Crankshaft Regrind Dimensions

[Steve P]

Good morning,

If one were to have a crankshaft ground 0.020" under size on the mains and crank pins what would be the new dimensions and tolerances?

 

Thanks

Steve

[harrytr5]

Your machine shop will know.

Regards Harry

[Steve P]

Just been reading "Tuning Standard Triumphs" by David Vizard.

 

I came across this comment

 

"When using a reground crank, we should avoid going below 0.010' under size if any reasonable degree of tuning is envisaged."

 

Is that comment worthy of consideration for a fast road engine?

 

Thanks

Steve

[Motorsport Mickey]

Steve,

 

You need to give more detailed information with your options and what you are considering, it's all a question of degree.

If you were regrinding to -60 then using it in motorsport is problematical (although I won my last TR Championship on a -60 big end crank ( 4 cylinder)but using it for fast road purposes with limited revs you may well find it ok (6 cylinder is not my expertise). When David Vizard wrote his tuning book Triumph cranks of any configuration were two a penny so why invest time into modding a crank that may have limitations ?. These days all cranks carry more of a cost and are much rarer on the ground (unless you want to go billeted) and so consideration should be given to considering these more used cranks still able to have a useful life.

 

Mick Richards

Edited January 12, 2016 by Motorsport Mickey

[Steve P]

Good morning Mick,

I am fitting a flowed head, fast road cam and a lightened flywheel. Your comments have answered my question. I feel comfortable fitting a -0.020" crankshaft.

 

Thanks

Steve

[Martti Ojanen]

I am just about to do the same. I am not sure they know these engines so well here in Finland and I have tried to find those figures myself.

First the clearance, I have got confirmation that 1,5 thou (0,038mm) if a good figure.

I have got Vandervell RTC1752-020V-VEL C77/VP2 main bearings and Clevite C77/VP2 big end bearings. Best you can get.

If you calculate -.020" undersize from std size, you get

main journal size = 58,191 - 58,204 mm => my planned grind 58,204

big end journal size = 47,117 - 47,130 mm => my planned grind 47,130

Both bearings thickness is 2,07 mm and I will measure the bearing ID before grinding and consult the workshop to be sure to get 1,5 thou clearance.

I will get the crank to workshop next week.

Martti

[Motorsport Mickey]

Hi Martti,

 

Yes, measuring the bearings diameter dimensions when they are fitted in the conrod or when into the main bearings of the block is the way to get the correct dimensions.

Make sure they are torqued up to the correct figures to ensure the bearing "crush" (the amount each bearing shell is forced into contact with the conrod surface and against the other bearing at the split) is allowed for. This will ensure the bearings are round and you have a correct figure to grind to.

 

Mick Richards

[Waldi]

I would appreciate your guidance:

I have dismantled my CP engine, "general wear" is what I have found so far, no major defects.

Oil pressure was ok with cold engine, but low when warm at idle (just under 20psi on new 20W50 oil).

I heard no strange sounds, knocks etc.

The 4+6 crankshaft journals are each 0.02 to 0.03 mm out of spec, based on the data in the Brown Bible (too small).

Surface finish of the journals is acceptable, after I polished them.

To my surprise, the main bearings do not have a "step"in the middle section, where the oil groove is in the bearing shells, where no wear should be expected?

It looks like the journals were ground too small when new, or is this a too easy assumption?

I am interested in your view if a reground is required. I face a similar dilemma with the pistons/bores.

The hex head plugs are removed for proper cleaning (but still visible in the picture) .

 

Also, if someone knows the main bearing ID (inside diameter of bearing shells) for std bearings, I would be interested, since I could not find that information, anywhere.

 

Thanks,

Waldi

 

 

 

[Motorsport Mickey]

It's important to grind and prepare crankshafts as best you can working from the brown book measurements. I don't run 6 cylinders so I use a Haynes for general information work, seems accurate enough under normal circumstances.
Using my Haynes TR5 manual it shows the crankshaft main bearing diameters as 58.699 - 58.712mm and a crankpin dia as being 47.625 - 47.638mm, however a clearance of 1.5 thou (0.038mm) for the journal to bearing clearance, a "running" clearance is ok in my book, and so the crank journals should be measured and compared against the conrod big end dia when torqued up with bearings fitted, as per a previous post made. Again I should be looking to see the running clearance is in that 1.5 thou area when comparing the bearing diameter with the crank journals. I wouldn't be pulling or trying to rework a crank that was running at those figures.

