) clearly indicate there is plenty of clearance between the crank-to-flexplate bolt and the face of the converter. The white sheet is a folder piece of paper stuck between the head of the bolt and the converter body. The 2nd picture shows a much thicker cardboard sheet that still fits quite fine.

Alright, so having said that all I came to discover that as I tightened the remaining motor-to-transmission case bolts, the clearance did actually end up pulling in, which is not something I anticipated, and let me explain what I mean by that.

The initial checks were done with the FOUR top bolts (small block) fully seated. However, as this appeared to be an issue and as I troubleshooted further I noticed that at the very edges where the locating dowels are there was still a bit of clearance, maybe 0.010"??? (I did not measure)

Subsequently once I added the two edge bolts and fully tighteneded ALL of them, it now appeared to almost completely close out the gap I had originally found. Mind you, it was not completely gone, but I would say down to about 0.025" (which I tried to measure with my gap blades, not quite handy getting one in there, but that's the closest I came to getting a good check).

Therefore - and this may be obvious to all of you who have done this countless times - the lesson I learned here is that ALL the engine-to-transmission case bolts must all be installed before anything else can be checked!

Honestly I feel a bit like an idiot to have missed this (I think this was a 'miss'). Thanks everyone for the feedback and troublshooting suggestions though as this allowed me to work through the issue!

Now off I got to fire-up the motor...keeping my fingers crossed we do not get rained out like yesterday!!!