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Stretch bolt, Torque To Yield (TTY) bolt theory discussed

Since the common introduction of torque to yield bolts, confusion has arisen, especially in regard to their reuse and as to when and if they can be re-torqued. Principally this is due to two sets of technologies existing side by side and often in the same engine.


Stretch bolt theory, torque to yield bolt theory, in a nutshell

stretch bolt image #2
Illustration stretch bolt #2

stretch bolt image #1
stretch bolt image #1

Stretch bolts for cylinder heads


Stretch bolts, or torque to yield bolts, when properly torqued, loose some of their diameter to the stretch, but are elastic, and if variations in the gasket material or other things occur they keep their clamping force by varying their diameter slightly, but frequently, like breathing. The clamping force of stretch bolts is necessary to retain the inevitable variations of cylinder head composite gasket material deformations due to heat etc. and they automatically keep the necessary clamping force on the head gaskets for them not to not leak.

Advantage of torque to yield bolts


The clamping force of Torque to Yield bolts is greater than non-stretch bolts size for size. Basically that allows for smaller engines with greater power to be produced, that is the power for weight ratio increases.


Disadvantage of torque to yield bolts


Once torqued, Torque to Yield bolts generally cannot be used again, consequently making repairs and correcting procedural errors very costly.
Illustration #1 Whenever a manufacturer says it's OK to reuse a TTY bolt, they will nevertheless give strict parameters regarding the bolts length before you do proceed. Thus adding to the labour costs of any repair.
Illustration #2 above shows connecting rod bolts, one new, the other stretched beyond its elastic limit, now fit only for the melting pot.


Complaints of cylinder heads leaking - Do stretch bolts work?


Of course, the real test of Torque to Yield bolts, is do they work as intended?
If there are chronic complaints of leaking cylinder heads on a particular engine, the owner of that engine would say a resounding No!
Still the theory is sound, make things smaller and more efficient is the creator's eternal call. Bugs will get sorted out, materials will improve. If Torque to Yield bolts themselves are not improved, both in application and in cost, they will be replaced by something more efficient and cheaper.

General rules for stretch bolts, torque to yield bolts


TTY (Torque To Yield) bolts, loose their clamping force on reuse, and unless they are strictly within the manufactures stretch limits, should not be used again, otherwise leaks will occur.

Do not reuse or retorque or retighten TTY (stretch) bolts unless the manufacturer SPECIFICALLY says it's OK to do so.

Do not buy non-genuine bolts unless they are clearly marked with grades that conform to recognizable international standards.
If you can't recognize the bolt as belonging to one of the following standards or some other standard that you trust, don't fit the bolt. I'll give you 10:1 it's sub-standard, only use OEM bolts.

Some examples of trustworthy standards are:
ISO: International Organization for Standardization
AISI: American Iron and Steel Institute
SAE: Society of Automotive Engineers
BS: British Standards
DIN: Deutsche Institut für Normung
JIS: Japanese Industrial Standards

Owner of Detroit MBE4000 engine replaces stretch bolts with non-stretch studs and nuts and solves chronic leaks.


Before discussing any particular case let's bear in mind the above rules. It is reported that all the rules were followed but let's caution that we still do not know for certain if or how they those rules were applied.

However, the above rules themselves testify to the necessity of finding more reliable and cheaper alternatives.

Development here would point to the type of bolts that are simple to use and don't need a complex set of rules for tightening. How about, for one, bolts that are truely elastic and will revert to their standard size after normal use and temperature variations? The science-fiction plasteel is an example of an ideal from a writers mind, knowing development is needed and will eventually come in this area, but postulating his fantastic solutions in the meantime. When will it come? Well, we shall see!

Chronic leaks, and wasting thousands of dollars trying to fix them by using Detroit Diesel agents who applied proper workshop manual procedures and used OEM parts to stop cylinder head leaks on Greg's MBE4000 engine led to one example of someone finding an alternative solution to a severe TTY problem. Actually he just reverted back to older technology using good quality materials and had studs and nuts manufactured. He fitted them in place of TTY bolts and they worked as planned, that is no leaks and no stretchy bolts.
Click this link to read more about it on our MBE4000 Page 

Feedback a year after replacing Stretch Bolts with standard (non-stretch) studs and nuts

Greg updated us a year after doing the job on his MBE4000 as follows (his own words):

"I had the valve covers taken off a month or so ago and according to the very experienced mechanic who did it, there was no evidence whatsoever of coolant leakage. Furthermore, I don't recall having to add any coolant in the past year, and if I did it would have only been a minor amount once, but I don't recall for sure at the moment."

Barrington Diesel Club

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