Lotus Elan

Head bolt differences

PostPost by: gjz30075 » Mon Feb 19, 2018 7:17 pm

What I have pictured are three different pairs of head bolts. The pair on the right came out of a mid 60s
4 bolt engine. It is the 'waisted' type. The other two are from 'stuff' I have laying about, and not 'waisted'.

So, a couple of questions: are they interchangable? If you notice the two pairs on the left, one set has
a bulge in the shank, just at the top of the threads and the other does not. Is there a functional difference
between the two? If I had a choice to use on a 4 bolt block, with matching (#'s) head, should I go with
either of the sets on the left? Or stick with what came out of the block?
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PostPost by: Quart Meg Miles » Mon Feb 19, 2018 10:16 pm

Stick with what came out!

Ten years ago I fitted a new set of bolts like your left hand pair on a major rebuild on my high mileage engine. A few years later two of them had broken and last year I found another two broken. The supplier, Fibreglass Services (Miles Wilkins), could not tell me the source of the bolts and no one on the forum confessed to ever breaking one, let alone four. I ordered a new set from QED and they looked like the ones that had broken and I sent them back and ordered a used set like your waisted ones on the right.

Make sure the threads on the bolts and sockets are clear and have sufficient depth for the bolts' length and there shouldn't be a problem. In my case the washers were rather conical where they had worn the head and I had recesses machined for them so the new ones could lie flat.

lotus-twincam-f39/lots-head-bolts-t38962.html "LOTs of US head bolts" has the whole of my saga.
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PostPost by: gjz30075 » Tue Feb 20, 2018 12:12 am

Quart Meg Miles wrote:Stick with what came out!


Interesting thread! My machinist does suggest the same as you.
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PostPost by: el-saturn » Tue Feb 20, 2018 8:55 am

i saved a few £££ which i would've spent on wine! so i spoiled my TC to a NEW set of new ARPs!! and don't have to worry about the consequences for saving 100quid.........while spending TWO thousand for the rest i'm doing (pulling, replacing, parts etc...) sandy
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PostPost by: rgh0 » Tue Feb 20, 2018 11:21 am

A bit of bolt and gasket engineering design practice.....

The reason you use a reduced shank bolt in a gasketed joint is to ensure that the bolt load remains close to constant as the gasket compresses over time. The reduced shank bolt is more flexible ( lower stiffness) and stretches more when tensioned. Over time as the head gasket compresses a little with multiple heat cycles the reduced shank bolts takes up this movement with little change in load compressing the gasket to create its seal

Full shank bolts that are stiffer lose tension quicker as the head gasket compresses overtime resulting in a gasket leak being more likely

However reduce shank bolts can fail more easily when being tensioned due to torsional loads being combined with the tension load on the slimmer bolt shank. Thus full shank bolts are better when tensioning but not as good in long term service

Lotus at some point concluded that a full shank bolt was OK with the head gaskets being used and they did not need to use reduced shank bolts with the consequent risk of failure when tensioning. Thus they change to full shank bolts....Or maybe they change because they were just cheaper :lol:

My recommendation
If reusing reduced shank bolts make sure they are crack tested and have no obvious defects. However despite careful checks I have broken a reduced shank bolt when installing a head and dont use them now.

If using full shank bolts check the tension a couple of extra times after installing the head to make sure the bolt tension is maintained as the gasket compresses overtime.

Personally I use ARP studs as the best current solution but reusing old bolts of both types is OK provided they are true original quality ( e.g. GKN brand stamping on the head or equivalent ) and in good condition. I would not use a no name brand or dodgy copy head bolt of any type.

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PostPost by: Quart Meg Miles » Tue Feb 20, 2018 11:29 am

Most important, which is why I forgot it, is that the slim shanked bolts have rolled threads. Three of my failed bolts broke in the turned threads section and the other at the head, which had sharp inside corners.

img_4818-broken-head-bolts-2017.jpg and

Followers of my earlier threads will be pleased to learn that the engine has covered 2000 miles since the rebuild and is almost oil tight though consumes water. It hasn't needed any other attention, the low mileage being a reflection of the wet weather, and I was driving it even last week.
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PostPost by: rgh0 » Tue Feb 20, 2018 11:35 am

Original GKN head bolts of both types and any decent non OEM bolt or stud such as from ARP will use rolled threads and have the right head to shank radius.