However when the engine is in use it's as well to remember that now we enter a dynamic phase, where the original measurements and tolerances are subject to imperfect circumstances wear and assume their own finishes and dimensions.
The 2 and 6 journals now showing a discrepancy of -0.02mm (around 6/10 ths of a thou) and -0.03mm (around 8/10 ths of a thou) if they have a decent finish may well have been ground slightly small, wear achieving that much material off would likely have been with abrasive particles which wouldn't wear evenly and would likely show heavy grooving, that journal doesn't look at all bad. The mains when in good condition are normally found in the condition you describe, it's a very high oil flow area from that centre groove and so it flushes most small crap particles away from the edges where it's seeping in, it's the highest pressure area on the surface of the crank. Different story though if the oil is heavily contaminated and has dirt in, then it's got a hydraulically loaded filing action against the crank surface and you'll see real damage on the surface groove area.


Mick Richards

Edited February 12, 2018 by Motorsport Mickey

[Waldi]

Hi Mick,

thanks for your thorough response.

I did measure the bearing ID's too, and then calculated the actual clearances (with the old bearing shells), and they are very big:

Mains: 0.091 mm average (3,6 thou)

Big end": 0,094 mm average (3,7 thou)
​I think the measurements are pretty accurate, my son and I were within 0.01 mm on all measurements (we had some "quality time" together this weekend)

There is not much spread in the clearances, which is good.

If I install new shells only (re-using the cranck shaft "as is", I did an attempt to calculate (guestimate) the expected clearances, these will be 0.067-0.081 mm (2,6 to 3,2 thou). This is a guessing since I do not have the new ID's of the shells as I did not order new shells yet pending a decision to regrind or not, so I estimated the ID of new shells to be the minimum and maximum diameter specified in the manuals for the BE bearing, and assumed a similar clearance for the mains. So it is a given that the clearances I will achieve will be bigger that the clearances in the WSM.

I have made an excell sheet with engine revison data, should anyone be interested, but know from experience it is difficult to "understand/follow the grey mass of another person".

 

I can have the crank reground, but that will not be a guarantee I have better (smaller) clearances.

Anyone, am I being penny wise, pound foulish, or does it make sence to leave the crank as is, with new shells only?

 

Thanks,

Waldi

Edited February 12, 2018 by Waldi

[Motorsport Mickey]

Too big Waldi, Id be regrinding.

 

Mick Richards

[Waldi]

Thanks Mick,

After a good day of work I came to the same conclusion.

I have asked a quotation from a respected Dutch company.

Best regards,

Waldi

[astontr6]

I would appreciate your guidance:

I have dismantled my CP engine, "general wear" is what I have found so far, no major defects.

Oil pressure was ok with cold engine, but low when warm at idle (just under 20psi on new 20W50 oil).

I heard no strange sounds, knocks etc.

The 4+6 crankshaft journals are each 0.02 to 0.03 mm out of spec, based on the data in the Brown Bible (too small).

Surface finish of the journals is acceptable, after I polished them.

To my surprise, the main bearings do not have a "step"in the middle section, where the oil groove is in the bearing shells, where no wear should be expected?

It looks like the journals were ground too small when new, or is this a too easy assumption?

I am interested in your view if a reground is required. I face a similar dilemma with the pistons/bores.

The hex head plugs are removed for proper cleaning (but still visible in the picture) IMG_7523.JPG.

 

Also, if someone knows the main bearing ID (inside diameter of bearing shells) for std bearings, I would be interested, since I could not find that information, anywhere.

 

Thanks,

Waldi

 

 

 

Hi Waldi, you carefully also check the camshaft bearing journals especially the one where the chain wheel is as this is the one that wears most and they are key to oil pressure at low revs.

 

Bruce.

[Waldi]

Hi Bruce,

Thanks and I did check the camshaft bearing clearances (with the exception of the middle bearing, which I could not access with the bore gage).

The clearances of the other 4 bearings are: 0,07 / 0,09 / 0,10 / 0,10 mm (front to rear).

The front one which gets the highest loading had 0,07 mm clearance.

Haynes indicates a nominal clearance of 0,066 - 0,116 mm.

This seems a lot, double to was is allowed for the crankshaft bearings...to be honest I do not understand why, maybe because there are no bearing shells and the journals are running against the block parent material?

 

The brown wsm does not provide a clearance, which seems a bit strange for such a comprehensive book, or am I looking in the wrong places?

 

Thanks,

Waldi

[RogerH]

Hi Waldi,

unlike the crank, the cam is always being forced in one direction.

This allows a good oil supply to the 12-o-clock position of the journals and so is always wiping oil around the bearing.

 

I read somewhere that the cam bearing just don;t wear out. Hard to believe but I have never found anything excessive in my limited rebuilds.

 

Roger

[Waldi]

Hi Roger,

Thanks.

for the crank journals yes, the conditions are very different, but not for the mains, I think.

At least Im glad mine appear to be ok, as lineboring is not exactly cheap.

Best regards,

Waldi