A dodgy quality bolt with cut threads and sharp head radius will fail whether full shank or reduced shank. Most dodgy bolts will be full shank as the dodgy suppliers will not go to the trouble of making a poor quality reduced shank bolt.

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PostPost by: Craven » Tue Feb 20, 2018 12:08 pm

Unless you have some information allowing you check the original length, I would not reuse the slim shank bolts. My experience has been these bolts are very easily over torqued, stretched beyond their elastic limit causing permanent weakening.
Weak bolts can often be detected when fitting, as you approach the final torque setting the stretch on the bolt will ‘feel’ quite different soft spongy compared to a good one.
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PostPost by: StressCraxx » Tue Feb 20, 2018 5:59 pm

Quart Meg Miles wrote:Most important, which is why I forgot it, is that the slim shanked bolts have rolled threads. Three of my failed bolts broke in the turned threads section and the other at the head, which had sharp inside corners.

IMG_4818 Broken head bolts 2017.JPG

Followers of my earlier threads will be pleased to learn that the engine has covered 2000 miles since the rebuild and is almost oil tight though consumes water. It hasn't needed any other attention, the low mileage being a reflection of the wet weather, and I was driving it even last week.


The head bolt on the left tells a "tale". The fracture is due to fatigue. There is evidence of a "beach" (initiation) and "ratcheting" in the fracture area. The underside of the bolt head should be radiused to the shank(grip). Even if the bolt head is radiused, the hardened washer must be chamfered in relief to clear the radius under the bolt head. If there is no chamfer in the washer, the radius is stressed heavily and failure comes quickly.
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PostPost by: Quart Meg Miles » Fri Feb 23, 2018 11:15 pm

StressCraxx wrote:
The head bolt on the left tells a "tale". The fracture is due to fatigue. There is evidence of a "beach" (initiation) and "ratcheting" in the fracture area. The underside of the bolt head should be radiused to the shank(grip). Even if the bolt head is radiused, the hardened washer must be chamfered in relief to clear the radius under the bolt head. If there is no chamfer in the washer, the radius is stressed heavily and failure comes quickly.

Indeed, and I ran a countersink bit into the new washers to creat that chamfer. I despair of some of the basic errors we encounter these days, especially at £15 per bolt.
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PostPost by: 512BB » Sat Feb 24, 2018 10:57 am

I am always amazed at people paying high prices or even buying at all, second hand head bolts on ebay and elsewhere, when they do not know the history of the bolts, no matter what the make.

Further, I do not know anyone who has ever had bolts crack tested. Why bother, for the cost of crack testing, just put the money towards a new set of bolts, for peace of mind. It seems penny pinching to me, that after you have rebuilt an engine, you would use head bolts of an unknown intergrity.

Its one thing to reuse bolts that have come out of an engine that you have owned an run for years, but quite another to buy s/h bolts that you know nothing about. Not for me.

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PostPost by: Quart Meg Miles » Sat Feb 24, 2018 10:01 pm

But,Leslie, I did just that, the bolts coming from the writer of the bible on the Twin Cam, and four of them broke - £60 cash gone and the cost and time of a rebuild owing to the failed head gasket. The new QED bolts looked the same with cut threads and sharp head joins.
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PostPost by: Craven » Sun Feb 25, 2018 12:12 am

Anyone involved in reliability studies will be aware of the Bath Tub Curve. Basically it shows the statistical failure rate of new components, a steep curve down known as Infant Mortality shows relatively high failure rate early in a components life.
Some engine builders actually prefer known used components that have in effect been tested and working within their useful life.
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PostPost by: SJ Lambert » Mon Feb 26, 2018 11:35 am

Is it your practice to torque ARP STUDS/nuts with normal/engine oil and to a somewhat lower than recommended with their Blue Lube on race heads Rohan?
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PostPost by: rgh0 » Mon Feb 26, 2018 12:58 pm

Just finished checking the torque the ARP head studs on my new engine tonight. I torque to the standard 55 to 60 ft lbs. I used the blue lube packet under the nuts and on the nut threads when I assembled. This will produce a higher bolt tension than the standard bolts on the head due to the finer thread on the ARP stud nuts.

I got about 5 degrees turn on each nut this time. This was the third time I have re torqued since building the engine and it has now had about 10 hours running. The nuts will not move anymore now based on past experience.

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